numerical simulations of low cape flows over a mountain ridge abstract in a recent study the authors performed numerical simulations of conditionally unstable flows past a mesoscale mountain ridge in order to investigate the statistically stationary features of the solution precipitation characteristics for intermediate to high values of convective available potential energy cape that study proposed a functional dependence of the rain rate on three parameters related respectively to the triggering and the orographic forcing of convection and to the ratio of the advective to convective time scales the present study extends that analysis to cover larger regions of the parameter space including experiments corresponding to a wider range of cape it is found here that the low cape moderate wind experiments do not fit the functional dependence for rain rate amount and location proposed in the authors previous study the analysis of the present solutions suggests that two additional nondimensional parameters should be taken into account

a numerical study of three catastrophic precipitating events over southern france i numerical framework and synoptic ingredients this study examines the simulation of three torrential rain events observed on 13 14 october 1995 the cevennes case 12 13 november 1999 the aude case and 8 9 september 2002 the gard case over the southeastern part of france using the meso nh non hydrostatic mesoscale numerical model these cases were associated with extreme heavy precipitation events hpes with significant precipitation amounts exceeding 500 mm in less than 24 hours several sets of numerical experiments were performed with 10 km and 2 5 km horizontal resolutions in part i of this study special attention is paid to the experimental design for obtaining realistic simulations of hpes with the meso nh model as well as the evolution of the synoptic patterns in which the rainfall events are embedded the best 2 5 km numerical simulations show the ability of the meso nh model to reproduce significant quasi stationary rainfall events moreover the model fairly reproduces the low level mesoscale environments associated with the three hpes the hpes formed in a slow evolving synoptic environment favourable for the development of convective systems diffluent upper level southerly flow pv anomalies etc at lower levels a southerly to easterly moderate to intense flow provided conditionally unstable and moist air as it moved over the relatively warm mediterranean sea typical for this time of the year late summer and autumn the two extreme cases gard and aude differ from the more classical event cevennes in terms of larger low level moisture fluxes weaker values of conditional convective instability as in the aude case is counterbalanced by a stronger warm and moist low level jet the mesoscale triggering and or sustaining ingredients for deep convection and the physical mechanisms leading to the stationarity of these rainfall events are presented and discussed in a companion paper

embedded cellular convection in moist flow past topography abstract marginally unstable air masses impinging upon a mountain ridge may lead to the development of a nominally stratiform orographic cloud with shallow embedded convection rainfall amounts and distribution are then strongly influenced by the convective dynamics in this study the transition from purely stratiform orographic precipitation to flow regimes with embedded convection is systematically investigated to this end idealized cloud resolving numerical simulations of moist flow past a two dimensional mountain ridge are performed in a three dimensional domain a series of simulations with increasing upstream potential instability shows that the convective dynamics may significantly increase precipitation amounts intensity and efficiency to an extent that cannot be replicated by two dimensional simulations under conditions of uniform upstream flow the embedded convection is of the cellular type it is demonstrated that simple stability measures of the upstream profile are poor predictors for

upstream effects of mesoscale mountains abstract the alpine experiment alpex has revealed that low level air is typically diverted around the alps without reaching the mountaintop in pursuit of an understanding of the physical basis of this phenomenon and of its generality we have explored the characteristics of orographic blocking of a rotating continuously stratified quid as revealed in a simple model problem retaining full nonlinear and transient effects hydrostatic dynamics is assumed and the obstacle is taken to be an infinitely long ridge with height h x the key questions treated are the strength of the upstream deceleration of cross mountain flow and the length scale over which the decelerated region extends by means of scale analysis the controlling parameters are found to be the rossby number ro u fl and the froude number fr nhm u where u is the speed of the oncoming flow f is the coriolis parameter l the mountain half width n the brunt vaisala frequency and hm is the maximum mountain height the scale analysis also

water column structure and circulation under the north water during spring transition april july 1998 abstract the north water is a large recurring polynya in northern baffin bay over 400 hydrographic ctd profiles taken over a grid of stations between april and july 1998 were analysed to understand the water column structures underlying the polynya during spring transition in the north water imported and dissimilar water masses converge over a complex bathymetry northern assembly na waters are derived from inflows through nares strait and jones sound southern assembly sa waters are derived from a northern moving branch of the west greenland current wgc na and sa waters mix primarily over the north south smith sound canyon between 76 5 n and 77 n latitude to form a distinct relatively thick halocline layer termed north water assembly nwa characterised by density compensating thermohaline intrusions in the southern portion of the north water a sharp front between nwa waters and sa waters roughly aligns with the 500 m isobath flow patterns deduced from potential temperature vs salinity s diagrams and from minima in stability frequency n profiles indicate that this front results from the convergence of nwa waters and a branch of the wgc that is cyclonically re directed as it approaches smith sound canyon not all sa waters are immediately re directed south towards the front as a second branch follows the greenland coast toward smith sound while s signatures show some evidence of upwelling along the greenland coast e g outcropping of isopycnal surfaces no surfacing of warm atlantic derived water was observed during the sampling period 1998 suggesting that mechanical removal of ice by winds and currents is the primary mechanism by which the north water forms some sensible heat exchange associated with brine rejection and penetrative convection may occur but only during periods of active ice formation

effects of moist froude number and cape on a conditionally unstable flow over a mesoscale mountain ridge abstract in this study idealized simulations are performed for a conditionally unstable flow over a two dimensional mountain ridge in order to investigate the propagation and types of cloud precipitation systems controlled by the unsaturated moist froude number fw and the convective available potential energy cape a two dimensional moist flow regime diagram based on fw and cape is proposed for a conditionally unstable flow passing over a two dimensional mesoscale mountain ridge the characteristics of these flow regimes are 1 regime i flow with an upstream propagating convective system and an early slowly moving convective system over the mountain 2 regime ii flow with a long lasting orographic convective system over the mountain peak upslope or lee slope 3 regime iii flow with an orographic convective or mixed convective and stratiform precipitation system over the mountain and a downstream propagating convective system and 4 regime iv flow with an orographic stratiform precipitatio

small scale rainfall mechanisms for an idealized convective southerly flow over the alps rain producing flows over the south side of the alps are investigated with an idealized high resolution 2 5 km numerical configuration where only three basic ingredients of typical heavy precipitation situations have been retained 1 a uniform and stationary southerly flow far upstream from the alps which drives 2 a conditionally unstable atmosphere towards 3 the real topography of the alps and the surrounding secondary mountains attention is given to small scale mechanisms of rainfall over the po plain and the alpine slopes these are linked with two orographically induced low level wind jets the easterly barrier wind jet along the southern flank of the alps and a south easterly jet formed over the sea along the south western flank of the apennines and continuing over land up to the alps the specific role of the secondary relief upstream from the alps in inducing or influencing these low level flow features and also in triggering organized convection is studied by means of experiments on the sensitivity to topography the idealized configuration also allows the advantages of a convection resolving model for modelling precipitation distributions at the alpine valley scale to be considered copyright 2007 royal meteorological society

a theoretical study of mountain barrier jets over sloping valleys abstract a shallow water model is developed to examine the dynamics of mountain barrier jets over a mesoscale sloping valley between two mountain ridges in this model the cold air trapped in the valley is represented by a shallow water layer that is coupled with the upper layer flow through a free slip interface the pressure gradient exerted from the upper layer plus the pressure gradient generated by the sloping cold layer itself drive the cold air down the valley forming a jet against the surface friction in the cross valley direction the jet induced coriolis force is balanced by the pressure gradient force for a given cross valley terrain profile the solution is controlled by eight dimensional environmental parameters in the nondimensional space the solution is controlled only by three parameters the froude number modified by the ratio between the valley width and the rossby radius of deformation the surface drag coefficient normalized by the ratio between the valley depth and the inertial r

cross barrier flow during orographic precipitation events results from map and improve abstract ground based and airborne doppler radar data collected during passage of frontal rainstorms over the european alps and cascade mountains of oregon are found to exhibit characteristic cross barrier flow and precipitation patterns a stably stratified layer of blocked or partially blocked low momentum flow below mountain crest level is separated from strong cross barrier flow aloft by a concentrated layer of vertical wind shear this shear layer slopes upward toward the mountain crest and persists for several hours as the storm passes over the mountain range this pattern is remarkably similar from case to case and from one mountain range to the other the orographically enhanced precipitation in these cases exhibits stratiform structure marked by a radar bright band which often drops to a lower height indicating locally cooler conditions immediately adjacent to the windward mountain slopes a terrain induced gravity wave produces strong downslope flow and spillover of precipitation onto the

effects of orography on the generation and propagation of mesoscale convective systems in a two dimensional conditionally unstable flow abstract effects of orography cold air outflow and gravity waves on the generation and propagation of convective systems in a conditionally unstable airstream over a mesoscale mountain are studied using a two dimensional cloud model based on the propagation of convective systems three regimes are identified i an upstream propagating convective system ii a quasi stationary convective system and iii quasi stationary and downstream propagating systems in regime i low moist froude number fw the convective cells are generated by upstream deceleration associated with orographic forcing by gravity waves associated with convective cells over the upslope area at earlier stages and by the upstream propagating density current at later stages when the density current is fully developed in this flow regime quasi continuous and heavy rainfall is produced over the upslope and plain areas as individual convective cells develop farther upstream at the head of the density current and then propagate d

