numerical simulation of a low level jet over complex terrain in southern iran the lut desert of iran is an elongated valley oriented north northwest to south southeast the valley descends southward to the jaz murian dry lake through a pass the navy s coupled ocean atmosphere mesoscale prediction system is used to study a northerly low level jet in the valley and across the dry lake the dynamics of the jet are investigated with force balance and froude numbers to determine the contribution of various mechanisms to the jet formation and maintenance the jet is initiated as a channeled gap flow in the convergent topography of the lut valley by the valley parallel pressure gradients generated by the large scale processes and by the presence of cold air over the valley s sloping terrain the pressure gradient is mainly counteracted by the frictional force the imbalance between them controls the intensity and persistence of the jet in the valley farther south the jet evolves into a downslope flow resembling a hydraulic jump on the steep slope of the dry lake a transition of subcritical situation to supercritical faster flow is found at the mountain crest between the lut valley and dry lake the depth of stably stratified cold layer the static stability of upstream inversion and magnitude of upstream winds all determine the jet configuration over the dry lake the lee troughing over the gulf of oman and the persian gulf as the large scale inland flow crosses the coastal mountains supports this low level jet through the increased along jet pressure gradient the jet is also influenced by diurnal forcing being strong at night and weak during daytime

structure of precipitating systems over taiwan s complex terrain during typhoon morakot 2009 as revealed by weather radar and rain gauge observations summary this study documents from an observational perspective the structure of precipitation systems over the complex topography of taiwan as typhoon morakot 2009 impinged on the island on 8 august 2009 an advanced multiple doppler radar synthesis technique particularly designed for dealing with non flat surfaces is applied to analyze the three dimensional wind fields over the ocean and terrain in the northern and southern portion of the analysis domain where the mountain slope is relatively gentle and steep respectively the radar reflectivity measurements indicate that the precipitation systems exhibit very distinct features namely horizontal translation in the north and abrupt intensification in the south while still far from the southern mountainous region a north south oscillation of an east west oriented band of strong radar reflectivity 40 dbz with a horizontal span of 20 km is observed along the mountain slopes the band of strong radar reflectivity has a much wider north south extent both the radar and rain gauge observations show that the major precipitation is primarily confined to the windward side of the mountains an analysis of the saturated brunt vaisala frequency reveals that the upstream atmosphere is statically unstable which implies that the lifting of the incoming convective cells by the topography will easily trigger precipitation thus most of the moisture will be consumed before the air reaches the leeward side of the mountains the long duration and the wide range of heavy precipitation in the mountainous regions resulted in a record breaking average over the gauges rainfall amount of 2000 mm over 4 days the prevailing winds approaching the mountains are from the west the cross barrier wind speed has a maximum 40 m s 1 above the mountain crest that can be reasonably explained by a simplified shallow water model the capability of applying the weather radar to provide a reliable quantitative estimate of the rainfall over a large area with high temporal and spatial resolution is demonstrated using dual polarimetric radar data the potential applications of the knowledge of the wind and precipitation characteristics in hydrology and other fields are addressed in this manuscript

observations of downslope winds and rotors in the falkland islands a field campaign aimed at observing the near surface flow field across and downwind of a mountain range on the falkland islands south atlantic is described the objective was to understand and eventually predict orographically generated turbulence the instrumentation was based primarily on an array of automatic weather stations awss which recorded 30 s mean surface pressure wind speed and direction at 2 m temperature and relative humidity for approximately one year these measurements were supported by twice daily radiosonde releases the densest part of the aws array was located to the south of the wickham mountain range across mount pleasant airfield mpa in northerly flow the array provides a detailed study of the flow downwind of the mountain range the dataset contains several episodes in which the flow downwind of the mountains is accelerated relative to the upwind flow during some of these episodes short lived typically 1 hour periods of unsteady flow separation are observed and these are associated with the formation of rotors aloft such events present a significant hazard to aviation at mpa examination of radiosonde profiles suggests that the presence of a strong temperature inversion at a height similar to the mountain height is a necessary condition for both downwind acceleration and the formation of rotors the data are used to show that the downwind fractional speed up is proportional to the non dimensional mountain height based on upstream near surface winds and a depth averaged brunt vaisala frequency diagnosed from radiosonde data similarly a relationship is established between a quantity that describes the spatial variability of the flow downwind of the mountains and the upstream wind and depth averaged brunt vaisala frequency the dependence of the flow behaviour on the froude number defined in the usual way for two layer shallow water flow and ratio of mountain height to inversion height is presented in terms of a flow regime diagram