a case study of an orographic cyclone south of the alps during the map sop the cyclonic formation of iop15 of the mesoscale alpine programme map 6 7 november 1999 is examined with respect to mesoscale features in the geopotential temperature wind and precipitation fields the effect of orography at different scales is evaluated using a mesoscale meteorological model bolam model errors are determined by comparing standard and special map sop observations with model output an optimal interpolation reanalysis of the initial condition aimed at assimilating mesoscale structures is applied resulting in an overall reduction of the forecast error including position of local scale precipitation maximum in the lago maggiore target area south of the alps different orography representations have an impact on the error patterns since it is shown that the alpine orography largely controls the cyclone growth over the mediterranean particular attention is devoted to the model verification of precipitation and wind fields in the vicinity of the alps where special observations allow for fine scale analysis copyright 2003 royal meteorological society

idealized numerical simulations of quasi stationary convective systems over the northwestern mediterranean complex terrain northwestern mediterranean coastal regions are frequently affected by torrential rainfall associated with quasi stationary mesoscale convective systems mcss the present work examines how the characteristics of a conditionally unstable flow impinging on the coastal complex terrain of the northwestern mediterranean can affect the location and intensity of quasi stationary mcss the study is based on idealized simulations but including the major ingredients of northwestern mediterranean heavy precipitation events a moist conditionally unstable marine flow of about 100 km width facing the true terrain composed of the massif central surrounded by the alps and the pyrenees we find that mcss are located upstream of the mountain range with a slow flow whereas with a rapid flow the heaviest precipitation is over the massif central slopes in a similar way when the lateral environment is drier the heaviest precipitation is located upstream whereas a humid environment favours precipitation over the slopes the dominant lifting mechanism is strongly related to the location of the system 1 direct orographic triggering for systems over mountain slopes and 2 cold pool triggering for upstream systems in addition the neighbouring mountains interplay through deflection of the flow and induced low level convergence favoured by slow or dry lateral environments and through cold pool blocking within valleys copyright 2012 royal meteorological society

paleoenvironmental change in tropical africa during the holocene based on a record from lake kifuruka western uganda a 10 000 year palynological record of vegetation and environmental changes in tropical africa is presented based on sediment data from a 230 cm long core recovered from lake kifuruka in western uganda the lake kifuruka record provides new sedimentary and paleoecological data for human environment interactions and anthropogenic disturbance in the tropical african region during the holocene the record shows evidence for prolonged dry conditions in the region at the terminus of the last glaciation period and the onset of the holocene period the early holocene and mid holocene periods were characterized by largely warm and wet climatic conditions the pollen record shows evidence of forest disturbance that is probably associated with agricultural activities near lake kifuruka shortly after ca 4300bp cal a bp coinciding with a time of increased desiccation in the region the forest disturbance in the record appears to have intensified and continued into the modern period but with no significant change in the overall vegetation diversity suggesting quick environmental recovery the lake kifuruka record is largely devoid of the abrupt climatic changes that have been observed from a couple of sites in the east african region which implies that the changes may be site specific rather than regional scale

climatology of heavy precipitation over corsica in the period 1985 2015 the island of corsica in the western mediterranean is frequently struck by devastating heavy precipitation events hpes which endanger property and lives alike in the framework of the hymex hydrological cycle in the mediterranean experiment project multiple hpes from 2012 were observed and studied and to provide context a climatology for hpes based on 31 years 1985 2015 of surface observations was derived based on 173 events hpes 100 mm d 1 strike the island around 5 6 times a year and show a distinct seasonal cycle they occur predominantly during autumn with more than half of the hpes occurring from september to december and almost 20 in october the most active month the eastern half of the island is most affected by hpes composite fields for all events show a cyclone over the western mediterranean west of corsica and warm and moist low level air upstream of the orography predominantly over the tyrrhenian sea southeast of the island the events are classified using their synoptic and mesoscale environment obtained from era interim data using equivalent orthogonal functions eofs and a principal component pc analysis together with the k means clustering algorithm three clusters of events are identified each has a different seasonal cycle dominant wind direction and precipitation distribution warm southeasterly events occur predominantly in autumn produce the most extreme events and affect the coasts and slopes of eastern corsica alike cool southeasterly events occur predominantly in winter and affect mostly the slopes along the east side of the orography southwesterly events are more often associated with large scale atlantic cyclones and occur throughout almost the entire year and with the highest precipitation over the mountains

airflow over a two dimensional escarpment ii hydrostatic flow hydrostatically dominated forced flow over the asymmetric topography of the darling scarp western australia is modelled using a hydrostatic numerical mesoscale model and compared with observations simulations reproduce the essential features of wave overturning and the development of a shooting hydraulic flow this response is dependent on the wind profile but not on the existence of a critical level the high backsheared environment leads to a high ratio of the brunt vaisala frequency to wind speed throughout the profile resulting in short wavelengths high nonlinearity and overturning with the ratio of the halfwidth of the topography to the boundary layer depth being small the flow fields are shown to be sensitive to the boundary layer

an easterly tip jet off cape farewell greenland i aircraft observations an easterly tip jet event off cape farewell greenland is described and analysed in considerable detail in part i of this study this paper comprehensive aircraft based observations are described while in part ii of this study numerical simulations and a dynamical analysis are presented the easterly tip jet of 21 february 2007 took place during the greenland flow distortion experiment it resulted through the interaction of a barotropic synoptic scale low pressure system in the central north atlantic and the high topography of southern greenland in situ observations reveal a jet core at the coast with peak winds of almost 50 m s 1 about 600 800 m above the sea surface and of 30 m s 1 at 10 m the depth of the jet increased with wind speed from 1500 m to 2500 m as the peak winds increased from 30 to 50 m s 1 the jet accelerated and curved anticyclonically as it reached cape farewell and the end of the barrier the easterly tip jet was associated with a tongue of cold and dry air along the coast of southeast greenland general cloud cover to the east and cloud streets to the south of cape farewell precipitation was observed during the low level components of the flight the very high wind speeds generated a highly turbulent atmospheric boundary layer and resulted in some of the highest surface wind stresses ever observed over the ocean copyright c 2009 royal meteorological society

further sensitivities of orographic precipitation to terrain geometry in idealized simulations abstractthis study examines how variations to the nondimensional mountain height and the horizontal aspect ratio of a straight ridge and a concave ridge influence orographic precipitation an idealized three dimensional model is used to simulate a moist flow impinging upon these two ridges with 0 66 2 0 and 1 0 8 0 the concave ridge generates substantially more precipitation than the straight ridge via an established precipitation enhancing funneling mechanism near the ridge vertex when the flow is unblocked based on previous work it was hypothesized that when the approaching flow becomes blocked the strength of the precipitation enhancement by the concave ridge relative to the straight ridge becomes negligible this study reveals that if is sufficiently large to induce flow reversal on the windward slope then a secondary circulation develops that is strengthened by the concave ridge with a subsequent enhancement of precipitation it is also shown that the competing effects of the ridg

ocean acidification in the three oceans surrounding northern north america 1 the uptake of anthropogenic co2 drives ocean acidification with attendant effects on the saturation state of calcium carbonate and marine ecosystems here we examine ocean acidification within the context of large scale water mass exchange and local physical and biogeochemical processes along a section around northern north america waters in the north pacific are preconditioned by the global scale circulation to be low source waters and as they move northward across the bering and chukchi seas they are modified by biological activity these waters then enter the canada basin and canadian arctic archipelago where cooling river discharge sea ice formation and biological activities modify water mass characteristics further continuing eastward into baffin bay relatively low waters extend from the surface to depth due to remineralization changes in are large along this pathway and the response of local marine ecosystems will likewise be shaped by the biogeography of species occupying a given region

terrain trapped airflows and orographic rainfall along the coast of northern california part ii horizontal and vertical structures observed by a scanning doppler radar abstractthis study documents the mean properties and variability of kinematic and precipitation structures associated with orographic precipitation along the coast of northern california in the con

orographic influence on basic flow and cyclone circulation and their impacts on track deflection of an idealized tropical cyclone abstracta series of idealized numerical experiments and vorticity budget analyses is performed to examine several mechanisms proposed in previous studies to help understand the orographic influence on track deflection over a mesoscale mountain range when an idealized tropical cyclone tc is embedded in a uniform easterly flow and passes over a mountain with a moderate froude number it is deflected to the south upstream moves over the mountain anticyclonically and then resumes its westward movement the vorticity budget analysis indicates that the tc movement can be predicted by the maximum vorticity tendency vt the orographic effects on the above tc track deflection are explained by the following 1 upstream of the mountain the easterly basic flow is decelerated as a result of orographic blocking that causes the flow to become subgeostrophic which advects the tc to the southwest analogous to the advection of a point vortex embedded in a flow the vt is primarily dominated by the horizontal vo

three dimensional effects in high drag state flows over long ridges abstract numerical simulations of nonrotating flow with uniform basic wind and stability past long three dimensional 3d ridges are compared to the corresponding two dimensional 2d limit to reveal the importance of 3d effects for mountain heights smaller than the threshold for breaking waves the low level flow over the interior of the ridge is well described by 2d theory when the horizontal aspect ratio is roughly 10 or greater by contrast in flows with wave breaking significant discrepancies between 2d and 3d results remain apparent even for 12 it is found that the onset of wave breaking and the transition to the high drag state is accompanied in 3d by an abrupt increase in deflection of the low level flow around the ridge the increased flow deflection is produced at least in part by upstream propagating columnar disturbances forced by the transition to the high drag state the deflection of the incident flow reduces the amplitude of the mountain wave aloft relative to 2d and acts as a neg