a new look at deformation as a diagnostic for large scale flow abstractdeformation plays a key role in atmospheric dynamics because it provides a dynamical measure of the interaction between different scales such as in frontogenesis a climatology of deformation constructed from interim ecmwf re analysis era interim data 1979 2013 reveals four main processes associated with deformation 1 frontogenesis at lower levels 2 movement and evolution of jet streams in the upper troposphere 3 orographic blocking and 4 rossby wave breaking the merits of deformation as an additional perspective are discussed for these processes on the basis of case studies and composite analyses in conjunction with analytic solutions this study shows that deformation can be used to unambiguously detect orographic blocking through the local strength of the flow diversion around orography moreover the deformation signature for orographic blocking observed in case studies and composites closely resembles the analytic solution for two dimensional flow around an obstacle the climatolo

the interaction of katabatic flow and mountain waves part ii case study analysis and conceptual model via numerical analysis of detailed simulations of an early september 1993 case night the authors develop a conceptual model of the interaction of katabatic flow in the nocturnal boundary layer with mountain waves mki a companion paper part i describes the synoptic and mesoscale observations of the case night from the atmospheric studies in complex terrain ascot experiment and idealized numerical simulations that manifest components of the conceptual model of mki presented herein the reader is also referred to part i for detailed scientific background and motivation the interaction of these phenomena is complicated and nonlinear since the amplitude wavelength and vertical structure of the mountain wave system developed by flow over the barrier owes some portion of its morphology to the evolving atmospheric stability in which the drainage flows develop simultaneously katabatic flows are impacted by the topographically induced gravity wave evolution which may include significantly changing wavelength amplitude flow magnitude and wave breaking behavior in addition to effects caused by turbulence including scouring perturbations to the leeside gravity wave structure at altitudes physically distant from the surface based katabatic flow layer can be reflected in the katabatic flow by transmission through the atmospheric column the simulations show that the evolution of atmospheric structure aloft can create local variability in the surface pressure gradient force governing katabatic flow variability is found to occur on two scales on the meso due to evolution of the mountain wave system on the order of one hour and on the microscale due to rapid wave evolution short wavelength and wave breaking induced fluctuations it is proposed that the mki mechanism explains a portion of the variability in observational records of katabatic flow

lee waves over the english lake district this paper presents the results of a field experiment which was conducted for three weeks during november 1991 the aim was to investigate the properties of internal gravity waves in the troposphere and lower stratosphere which were generated by the mountains of the lake district in north west england uk the main technique used was that of multiple sounding using radiosondes throughout the experiment measurements obtained from groups of five radiosondes launched nearly simultaneously indicate that the waves are quasi steady and trapped within the troposphere this is illustrated by a single detailed case study for 26 november in which the radiosonde ascent rate fluctuations are as large as 6 5 m s 1 peak to peak these measurements are supported by aircraft observations simple techniques for analysing the structure of the wave field are presented and these include the calculation of a phase line tilt and phase speed and comparisons with simple analytical and numerical solutions for trapped gravity waves a three dimensional linear numerical model is used to simulate the waves observed on 26 november and three other cases also the model predictions are shown to compare well with the observations this is particularly true in the mid troposphere where the model reproduces both the positions and amplitudes of the peaks and troughs in the vertical velocity field spectral analysis of the local orography and examination of typical variations of the wind and buoyancy frequency with height show that gravity waves generated by the orography in the lake district will frequently be trapped in the troposphere