warming of the polar water layer in disko bay and potential impact on jakobshavn isbrae a number of recent studies have shown enhanced retreat of tidewater glaciers over much of southern and western greenland one of the fastest retreats has occurred at jakobshavn isbrae with the rapid retreat linked to the arrival of relatively warm and saline irminger water along the west coast of greenland similar links to changes in ocean water masses on the coastal shelf of greenlandwere also seen on the east coast this study presents hydrographic data from disko bay additionally revealing that there was also a significant warming of the cold polar water entering disko bay from the mid to late 1990s onward this layer which lies at a depth of 30 200m warmedby 18 28c the heat content of the polar water layer increased by a factor of 3 6 for the post 1997 period compared to the period prior to 1990 the heat content in the west greenland irminger water layer between the same periods increased only by a factor of 2 but contained more total heat the authors suggest that the changes in the polar water layer are related to circulation changes in baffin bay

origins of aircraft damaging clear air turbulence during the 9 december 1992 colorado downslope windstorm numerical simulations and comparison with observations results from numerical simulations of the colorado front range downslope windstorm of 9 december 1992 are presented although this case was not characterized by severe surface winds the event caused extreme clearair turbulence cat aloft as indicated by the severe structural damage experienced by a dc 8 cargo jet at 9 7 km above mean sea level over the mountains detailed measurements from the national oceanic and atmospheric administration environmental research laboratories environmental technology laboratory doppler lidar and wind profilers operating on that day and from the defense meteorological satellite program satellite allow for a uniquely rich comparison between the simulations and observations four levels of grid refinement were used in the model the outer domain used national centers for environmental prediction data for initial and boundary conditions the finest grid used 200 m in all three dimensions over a 48 km by 48 km section the range of resolution and domain coverage were sufficient to resolve the abundant variety of dynamics associated with a time evolving windstorm forced during a frontal passage this full range of resolution and model complexity was essential in this case many aspects of this windstorm are inherently three dimensional and are not represented in idealized models using either 2d or so called 2d 3d dynamics both the timing and location of wave breaking compared well with observations the model also reproduced cross stream wavelike perturbations in the jet stream that compared well with the orientation and spacing of cloud bands observed by satellite and lidar model results also show that the observed cat derives from interactions between these wavelike jet stream disturbances and mountain forced internal gravity waves due to the nearly east west orientation of the jet stream these two interacting wave modes were orthogonal to each other thermal gradients associated with the intense jet stream undulations generated horizontal vortex tubes hvts aligned with the mean flow these hvts remained aloft while they propagated downstream at about the elevation of the aircraft incident and evidence for such a vortex was seen by the lidar the model and observations suggest that one of these intense vortices may have caused the aircraft incident reports of strong surface gusts were intermittent along the front range during the period of this study the model showed that interactions between the gravity waves and flow aligned jet stream undulations result in isolated occurrences of strong surface gusts in line with observations the simulations show that strong shears on the upper and bottom surfaces of the jet stream combine to provide an episodic downburst of turbulence in the present case the perturbations of the jet stream provide a funnel shaped shear zone aligned with the mean flow that acts as a guide for the downward transport of turbulence resulting from breaking gravity waves the physical picture for the upper levels is similar to the surface gusts described by clark and farley resulting from vortex tilting the cat feeding into this funnel came from all surfaces of the jet stream with more than half originating from the vertically inclined shear zones on the bottom side of the jet stream visually the downburst of turbulence looks similar to a rain shaft plummeting to the surface and propagating out over the plains leaving relatively quiescent conditions behind

structure and mixing across an arctic atlantic front in northern baffin bay 1 a front forms in northern baffin bay 75 25 n between atlantic derived water carried by the west greenland current and arctic derived waters exiting southward through nares strait and jones sound via the baffin current subsurface waters e g below 100 m of the west greenland current are as much as 2 c warmer than those of the baffin current confluence of these waters leads to a frontal transition between 100 500 m wherein cross frontal gradients of potential temperature l 0 06 0 07 c km 1 and salinity s l 0 005 0 007 psu km 1 are largely density compensating l 0 001 km 1 subsequent thermohaline interleaving establishes conditions conducive to mixing via cabelling and double diffusion the front s location would allow it to play a contributing role in the formation of baffin current water structure and eventual export of freshwater to the north atlantic

holocene vegetation history and quantitative climate reconstructions in a high elevation oceanic district of the italian alps evidence for a middle to late holocene precipitation increase abstract we reconstructed the vegetation and climate history during the last 10 ka in a high elevation sedimentary record armentarga peat bog 2345 m asl on the southern flank of the european alps through the study of paleoecological and sedimentary proxies we included a specific elevational transect of modern pollen accumulation rates for timberline forming trees and shrubs alnus viridis pinus sylvestris mugo pinus cembra quantitative reconstructions of july temperature and annual precipitation were obtained by applying numerical transfer functions built on an extensive pollen climate calibration set from the european alps changes in elevational vegetation arrangement were primarily driven by phases of precipitation increase and to a lesser extent by millennial scale temperature changes already known from glacier timberline chironomids and speleothem records at alpine scale changes in pollen inferred annual precipitation occurred in three main steps an early holocene moderately humid phase is mirrored by the early spread of alnus viridis dwarf forests precipitation started to increase at 6 2 ka cal bp a further prominent step forward at the middle to late holocene transition led to the high values of snowfall and runoff characterizing today s oceanic elevational climates of the outer italian alps this change led to timberline depression and grassland expansion locally human impact was weak at the late neolithic bronze age transition this event correlates with lake level oscillations in the northern mediterranean borderlands suggesting intensification of southern air masses conveyed by tyrrhenian cyclones towards windward districts

moist dynamics and orographic precipitation in northern and central california during the new year s flood of 1997 abstract the dynamics of moist orographic flows during the january 1997 floods in northern and central california are investigated using numerical simulations computed with the fifth generation pennsylvania state university national center for atmospheric research psu ncar mesoscale model mm5 early in the event 31 december 1996 1 january 1997 the low level winds offshore of california s central coast were blocked by the topography of the santa lucia range and the low level winds in the central valley were blocked by the topography of the central sierra nevada range in contrast moisture laden winds along the northern coast ranges and the northern sierra nevada flowed over topographic barriers as the core of humid air migrated to the south over 24 h the low level barrier jets weakened as the atmospheric stability decreased bringing heavy rainfall to the central and southern sierra nevada at the end of the event the heavy precipitation during this event was largely controlled by the interactio

low froude number flow past three dimensional obstacles part i baroclinically generated lee vortices abstract we study the flow of a density stratified fluid past a three dimensional obstacle using a numerical model our special concern is the response of the fluid when the froude number is near or less than unity linear theory is inapplicable in this range of froude number and the present numerical solutions show the rich variety of phenomena that emerge in this essentially nonlinear flow regime two such phenomena which occupy parts i and ii of this study are the formation of a pair of vertically oriented vortices on the lee side and a zone of flow reversal on the windward side of the obstacle the ice vortices have been explained as a consequence of the separation of the viscous boundary layer from the obstacle however this boundary layer is absent by design in the present experiments and lee vortices still occur we argue that a vertical component of vorticity develops on the lee side owing to the tilting of horizontally oriented vorticity produced baroclinically as the isentropes deform in r

simulations of moist nearly neutral flow over a ridge although a fairly common atmospheric condition in orographic rain scenarios there is relatively little known about moist neutral flows over a ridge from theory and modeling presented in this paper are numerical simulations of the orographic flow modification occurring for a two dimensional moist nearly neutral flow over a ridge in the regime where the coriolis force can be neglected if an initially saturated moist neutral flow were to remain everywhere saturated as it flows over an obstacle then the expected solution would be the linear solution because the condition for linearity hill height less than the ambient wind velocity static stability is always met however for higher mountains the solutions indicate the development of areas of unsaturated air with correspondingly larger values of local static stability this internal switching from small to large values of static stability is an inherent nonlinearity which has far reaching consequences for understanding the orographic flow modification in this regime the sensitivity of the solution to the mountain height and to the initial cloud water content is analyzed here the authors find that the solutions fall into three basic categories if the mountain height is small enough a saturated flow can be maintained everywhere given sufficient initial cloud water for tall mountains the atmosphere upwind of the mountain is maintained in a saturated state and transitions to an unsaturated downslope flow on the lee side which has characteristics associated with downslope windstorms for mountains of intermediate height the solutions show the existence of an upwind propagating disturbance that has the effect of desaturating the atmosphere above the mountain

formation mechanisms for convection over the ligurian sea during map iop 8 abstract the dynamical impacts of an unusually strong stable layer that developed over the po valley and northern ligurian sea during mesoscale alpine program map intensive observation period 8 iop 8 on the formation of convection over the ligurian sea are explored based on numerically simulated equivalent potential temperature wind vectors and by a trajectory analysis of parcels both beneath and above the stable layer it is shown that the stable layer behaved as a material surface or effective mountain to the airstreams impinging on it from the south additional analyses show that the leading edge of the stable layer was collocated with maxima in upward motion and a strong positive moisture flux hence it was further argued and demonstrated through inspection of soundings upstream of the cold dome and trajectory analyses that lifting by the stable layer enhanced convective activities over the ligurian sea finally processes contributing to the maintenance of the stable layer during iop 8 were

a numerical study of moist stratified flow regimes over isolated topography numerical simulations of moist flows over simply shaped three dimensional mountains were undertaken using a mesoscale meteorological model with the purpose of examining the existence of different flow regimes near obstacles and the transitions among them moving the system in parameter space resulted in different solutions for a particular set of parameters this method was adopted to explore numerically multiple regimes stable for at least meteorologically interesting time periods the evolution of the flow was studied in two different experimental set ups in the first case the height of the obstacle was progressively changed over time in the second case the effect of an advective change of the humidity content inside the channel was analysed in both experiments a dependence on the history of the flow became apparent a comparison among cases of obstacles with the same aspect ratio and non dimensional height but with different horizontal cross sections highlighted the importance of the obstacle geometry in perturbing the upstream flow favouring the persistence of upstream blocked regions in the case of an arc shaped mountain the sensitivity of the reversal flow volume to several control parameters was analysed with additional experiments in particular the importance of the cooling associated with the evaporation of precipitation was emphasized copyright 2004 royal meteorological society

roll circulations in the convective region of a simulated squall line abstract in high resolution numerical simulations using horizontal grid spacing less than 1 km the convective region of squall lines will sometimes overturn as quasi horizontal convective rolls the authors study one case in detail using output from a simulation with 125 m grid spacing the rolls have an average spacing of 3 km and are aligned parallel to the vertical wind shear individual convective cells often have long lived undiluted cores that entrain primarily on the sides of the rolls i e between the roll updraft and downdraft the following set of conditions is proposed for obtaining roll overturning the formation of a moist absolutely unstable layer maul vertical shear of the horizontal wind within the maul an environment without large amplitude perturbations and quasi horizontal flow over the squall line s surface based cold pool further insight is gained through a series of more idealized simulations wherein a specified maul is perturbed by analytic potential temperature pertur

characteristics of summer airflow over the antarctic peninsula in response to recent strengthening of westerly circumpolar winds abstract summer near surface temperatures over the northeast coast of the antarctic peninsula have increased by more than 2 c over the past 40 years a temperature increase 3 times greater than that on the northwest coast recent analysis has shown a strong correlation between this striking warming trend and significant change in the summer southern hemisphere annular mode sam which has resulted in greatly increased summer westerlies across the northern peninsula it has been proposed that the strengthening westerlies have resulted in increased vertical deflection of relatively warm maritime air over the northern peninsula contributing significantly to the observed warming and the recent collapse of northern sections of the larsen ice shelf in this study laboratory and numerical modeling of airflow incident to the peninsula are employed to further understand this mechanism it is shown that the effect of the strengthening westerlies has led to a distinct transition from a blocked regime to a flow

mauritius since the last glacial environmental and climatic reconstruction of the last 38 000 years from kanaka crater a 10 m long peat core from the kanaka crater 20 25 s 57 31 e located at 560 m elevation in mauritius was analyzed for microfossils eight radiocarbon ages show the pollen record reflects environmental and climatic change of the last ca 38 cal ka bp the record shows that the island was continuously covered by forest with erica heath philippia in the uplands cyperaceous reedswamp with pandanus trees was abundant in the coastal lowlands as well as locally in the waterlogged crater the record shows changes in climatic humidity wet from 38 0 to 22 7 cal ka bp drier from 22 7 to 10 6 cal ka bp and wetter again from 10 6 cal ka bp to recent as the main response to climate change a high turnover in montane forest species is evidenced at 22 7 cal ka bp and at the start of the holocene the limited altitudinal ranges in the mountains of mauritius maximum altitude 828 m and changing humidity being more important than changing temperature suggests that in response to climate change a reassortment in taxonomic composition of montane forests might be equally important as displacement of forest types to new altitudinal intervals we found weak impact of the latitudinal migration of the intertropical convergence zone and data suggest that the indian ocean dipole is a more important driver for climatic change in the southwest indian ocean

great salinity anomalies in the north atlantic abstract we revisited the great salinity anomaly of the 1970s gsa 70s dickson et al 1988 and documented the newly identified great salinity anomaly of the 1980s hence termed gsa 80s both propagated around the north atlantic in a similar fashion the advective mechanism initially proposed to explain the observed sequence of low salinity low temperature events during the gsa 70s apparently holds also for the gsa 80s the latter was successively observed in the west greenland current 1982 the labrador current 1983 the flemish pass 1984 near the charlie gibbs fracture zone 1984 1985 in the rockall channel 1985 south of iceland 1985 1988 in the north sea 1986 1987 norwegian sea 1987 1988 barents sea 1988 1989 and iceland sea 1989 1990 the advection speed of the gsa 80s seems to be greater than the one of the gsa 70s the 1980s anomaly reached the barents sea 6 to 7 years after peaking in the west greenland current while the 1970s anomaly traveled the same route in 8 to 10 years these anomalies however seem to be of different origin the gsa 70s was apparently boosted remotely by a freshwater sea ice pulse from the arctic via fram strait consequently the gsa 70s was accompanied by a large sea ice extent anomaly in the greenland and iceland seas which propagated into the labrador sea in contrast the gsa 80s was likely formed locally in the labrador sea baffin bay mainly because of the extremely severe winters of the early 1980s but supplemented with a possible contribution of the arctic freshwater outflow via the canadian archipelago facilitated by strong northerly winds which would have enhanced stability and ice formation this anomaly was also associated with a positive sea ice extent anomaly in the labrador sea baffin bay which however had no upstream precursor in the greenland sea thus the gsas are not necessarily caused solely by an increased export of freshwater and sea ice from the arctic via fram strait these results are corroborated by the early 1990s data when a new fresh cold anomaly was formed in the labrador sea and accompanied by a large positive sea ice extent anomaly the harsh winters of the early 1990s were however confined to the labrador sea baffin bay area while the atmospheric and oceanic conditions in the greenland iceland and irminger seas were normal the labrador sea baffin bay area appears therefore to play a key role in formation of gsas as well as in propagation of the gsas formed upstream a likely contribution of the enhanced canadian archipelago freshwater outflow to the gsa formation also seems to be significant two major modes of the gsa origin are thus identified remote generated by an enhanced arctic ocean freshwater export via either fram strait or the canadian archipelago and local resulting from severe winters in the labrador sea baffin bay both modes should be taken into account to model decadal variability of the coupled ocean ice atmosphere system in the north atlantic arctic basin

an overview of physical processes in the north water the presence of a polynya strongly affects biological processes by influencing underwater light levels water column stratification the upwelling of nutrients and the timing of production cycles an overview of the existing literature on the oceanography meteorology and sea ice conditions of the north water a large recurring polynya in northern baffin bay is presented the north water is influenced by a cold inflow of arctic ocean water from the north and in some areas a warmer inflow of atlantic water from the south earlier observations and modeling studies suggested that both wind and ocean advection of sea ice as well as heat input to the surface waters by either upwelling or mixing were responsible for the formation and maintenance of the polynya recent data indicate the north water to be primarily generated by the southward drift of ice by winds and currents as opposed to melting ice by sensible heat input over large areas localized sensible heat effects occur in fall winter and spring along the greenland side of the polynya

upstream propagating wave modes in moist and dry flow over topography abstractprevious studies have observed upstream propagating modes in two dimensional numerical simulations of idealized flow over topography with moist nearly neutral conditions in the troposphere topped by a stable stratosphere the generation and propagation mechanisms for these modes were attributed to localized and dramatic changes in stability induced by the desaturation of the flow impinging on the mountain in the present paper it is shown that these modes are transient upstream propagating gravity waves which are a fundamental feature of both moist and dry flow over topography of a two layer troposphere stratosphere atmospheric profile impulsively started from rest the mode selection and propagation speeds of these transient waves are highly dependent on the tropospheric stability as well as the wind speed and tropopause depth in the moist case these modes appear to propagate according to an effective static stability that is intermediate to the normal dry stability and the lower moist stabi

the denver cyclone part i generation in low froude number flow abstract a numerical model is used to study the denver cyclone a mesoscale vortex that develops in eastern colorado under southerly to southeasterly flow diurnal effects e g surface heating cooling have been excluded from these simulations but are discussed in part ii of this study simulations are first performed with a southerly ambient wind i e parallel to the continental divide if the froude number of the flow based on the height of the palmer divide an east west ridge to the south of denver is small then a vortex forms over the denver region this circulation then propagates northward at just over half the upstream flow velocity leaving essentially stagnant air in the lee of the palmer divide it is shown that coriolis turning has a negligible effect on the flow when the ambient wind is from the south however when the ambient wind is at an angle to the continental divide northerly winds can develop along the foothills due to coriolis turning of the flow as it approaches the divid