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

another look at downslope windstorms part i the development of analogs to supercritical flow in an infinitely deep continuously stratified fluid abstract numerical simulations are conducted to examine the role played by different amplification mechanisms in the development of large amplitude mountain waves it is shown that when the static stability has a two layer structure the nonlinear response can differ significantly from the solution to the equivalent linear problem when the parameter nh u is as small as 0 3 in the cases where the nonlinear waves are much larger than their linear counterparts the highest stability is found in the lower layer and the flow resembles a hydraulic jump simulations of the 11 january 1972 boulder windstorm are presented which suggest that the transition to supercritical flow forced by the presence of a low level inversion plays an essential role in triggering the windstorm the similarities between breaking waves and nonbreaking waves which undergo a transition to supercritical flow are discussed

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

some aspects of the effect of surface friction on flows over mountains the effect of surface friction on flow over mountains in stratified fluids is investigated using a two dimensional version of the naval research laboratory s non hydrostatic coupled ocean atmosphere mesoscale prediction system coamps without surface friction flow over a mountain attains a maximum speed at the surface on the lee slope with surface friction the flow speed vanishes at the surface and reaches a maximum near the top of the boundary layer bl corresponding to this wind distribution is a shallow layer of very large vertical wind shear beneath the jet maximum and a weaker but deeper sheared layer elsewhere this strong shear generates maximum turbulent kinetic energy tke at the jet maximum region meanwhile the buoyancy effect associated with the mountain gravity wave inhibits the production of the tke above the jet maximum region the combined effects of shear and buoyancy lead to a very shallow bl on the lee slope in general surface friction reduces the amplitude of mountain gravity waves comparison of the flow fields with the outline of the bl height indicates that flow characteristics correspond closely to the distribution of the bl height it is hypothesized that the reduction in mountain wave amplitude in the presence of surface friction is due to the reduction in the slope of the bl height as compared to the terrain height for narrower and or higher mountains the bl height exhibits a secondary peak on the lee side that further modifies the wave simulations using the bl height as a solid mountain without surface friction confirm that characteristics of flow over the mountain are associated with the distribution of bl height rather than the terrain height field as the non dimensional mountain height increases the magnitude of the effect of surface friction decreases due to decreasing bl depth on the lee side with the result that the bl height profile becomes closer to the terrain height field the effect of surface friction on asymmetric mountains is also investigated without surface friction the pressure drag shows distinctive reduction with a secondary peak on the downstream side but the effect is less when the secondary peak is on the upstream side with surface friction all three mountains have similar pressure drag corresponding to their similar bl height profiles copyright 2003 royal meteorological society

hydraulic and wave aspects of novorossiysk bora bora in novorossiysk seaport on the black sea coast of the caucasus is one of the strongest and most prominent downslope windstorms on the territory of russia in this paper we evaluate the applicability of the hydraulic and wave hypotheses which are widely used for downslope winds around the world to novorossiysk bora on the basis of observational data reanalysis and mesoscale numerical modeling with wrf arw it is shown that mechanism of formation of novorossiysk bora is essentially mixed which is expressed in the simultaneous presence of gravity waves breaking and a hydraulic jump as well as in the significant variability of the contribution of wave processes to the windstorm dynamics effectiveness of each mechanism depends on the elevated inversion intensity and mean state critical level height most favorable conditions for both mechanisms working together are moderate or weak inversion and high or absent critical level

the impact of greenland on cyclone evolution in the north atlantic in order to understand better the role of greenland s orography in determining the position and strength of the icelandic low we have carried out a number of experiments using a numerical weather prediction model in those experiments greenland s orography was modified and the impact on cyclone developments over the north atlantic was investigated we have focused on one fairly characteristic winter case from january 1995 but another one from january 1993 gave qualitatively similar results we have found evidence that the deepening of baroclinic cyclones near iceland is hampered by the presence of greenland s orography this has been shown to be related to the inability of cold air from the north to cross over greenland leading to a distortion of the thermal field associated with the disturbance and a halting of the progression of the cold front in the january 1995 case a characteristic secondary cyclone forming between greenland and iceland was shown to be entirely orographic in that it was absent in runs where greenland s orography was removed the results are considered in the context of recent theoretical studies of flow splitting in stratified flow impinging on high mountains