multi proxy reconstruction of environmental dynamics and colonization impacts in the mauritian uplands a 115 cm long sediment core retrieved from the exposed uplands of mauritius a small oceanic island in the indian ocean shows environmental change from the uninhabited era into post colonization times well preserved fossil pollen and diatoms in the uppermost 30 cm of the core reflect environmental conditions during the last 1000 years granulometric analysis along the core shows that the sediments below 30 cm consist of weathered material and that the record may contain hiatuses this is also illustrated by a 14c date at 96 cm depth of 35 000 calibrated years before ad 1950 35 0 cal ka the pollen record shows that pristine vegetation at 650 m elevation consisted of ericaceous heathland and pandanus marsh around 0 9 cal ka wet montane forest and fern rich marsh replaced heathland vegetation indicating higher moisture availability natural changes in upland vegetation associations are mainly driven by changes in sediment accumulation causing changes in soil properties and drainage conditions the historically well dated start of colonization ad 1638 is reflected by the sudden arrival of exotic plant taxa camellia sinensis tea pinus spp pine casuarina equisetifolia coastal she oak psidium cattleianum strawberry guava homalanthus queensland poplar and saccharum officinarum sugar cane as well as an increase in charcoal indicating deforestation

low froude number flow past three dimensional obstacles part ii upwind flow reversal zone abstract the present paper contains a continuation of our study of the flow of a density stratified fluid past three dimensional obstacles for froude number o 1 linear theory large froude number and potential flow type theory small froude number are both invalid in this range which is of particular relevance to natural atmospheric flows past large mesoscale mountains the present study was conceived to provide a systematic investigation of the basic aspects of this flow thus we have excluded the effects of friction rotation nonuniform ambient flow and the complexity of realistic terrain in part i of this study we focused on the pair of vertically oriented vortices forming on the lee side when the froude number decreases below 0 5 approximately and argued that their formation may be understood in terms of nonlinear aspects of inviscid gravity waves i e without invoking traditional arguments on the separation of the friction boundary layer herein we examine the zone of flow reversal on

orographic clouds in terrain blocked flows an idealized modeling study abstract idealized numerical simulations of moist strongly stratified flow over topography are used to study the processes that control orographic clouds in terrain blocked flows as a joint function of the nondimensional flow parameter nh u the horizontal topographic aspect ratio and the rossby radius of deformation nh f the simulations show the competition between enhanced upstream condensation in the secondary vertically propagating gravity wave and the reduction of condensation owing to enhanced low level flow deflection as nh u increases above about 1 5 the tendency for flow to be deflected around the barrier reduces cloud formation in the primary gravity wave over the ridge while increasing expands low level clouds over a broader upstream area ice clouds may form aloft in the secondary vertically propagating gravity wave and extend upstream for several hundred kilometers in terrain blocked flows more than half of the condensate mass develops upstream of the barrier in the secondary gravi

a gravity wave drag matrix for complex terrain abstractwe propose a simplified scheme to predict mountain wave drag over complex terrain using only the regional average low level wind components u and v the scheme is tuned and tested on data f

role of eastern ghats orography and cold pool in an extreme rainfall event over chennai on 1 december 2015 chennai and its surrounding region received extreme rainfall on 1 december 2015 a rain gauge in the city recorded 494mm of rainfall within a span of 24 h at least a 100 yr event the convective system was stationary over the coast during the event this study analyzes how the eastern ghats orography and moist processes localized the rainfall era interim data show a low level easterly jet llej over the adjacent ocean and a barrier jet over the coast during the event acontrol simulation with the nonhydrostatic weather research and forecasting wrf model shows that the eastern ghats obstructed the precipitation driven cold pool from moving downstream resulting in the cold pool piling up and remaining stationary in the upwind direction the cold pool became weak over the ocean it stratified the subcloud layer and decelerated the flow ahead of the orography hence the flow entered a blocked regime maximum deceleration of the winds and uplifting happened at the edge of the cold pool over the coast therefore a stationary convective system and maximum rainfall occurred at the coast as a result of orographic blocking propagation of a low pressure system lps was obstructed because of the topographic beta effect the lps subsequently traveled a southward path in a sensitivity experiment without the orography the cold pool was swept downstream by the winds clouds moved inland in the second experiment with no evaporative cooling of rain the cold pool did not form flow as well as clouds moved over the orography

a theory for strong long lived squall lines abstract we study herein the mechanics of long lived line oriented precipitating cumulus convection squall lines using two and three dimensional numerical models of moist convection these models used in juxtaposition enable us to address the important theoretical issue of whether a squall line is a system of special long lived cells or whether it is a long lived system of ordinary short lived cells our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but on occasion a squall line may be composed of essentially steady supercell thunderstorms the numerical experiments presented herein show that either type of squall line may develop from an initial line like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line with shallow shear oriented perpendicular to the line a long lived line evolves containing individually short lived cells our analysis of this type o

the effects of a mountain on the propagation of a preexisting convective system for blocked and unblocked flow regimes abstract observations and previous research of squall lines impinging on mountain ranges have revealed that the squall lines sometimes stall upstream of the mountains for several hours leading to copious accumulations of precipitation it has been hypothesized that squall line stagnation may be more prone to occur in flows where the froude number f u nh where u is the basic state wind n is the brunt vaisala frequency and h is the mountain height is low this hypothesis is tested herein through a series of idealized two dimensional experiments where a convective system was triggered upstream of a mesoscale mountain in conditionally unstable flow for simulations with relatively low froude numbers stagnation of the preexisting convective system was not observed in the simulations with high values of f squall lines were noted to stagnate between 100 and 200 km upstream of the mountain this result indicates that squall line stagnation may be more favored for moderate to large values of f for condi

orographic precipitation isotopic ratios in stratified atmospheric flows implications for paleoelevation studies an idealized fully nonlinear atmospheric model was used to evaluate the effects of stratified atmospheric flows on orographic precipitation isotopic ratios with the aim of improving the foundation for interpreting proxy records used for paleoelevation studies orographic precipitation isotopic ratios can be understood in terms of two nondimensional parameters the flow parameter nh u where n is the moist brunt vaisala frequency a measure of atmospheric stratification h is the large scale relief and u is the upstream horizontal wind speed perpendicular to the orogen and the horizontal aspect ratio the ratio of the along strike length of the orogen to its across strike width existing rayleigh condensation models for paleoaltimetry studies are only strictly applicable when nh u 0 much of the time high values of both nh u and favor precipitation upstream of the terrain leading to isotopic ratios that are more depleted than predicted by the rayleigh model high nh u and low favor low level deflection of winds around the terrain which substantially reduces lifting and condensation leading to precipitation isotopic ratios in that are less depleted than predicted by the simple rayleigh model changes in climate or the horizontal terrain aspect ratio can change precipitation isotopic ratios at least as much as changes in surface elevation and surface uplift need not always affect precipitation isotopic ratios suggesting that proxy records must be interpreted within a broad context of climate variability and landscape evolution

on the role of sloping terrain in the forcing of the great plains low level jet abstract the summertime great plains low level jet llj has been the subject of numerous investigations during the past several decades characteristics of the llj include nighttime development of a pronounced wind maximum of typically 15 20 m s 1 at levels 300 800 m above the surface and a clockwise rotation of the wind maximum during the course of the night maximum frequency of occurrence of the llj is found in the southern great plains theories proposed to explain the diurnal wind maximum of the great plains llj include inertial oscillation of the ageostrophic wind the diurnal oscillation of the horizontal pressure field associated with heating and cooling of the sloping terrain and the western boundary current interpretations a simple equation system and output from the 12 km horizontal resolution weather research and forecasting nonhydrostatic mesoscale model nam for july 2008 are used to provide evidence as to the importance of the great plains topography in driving the llj summertime heati

airflow within major alpine river valleys under heavy rainfall summary this study documents the aire ow within major alpine river valleys in the presence of precipitation the analysis seeks a conceptual understanding of the effect of rain on the valley e ow ground based and airborne doppler radar and surface data from the mesoscale alpine programme show that during persistent rainy periods the subsidence caused by melting and evaporation of precipitation particles contributes to the formation of a down valley e ow this down valley eow opposes moist southerly e ow aloft which is lifted over the topographic barrier and in which precipitation particles are formed

characteristics and predictability of a supercell during hymex sop1 an analysis is presented here of intense convection affecting the friuli venezia giulia region fvg northeastern italy during the intensive observation period 2b iop2b in the first special observation period sop1 of hymex hydrological cycle in mediterranean experiment the present study focuses on the first of three severe convection episodes that affected fvg on the morning of 12 september 2012 in the first episode a supercell which produced hail and severe damage to trees and buildings was observed on the plain of fvg the available observations are analysed together with a high resolution mesoscale model in order to identify the relevant mechanisms for the formation and development of the cell six different simulations were performed starting at three different initial times using respectively two different analysis forecasts as initial boundary conditions a large spread in forecast precipitation is found among the six simulations only a few of the simulations were able to reproduce intense rainfall on the plain of fvg during the morning although with significant differences in the rainfall distribution among them one of the six simulations well reproduces the observed elongated distribution of the intense rainfall maximum the characteristics of the cell responsible for this distribution are consistent with those expected for a supercell and its simulated evolution near the adriatic coast agrees well with the other observations some instability parameters over the fvg plain and offshore over the northern adriatic sea are analysed every 5 min showing that during this event the potential instability varies significantly over small space and time intervals and among the simulations the best simulations have the best match to the observed potential instability calculated using the mean characteristics of the lowest 500 m layer

rapid succession of plant associations on the small ocean island of mauritius at the onset of the holocene the island of mauritius offers the opportunity to study the poorly understood vegetation response to climate change on a small tropical oceanic island a high resolution pollen record from a 10 m long peat core from kanaka crater 560 m elevation mauritius indian ocean shows that vegetation shifted from a stable open wet forest last glacial state to a stable closed stratified tall forest holocene state an ecological threshold was crossed at w11 5 cal ka bp propelling the forest ecosystem into an unstable period lasting w4000 years the shift between the two steady states involves a cascade of four abrupt 150 years forest transitions in which different tree species dominated the vegetation for a quasi stable period of respectively 1900 1100 and 900 years we interpret the first forest transition as climatedriven reflecting the response of a small low topography oceanic island where significant spatial biome migration is impossible the three subsequent forest transitions are not evidently linked to climate events and are suggested to be driven by internal forest dynamics the cascade of four consecutive events of species turnover occurred at a remarkably fast rate compared to changes during the preceding and following periods and might therefore be considered as a composite tipping point in the ecosystem we hypothesize that wet gallery forest spatially and temporally stabilized by the drainage system served as a long lasting reservoir of biodiversity and facilitated a rapid exchange of species with the montane forests to allow for a rapid cascade of plant associations

comparison of laser grain size analysis with pipette and sieve analysis a solution for the underestimation of the clay fraction classically the grain size of soil and sediment samples is determined by the sieve method for the coarse fractions and by the pipette method based on the stokes sedimentation rates for the fine fractions results from the two methods are compared with results from laser diffraction size analysis which is based on the forward scattering of monochromatic coherent light from a point of view of laboratory efficiency the laser sizing technique is far superior accuracy and reproducibility are shown by measurements on certified materials it appears that laser grain size measurements of certified materials correspond very well with the certificated measurements tests were also done on a set of randomly selected sediments of fluvial aeolian and lacustrine origin except for the 2 m clay fraction there is a coarsening of the mean diameter of one to two size classes 0 25 caused by the non sphericity of the particles the platy form of the clay particles induces considerable differences eight size classes between pipette and laser measurements the 2 m grain size defined by the pipette method corresponds with a grain size of 8 m defined by the laser particle sizer for the studied sediments using a higher grain size level for the clay fraction when laser analysis is applied enables workers in the geological and environmental field to compare classical pipette analysis with a laser sizing technique

mechanisms of intense alpine rainfall abstract numerical studies by the authors and others of the 1994 piedmont flood show that the orographically modified flow was a critical element for the production of extraordinary rainfall to uncover the precise mechanism of orographic rainfall occurring in full physics mm5 simulations of the 1994 piedmont flood the authors carried out simulations with the same real data initial and boundary conditions but with the real orography replaced by an idealized one with excellent agreement between real and idealized orography on the rainfall rate versus time in the piedmont area analysis of the idealized orography simulation provides a clear picture of the model s mechanism of orographically induced rainfall as noted in previous studies of the 1994 piedmont case a moist saturated airflow has a reduced effective static stability and tends to flow over the mountains while an unsaturated airstream is stable and tries to flow around toward the left in the northern hemisphere in the 1994 piedmont case t

effects of the alps and apennines on forecasts for po valley convection in two hymex cases two intensive observation periods iops of the hydrological cycle in the mediterranean experiment hymex are examined in this study iop6 and iop13 were characterized by troughs with associated cold fronts entering the western mediterranean and in both cases organized frontal convection in the po valley was observed these similarities notwithstanding predictability of the po valley convection was limited for iop6 for most of the forecast models available during hymex but not for iop13 using the weather research and forecast wrf model in research mode the present study confirms the relatively good forecast for frontal convection in iop13 is not very sensitive to modeling assumptions in contrast it is found that only a two way nested simulation initialized close to the event was able to produce a realistic representation of the squall line in the po valley for iop6 a comparison between a successful and an unsuccessful simulation of the po valley convection in iop6 suggests the sensitivity lies with the orographic flow modification which was at the threshold between flow over and flow around responses to the maritime alps in particular in the flow over regime downslope winds from the maritime alps and apennines suppress the convergence uplift where the front encounters the barrier wind in the po valley while in the flow around regime it is unimpeded a delicate balance between the competing mechanisms of orographically induced subsidence on the lee side of the apennines and frontal uplift in the po valley is found to be crucial for squall line survival

heavy precipitation on the alpine southside an upper level precursor analyses of four separate autumnal events of very heavy precipitation on the southern slopes of the european alps indicate that each was accompanied by a strikingly similar anomaly at upper tropospheric levels it took the form of a narrow 500km deep 4km and elongated 2000 km streamer of intruded stratospheric air sic high potential vorticity air extending north south from the british isles to the western mediterranean in each case the streamer translated comparatively slowly eastward and the storm event ensued as its forward flank approached the alpine ridge empirical indicators and theoretical considerations support the thesis that the streamer is an ubiquitous dynamically significant and distinctive precursor of storms on the alpine southside

the persian gulf summertime low level jet over sloping terrain there are few observational and numerical studies of the summertime low level jet llj over the persian gulf known as the shamal in this study the weather research and forecasting wrf model accurately simulates the llj s vertical structure nocturnal features and strong diurnal variation of the wind the case study period is divided into a period during which the shamal is spatially extended strong winds over the gulf and iraq and a period during which it is less spatially extended strong flow only over the gulf orography mountain slope and land sea breeze are found to be key factors for the shamal diurnal variation of wind speed and wind direction the blackadar mechanism appears to be secondary to the heating in forcing the shamal the zagros mountains not only channel the northwesterly winds but also provide a barrier for the easterly monsoon airflow which maximizes the wind speed only during the less extensive period does the zagros mountains slope significantly affect the shamal wind intensity and direction both shallow and steep mountain slope sensitivity studies feature a jet like flow over the persian gulf the steep slopes cause larger wind speeds however the shallow slopes reveal a stronger diurnally varying wind direction due to larger heating and cooling of the sloping terrain it is also shown that during the less extensive period the land breeze and the lower friction over the sea increase the intensity of the nocturnal jet over the gulf copyright 2011 royal meteorological society

numerical simulations of conditionally unstable flows over a mountain ridge abstractin two recent papers the authors performed numerical simulations with a three dimensional explicitly cloud resolving model for a uniform wind flowing past a bell shaped ridge and using an idealized unstable weisman klemp sounding with prescribed values of the relevant parameters more recently some observed cases of orographically forced wind profiles were analyzed showing that in order to reproduce larger rainfall rates it was necessary to initialize the sounding with low level flow toward the mountain with weak flow aloft as observed additional experiments using the weisman klemp sounding but with nonuniform wind profiles are performed here to identify the conditions in which the presence of a low level cross mountain flow together with calm flow aloft may increase the rain rates in conditionally unstable flows over the orography the sensitivity of the solutions to the wind speed at the bottom and the top of a shear layer and the effect of different mountain widths and heights are

formation and maintenance of a long lived taiwan rainband during 1 3 march 2003 abstracttaiwan rainbands trs defined here as convective lines which form off the mountainous eastern coast of taiwan under weakly synoptically forced weather conditions are a well known mesoscale phenomenon but their formative processes remain the subject of debate this study uses surface and radar observations within the coastal zone of eastern taiwan and ncep reanalysis data to document a long lived tr with a lifetime of 36 h during 1 3 march 2003 to advance the current general understanding of mechanisms responsible for the tr s formation and maintenance detailed analyses indicate that the rainband was initiated by convergence that was produced as low level environmental northeasterly easterly onshore flow encountered topographically blocked northerlies that developed nearshore the northerly blocked flow was observed to weaken and subsequently dissipate because of changing synoptic pressure patterns that caused prevailing southeasterlies southerlies at low levels however colder nearshore ai

dynamics of a catalina eddy revealed by numerical simulation abstract through numerical simulations with the pennsylvania state university ncar mesoscale model the dynamics of a catalina eddy event that formed during the period 26 30 june 1988 off the coast of southern california is examined a strengthening and veering of the low level synoptic scale winds from climatological northwesterlies to northerlies results in a more pronounced effect of the coastal orography around the california bight region in particular relative vorticity formed by flow over the coastal terrain remains offshore prior to the formation of a quasi steady eddy within the bight the northerlies are strong enough to advect anomalously high vorticity out of the region the formation of a mature catalina eddy relies on a rapid deceleration of the synoptic scale northerlies on 30 june such that vorticity once formed remains in the bight the eddy is also strongly modulated by the diurnal cycle northwesterly flow around 500 m above mean sea level impinging on the mountains north of the b

analysis and simulation of local and regional conditions for the rainfall over the lago maggiore target area during map iop 2b the mesoscale alpine programme map operational and research network collected a large variety of measurements during the two days of intensive observing period iop 2b 19 and 20 september 1999 which is one of the most intense rainfall episodes during the special observing period over the lago maggiore area the synoptic situation is a typical one of a southerly flow and associated convection over alpine orography simulations with the non hydrostatic meso nh model are used to describe the rainfall conditions and are successfully validated at the different scales allowed by the map data the two types of pre frontal orographically triggered convective episodes are described 1 sporadic and shallow convection occurs over the coastal mountains and propagates over the po valley the chronology and the intensity of these convective episodes are driven by two main features the low level humidity anomalies and the middle troposphere dry anomaly present over the mediterranean sea 2 the lago maggiore orographic convection has a very long duration and is mainly controlled by both the intensity of the easterly low level wind in the north part of the po valley and the upper level clouds of the cold front the different origins of the air masses transported by this easterly barrier wind and the associated local circulations are detailed copyright 2003 royal meteorological society