diurnal variation of downslope winds in owens valley during the sierra rotor experiment abstract the impact of diurnal forcing on a downslope wind event that occurred in owens valley in california during the sierra rotors project srp in the spring of 2004 has been examined based on observational analysis and diagnosis of numerical simulations the observations indicate that while the upstream flow was characterized by persistent westerlies at and above the mountaintop level the cross valley winds in owens valley exhibited strong diurnal variation the numerical simulations using the coupled ocean atmosphere mesoscale prediction system coamps capture many of the observed salient features and indicate that the in valley flow evolved among three states during a diurnal cycle before sunrise moderate downslope winds were confined to the western slope of owens valley shallow penetration state surface heating after sunrise weakened the downslope winds and mountain waves and eventually led to the decoupling of the well mixed valley air from the westerlies aloft around local noon decoupled

high resolution observations and numerical simulations of easterly gap flow through the strait of juan de fuca on 9 10 december 1995 abstract this paper examines the three dimensional flow and dynamics of easterly gap flow through the strait of juan de fuca on 9 10 december 1995 using high resolution observations and model simulations the observations were obtained during the fourth intensive observing period during the second phase of the coastal observation and simulation with topography field experiment one of the most important data sources was the noaa p 3 aircraft which provided flight level data reflectivity and doppler winds as it flew down the strait of juan de fuca below 900 m the flow accelerated from 7 to 14 m s 1 at the entrance to the strait with a significant cross gap component and confluence over the eastern sections of the strait below 500 m the easterly flow approaching the western exit of the strait accelerated from 15 to 20 m s 1 as the 900 m deep easterly flow exited the gap it decreased in depth and accelerated to 25 m s 1 creating a shallow swath of diffluent winds below 300 m later in the event whe

gravity wave drag on two mountains the generation of linear internal gravity waves by the flow over two idealized mountains whose summits are aligned in the same direction as the mean wind is studied in particular the sensitivity of the wave drag to the downwind distance between the mountains is examined by using a three dimensional numerical model based on the linearized steady state boussinesq equations of motion both two and three dimensional orography is considered when the upstream conditions are such that the wind and buoyancy frequency do not vary with height and the mountains are wide enough for non hydrostatic effects to be unimportant the model predicts the same drag dependence on two dimensional ridge spacing as was derived analytically by grisogono et al the corresponding three dimensional numerical result predicted by the model is qualitatively very similar to the two dimensional result when the mountains are narrow and the waves are non hydrostatic the drag dependence on mountain spacing is more complicated maxima and minima in the drag can occur when the distance between the two mountains is such that waves generated by the mountain furthest upstream are in phase maxima in the drag or out of phase minima in the drag with the waves generated by the downstream mountain it is shown that this is an important effect when the waves are trapped by upstream profiles whose wind speed varies with height

windstorms along the western side of the washington cascade mountains part i a high resolution observational and modeling study of the 12 february 1995 event abstract this paper describes the leeside wind event of 12 february 1995 in which a swath of strong low level easterly flow with gusts to 35 40 m s 1 extended downwind of a major gap in the cascade mountains over the puget sound lowlands low level easterly flow approaching the olympics was decelerated to less than 5 m s 1 resulting in enhanced over 30 cm snowfall along the windward side of the barrier in addition to conventional synoptic observations high resolution data sources were utilized profilers on both sides of the cascades a wsr 88d doppler radar located in northern puget sound and nonhydrostatic model simulations down to 1 km resolution with the exception of underpredicting heavy snowfall to the east of the olympics many of the mesoscale features of this event were realistically simulated including cold air damming over eastern washington strong winds downwind of the gaps a mountain wake downstream of mount rainier and flow splitting around the olympics a major objective of th