late holocene environmental change and the anthropization of the highlands of santo antao island cabo verde abstract cabo verde was the first tropical archipelago colonized by europeans historians have suggested that the first colonizers initiated archipelago wide ecosystem degradation loss of vegetation cover and erosion however the human environment interactions that led to the archipelago s current environmental status remain poorly understood here we report the first palaeoecological study of past vegetation change and disturbance regimes for cabo verde we present a 2130 yr old sediment sequence from a volcanic caldera cova de paul located at 1200 m asl on santo antao island for which we analyzed fossil pollen non pollen palynomorphs npps charcoal silica bodies and grain size distribution our analyses do not show evidence of the presence of temperate tropical or subtropical forests growing on the summits of santo antao in pre human times the pollen record shows that scrubland and grasslands dominated the highlands and underwent compositional changes ca 1850 and 1300 cal yr bp these shifts overlap with erosion phases and are linked to intensified seasonality steady rates of sedimentation marked the period 1230 to ca 350 cal yr bp but an increase in charcoal concentrations indicate a drying phase increases in regional and local fire peaks of coprophilous fungi and the presence of new world crop pollen zea mays are interpreted as the onset of portuguese settlement of the highlands after 450 bp sustained erosion between ca 350 and 100 cal yr bp indicate soil degradation and the pollen record shows the increase in introduced herbaceous taxa rosaceae centaurea verbenaceae and exotic tree taxa e g pinus while shifting abundances of coprophilous fungi indicate changes in land use the record shows stabilization of soils in the last century due to recent afforestation of the highlands further palaeoecological studies have the potential to provide further detail of the long term dynamics of cabo verde ecosystems and to inform conservation initiatives

modeling wind driven circulation and landfast ice edge processes during polynya events in northern baffin bay abstract a high resolution sea ice ocean numerical model of the north water polynya has been developed to study the wind driven circulation during polynya events an idealized three layer stratified ocean is used to initialize the model to characterize the baroclinic response to realistic wind and ice conditions the model general circulation pattern is mainly forced by an along channel sea level gradient between the arctic ocean and baffin bay which determines the magnitude of the southward baffin current and by an across channel sea level gradient in baffin bay which drives the northward west greenland current wgc these two currents are found to be anticorrelated to each other in the smith sound area during strong northerly wind events occurring quite frequently in the winter spring period in the polynya nutrient rich baffin bay waters transported by the wgc are forced toward the greenland shelf coinciding with upwelling events along the greenland coast whenever an ice bridge is present i e

a numerical study on the effects of taiwan topography on a convective line during the mei yu season abstract during the morning hours on 23 may 2002 a convective line associated with a mei yu front brought heavy rainfall along the coast of central taiwan under favorable synoptic conditions of warm air advection and large convective available potential energy cape of over 3000 m2 s 2 doppler radar observations indicated that deep convection was organized into a linear shape with a northeast southwest orientation along the front about 70 km offshore from taiwan over the northern taiwan strait the system then moved toward taiwan at a slow speed of about 4 m s 1 in the present study the effects of taiwan topography on this convective line and subsequent rainfall distribution were investigated through numerical modeling using the nagoya university cloud resolving storm simulator cress at a 2 km horizontal grid size experiments with different terrain heights of taiwan including full terrain ftrn half terrain htrn and no terrain ntrn were performed the control run using full terrain and co

interaction of low level flow with the western ghat mountains and offshore convection in the summer monsoon abstract seven year averaged values of percent frequency of occurrence of highly reflective cloud for the months june july and august indicate that offshore convection is a major component of the cloudiness of the southwest monsoon principal areas of convection occur off of the western coats of india burma thailand and the philippines this study concentrates on the area upstream of the western ghats mountains of india analysis of a special boundary layer mission flown during the wmo icsu summer monsoon experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection data obtained on this mission allow a large scale momentum budget computation for the subcloud layer which shows pressure deceleration to be significant the budget dominated by advection predicts an increase of average wind speed which is observed the pressure decelera

numerical simulations of waves over large crater topography in the atmosphere abstractnumerical simulations are used to investigate waves generated by flow over crater topography of diameter 100 km in an idealized atmosphere the atmosphere is stratified with a constant buoyancy frequency profile and the background wind is constant this study describes the development of a low level jet along the upstream crater slope and its interaction with the cooler air within the crater valley this interaction results in a hydraulic jump like structure that acts as a modified topography forcing a beam of secondary waves the hydraulic jump is formed by a retreating gravity current as the cool air within the crater readjusts after the initial tilting of potential temperature contours a two dimensional simulation is used to compare features such as wave overturning in two and three dimensions several variations on the atmosphere s profile are considered including an atmosphere with reduced constant stratification and an atmosphere that is unstratified within the crater these results indic

factors governing cellular convection in orographic precipitation the development of shallow cellular convection in warm orographic clouds is investigated through idealized numerical simulations of moist flow over topography using a cloud resolving numerical model buoyant instability a necessary element for moist convection is found to be diagnosed most accurately through analysis of

effect of stable layer formation over the po valley on the development of convection during map iop 8 during intensive observation period iop 8 of the mesoscale alpine program there was a stable layer of air in the lowest levels of the po valley and just upstream of the apennines in this study the effects of the stable layer on the formation of convection in the southern alpine region were investigated through a series of two dimensional idealized experiments the goals of this study were twofold 1 to determine if stable layer strength affected the placement of convection and 2 to test the notion that the stable layer during iop 8 behaved as an effective mountain to accomplish the first objective three simulations were compared in which the strength of the inversion and low level cooling in the po valley and upstream of the apennines was varied the results of these simulations show that the stronger the inversion and low level cooling the farther south the convection was positioned additionally it was found that convection developed as a result of the formation of a broad region of moist instability over the stable layer cellular convection developed in this region of moist instability the second objective was tested through a simulation where the cold pool was replaced by terrain mmtn as in the reference or stb10 simulation the upslope of the terrain in the mmtn simulation was characterized by a wide zone of moist instability however wave structures to the lee of the apennines were markedly different in the stb10 and mmtn simulations this led to different convective and precipitation patterns with the mmtn simulation exhibiting heavier convection over the po valley while the heaviest convection in stb10 was upstream of the apennines these results suggest that at best the stable layer in the stb10 simulation can only be roughly approximated by terrain

synoptic versus orographic control on stationary convective banding quasi stationary convective bands can cause large localised rainfall accumulations and are often anchored by topographic features here the predictability of and mechanisms causing one such band are determined using ensembles of the met office unified model at convection permitting resolution 1 5 km grid length the band was stationary over the uk for 3 h and produced rainfall accumulations of up to 34 mm the amount and location of the predicted rainfall was highly variable despite only small differences between the large scale conditions of the ensemble members only three of 21 members of the control ensemble produced a stationary rain band these three had the weakest upstream winds and hence lowest froude number band formation was due to the superposition of two processes lee side convergence resulting from flow around an upstream obstacle and thermally forced convergence resulting from elevated heating over the upstream terrain both mechanisms were enhanced when the froude number was lower by increasing the terrain height thus reducing the froude number the band became more predictable an ensemble approach is required to successfully predict the possible occurrence of such quasi stationary convective events because the rainfall variability is largely modulated by small variations of the large scale flow however high resolution models are required to accurately resolve the small scale interactions of the flow with the topography upon which the band formation depends thus although topography provides some predictability the quasi stationary convective bands anchored by it are likely to remain a forecasting challenge for many years to come

bayesian exploration of multivariate orographic precipitation sensitivity for moist stable and neutral flows abstractrecent idealized studies examined the sensitivity of topographically forced rain and snowfall to changes in mountain geometry and upwind sounding in moist stable and neutral environments these studies were restricted by necessity to small ensembles of carefully chosen simulations research presented here extends earlier studies by utilizing a bayesian markov chain monte carlo mcmc algorithm to create a large ensemble of simulations all of which produce precipitation concentrated on the upwind slope of an idealized gaussian bell shaped mountain mcmc based probabilistic analysis yields information about the combinations of sounding and mountain geometry favorable for upslope rain as well as the sensitivity of orographic precipitation to changes in mountain geometry and upwind sounding exploration of the multivariate sensitivity of rainfall to changes in parameters also reveals a nonunique solution multiple combinations of flow topography and environment produce similar surface rainfall amo

application of theory to simulations of observed cases of orographically forced convective rainfall abstractin two recent papers the authors reported on numerical simulations of conditionally unstable flows past an idealized mesoscale mountain ridge these idealized simulations which were performed with a three dimensional explicitly cloud resolving model allowed the investigation of simulated precipitation characteristics as a function of the prescribed environment the numerical solutions were carried out for a uniform wind flowing past a bell shaped ridge and using an idealized unstable sounding with prescribed values of the relevant parameters in the present work the application of these theoretical results to observed cases of orographically forced convective rainfall including the big thompson flood 1976 colorado the oahu flood 1974 hawaii and the gard flood 2002 france is reported specifically numerical simulations have been carried out using observed and idealized soundings relevant to these cases but with idealized topography it is found that using the observed soundings but

sensitivities of orographic precipitation to terrain geometry and upstream conditions in idealized simulations abstractthis study examines how variations in relatively simple terrain geometries influence orographic precipitation and its spatial patterns of sensitivity to small changes in upstream conditions an idealized three dimensional model is used to simulate a moist flow impinging upon three alpine scale terrain shapes a straight ridge a concave ridge and a convex ridge a variety of simulations are conducted to investigate the sensitivity of precipitation patterns to ridge length and upstream thermodynamic and wind conditions for an impinging flow with a nondimensional mountain height of approximately unity it is found that for the straight and convex ridges the flow response is mostly linear for the conditions examined here and passes over the obstacles with little lateral deflection the concave ridge however exhibits strengthened flow deceleration wave breaking in the lee and flow confluence between the ridge arms the concave ridge also generates substantially more precipitation than the other

observations and impacts of upstream blocking during a widespread orographic precipitation event a unique combination of airborne and ground based doppler radar observations stretching continuously from the alps south to the mediterranean sea is assembled to document profoundly blocked airflow and associated modification of precipitation processes upstream of the alps during intensive observing period 8 of the mesoscale alpine programme this diversion of incident southerly low level winds in advance of a deep baroclinic trough served to reduce markedly the transport of warm moist mediterranean air toward the steep alpine slopes heavy rains forecast in this flood prone region thus failed to materialize with lighter though protracted stratiform precipitation extending over the comparatively flat reaches of the adjacent po valley release of convective instability did occur but was displaced well south of the alps over the coastal apennine mountain chain and adjacent waters of the mediterranean sea influences on a variety of scales that evidently led to development of this blocked flow are examined and multiple expressions of upstream blocking are identified these range from pervasive down valley flows observed within multiple deep alpine river drainages to a larger scale cool easterly low level flow channelled alongside the curving south face of the alps which was ultimately extruded southward over the mediterranean observed thermodynamic profiles in these contrasting regimes are presented and related to the observed precipitation structure copyright 2003 royal meteorological society

further results on moist nearly neutral flow over a ridge abstract in a recent study the authors performed numerical simulations of moist nearly neutral flows over a ridge using the weather research and forecasting wrf model in a regime where the coriolis force can be neglected and with the simple kessler warm rain microphysical scheme in the present work further numerical solutions using more general and realistic experimental conditions are discussed the upstream propagating disturbance which was found in the author s previous study to desaturate the initially saturated sounding for intermediate mountain heights is present for all the simulations with taller mountains considered in the present work the inclusion of the coriolis force however suppresses the upwind propagation of the dry region and weakens the downstream development of convective cells the sensitivity to different microphysical schemes has also been investigated the simple kessler scheme was compared with a more complete scheme by lin et al which includes ice species some differ

mechanisms producing different precipitation patterns over north eastern italy insights from hymex sop1 and previous events during the first hymex special observation period sop1 field campaign the target site of north eastern italy nei experienced a large amount of precipitation locally exceeding the climatological values and distributed among several heavy rainfall episodes in particular two events that occurred during the last period of the campaign drew our attention these events had common large scale patterns and a similar mesoscale setting characterised by southerly low level flow interacting with the alpine orography but the precipitation distribution was very different during intensive observing period iop18 31 october 1 november 2012 convective systems were responsible for intense rainfall mainly located over a flat area of the eastern po valley well upstream of the orography conversely during iop19 4 5 november 2012 heavy precipitation affected only the alpine area in addition to iop18 and iop19 the present study analyses other heavy precipitation episodes that display similar characteristics and which occurred over nei during the autumn of recent years a high resolution 2 km grid spacing non hydrostatic nwp model and available observations are used for this purpose the two different observed precipitation patterns are explained in terms of interaction between the impinging flow and the alps depending on the thermodynamic profile convection can be triggered when the impinging flow is forced to rise over a pre existing cold air layer at the base of the orography in this situation a persistent blocked flow condition and upstream convergence are responsible for heavy rain localized over the plain conversely if convection does not develop flow over conditions are established and heavy rain affects the alps numerical parameters proposed in the literature are used to support the analysis finally the role of evaporative cooling beneath the convective systems is evaluated it turns out that the stationarity of the systems upstream of the alps is mainly attributable to persistent blocked flow conditions while convective outflow slightly modifies the location of precipitation

orographic effects on rainfall in map cases iop 2b and iop 8 although the large scale flow was similar important differences in mesoscale atmospheric structure made the difference between moderately intense rain in intensive observing period iop 2b and relatively light rain in iop 8 in the lago maggiore target area lmta this conclusion is supported here through analysis of data from and numerical simulations of these two cases analysis of the large scale data shows that there was in both cases a moist tongue of southerly flow moving from the west to the east of the alpine south side mesoscale data analysis and numerical simulations suggest that the most important difference between the two cases was the presence of a cold stable air mass in the po valley in iop 8 which persisted through the period in which the large scale moist tongue was progressing eastward and prevented the most humid air from reaching the lmta another difference contributing to the greater rainfall in the lmta in iop 2b was the development of conditional instability with associated convective rain due to the effect of the alps on the eastward passage of the moist tongue in iop 8 the atmosphere remained locally stable throughout the period copyright 2003 royal meteorological society

rain shadow development during the growth of mountain ranges an atmospheric dynamics perspective 1 an idealized atmospheric model is used to explore the links between climate and topography in the development of orographic rain shadows during orogenesis the atmospheric dynamics theory of density stratified fluid flow over topography is used to interpret the results the controlling nondimensional parameter is nh u where n is the brunt vaisala frequency a measure of atmospheric stability h is the terrain relief and u is the initial horizontal wind speed rain shadow development is found to be a nonlinear and nonunique function of both topography and atmospheric state indicating that geological records of orographic aridity cannot be interpreted in terms of relief alone when upstream topography exceeds nh u 1 during surface uplift downstream orographic precipitation vanishes and downstream orographic cloud mass decreases by as much as 90 upstream blocking of air flow can generate a forward projecting rain shadow in which a relatively low ridge nh u 1 may be decoupled from the atmospheric flow by a zone of flow stagnation extending upstream of the high terrain such an effect may occur if the valley separating the two ranges is narrower than the length scale of flow stagnation in the model configuration used here lateral widening of a relatively low nh u 1 range increases downstream cloud mass by up to a factor of 3 these results help to refine interpretations of climate tectonic interactions in shaping the geological record of the sierra nevada and andes

internal gravity waves in a saturated moist neutral atmosphere abstractthis work is motivated by an unusual feature associated with the start up of a moist nearly neutral atmospheric flow over a mountain ridge that was previously observed in a full physics numerical model in that study the upstream propagation of a wave of subsidence precluded the establishment of upward displaced and saturated flow that might be expected upstream of the topography this phenomenon was hypothesized to be a consequence of the peculiar property of saturated moist neutral flow an upward air parcel displacement produces zero buoyancy while a downward displacement desaturates the air parcel and produces a positive buoyancy anomaly in the present study this hypothesis is confirmed within numerical solutions to a reduced system of equations that incorporates the saturated atmosphere property in a particularly simple manner the relatively uncomplicated nature of these solutions motivates the numerical solution of a further simplified initial value problem for both nonhydrostatic and h

effect of the andes cordillera on precipitation from a midlatitude cold front the effects of the andes cordillera the major mountain range in south america on precipitation patterns of baroclinic systems approaching from the southeast pacific remain largely unstudied this study focuses on a case in late may 2008 when an upper level trough and surface cold front produced widespread precipitation in central chile the primary goal was to analyze the physical mechanisms responsible for the structure and evolution of the precipitation weather research and forecasting wrf model simulations indicate that as an upper level trough approached central chile midtropospheric flow below 700 hpa was blocked by the high topography and deflected poleward in the form of a barrier jet this northerly jet had wind maxima in excess of 15 m s 21 was centered around 925 hpa and extended westward 200 km from the mountains it intersected the cold front which approached from the south near the coast thereby increasing convergence along the frontal surface slowing its equatorward progress and enhancing rainfall over central chile another separate region of heavy precipitation formed over the upwind slopes of the cordillera a trajectory analysis confirmed that the barrier jet moved low level parcels from their origin in the moist southeast pacific boundary layer to the coast when model topography was reduced to twenty percent of its original height the cold front advanced more rapidly to the northeast generated less precipitation in central chile between 338 and 368s and produced minimal orographic precipitation on the upwind andean slopes based on these findings the high topography appears responsible for not only orographic precipitation but also for substantially increasing precipitation totals over the central coast and valley

precipitation over concave terrain abstract many topographic barriers are comprised of a series of concave or convex ridges that modulate the intensity and distribution of precipitation over mountainous areas in this model based idealized study stratiform precipitation associated with stratified moist airflow past idealized concave ridges is investigated with a focus on windward blocking flow confluence and the associated precipitation enhancement it is found that flow confluence and precipitation enhancement by a concave ridge are controlled by the nondimensional ridge height m m nmhm u where nm is the moist buoyancy frequency hm is the maximum ridge height and u is the wind speed based on which three dynamical regimes can be defined in the linear regime m 0 4 a flow confluence zone is present over the upwind slope of the ridge vertex where precipitation is significantly enhanced the precipitation enhancement is due to the additional updraft driven by the horizontal flow convergence with a considerable contribution fr