gap flow in an alpine valley during a shallow south fohn event observations numerical simulations and hydraulic analogue this paper examines the three dimensional structure and dynamics of southerly hybrid gap mountain flow through the wipp valley wipptal austria observed on 30 october 1999 using high resolution observations and model simulations the observations were obtained during a shallow south fohn event documented in the framework of the mesoscale alpine programme map three important data sources were used the airborne differential absorption lidar leandre 2 the ground based doppler lidar teaco2 and in situ measurements from the national oceanic and atmospheric administration p 3 aircraft this event was simulated down to 2 km horizontal resolution using the non hydrostatic mesoscale model meso nh the structure and dynamics of the flow were realistically simulated the combination of high resolution observations and numerical simulations provided a comprehensive three dimensional picture of the flow through the wipptal in the gap entrance region brenner pass austria the low level jet was not solely due to the channelling of the southerly synoptic flow through the elevated gap part of the wipptal flow originated as a mountain wave at the valley head wall of the brenner pass downstream of the pass the shallow fohn flow had the characteristics of a downslope windstorm as it rushed down towards the inn valley inntal and the city of innsbruck austria downhill of the brenner pass the strongest flow was observed over a small obstacle along the western side wall the nosslachjoch rather than channelled in the deeper part of the valley just to the east further north the low level jet was observed in the centre of the valley approximately halfway between brenner pass and innsbruck where the along axis direction of the valley changes from north to north north west the low level jet was observed to be deflected to the eastern side wall of the wipptal interaction between the stubaier alpen the largest and highest topographic feature to the west of the wipptal and the south westerly synoptic flow was found to be the primary mechanism responsible for the deflection the along and cross valley structure and dynamics of the flow were observed to be highly variable due to the influence of surrounding mountains localized steep slopes within the valley and outflows from tributaries the gschnitztal and the stubaital to the west of the wipptal for that shallow fohn case observations and simulations provided a large body of evidence that downslope flow created thinning thickening fluid and accelerations decelerations reminiscent of mountain wave hydraulic theory along the wipptal two hydraulic jump like transitions were observed and simulated i on the lee slope of the nosslachjoch and ii in the gschnitztal exit region a hydraulic solution of the flow was calculated in the framework of reduced gravity shallow water theory the down valley evolution of the froude number computed using leandre 2 p 3 flight level and teaco2 measurements confirmed that these transitions were associated with super to subcritical transitions copyright 2002 royal meteorological society

effects of inversion height and surface heat flux on downslope windstorms abstractsimulations are presented focusing on the role of temperature inversions in controlling the formation and strength of downslope wind storms three mechanisms are examined depending on the relative height of the inversion with respect to the mountain and the stability of vertically propagating mountain waves for low level inversions flows are generated that closely resemble a reduced gravity shallow water hydraulic response with a large vertical displacement of the inversion on the lee side of the mountain for higher level inversion cases simulated flows more closely followed a stratified hydraulic behavior with the inversion acting as a rigid reflective lid in the third mechanism downslope winds were forced by a self induced critical layer located below the inversion height the presence of the inversion in this case had little effect on the resulting downslope winds observations made on the falkland islands show that downslope windstorms may preferentially occur in early morning even withou

small amplitude coastally trapped disturbances and the reduced gravity shallow water approximation solutions are obtained for linear hydrostatic disturbances propagating parallel to the face of an uninterrupted topographic step in an infinitely deep stably stratified fluid on an plane these waves are vertically trapped because their frequencies are smaller than the coriolis parameter and the height of the topographic step is finite these waves are referred to as step trapped kelvin waves because they are dynamically similar to internal kelvin waves throughout the layer of fluid below the top of the topographic step these waves appear to provide an idealized semi analytic model for the coastally trapped disturbances observed to propagate parallel to mountainous coastlines in several parts of the world computations are performed for a basic state with uniform static stability and for a three layer basic state in which the two lowest layers represent the marine boundary layer and a strong capping inversion one might suppose that the linear dynamics of hydrostatic disturbances in the three layer basic state could be well approximated by a reduced gravity shallow water model but this is not the case in particular the reduced gravity shallow water model does not provide reliable estimates for the phase speed of linear step trapped kelvin waves this defect suggests that detailed quantitative comparisons between marine boundary layer flows and the reduced gravity shallow water system may not have any intrinsic physical significance nevertheless these results do not preclude the possibility of constructing useful qualitative analogies between marine boundary layer flows and the reduced gravity shallow water model

assessing the impact of the tropopause on mountain waves and orographic precipitation using linear theory and numerical simulations abstractthe partial reflection of mountain waves at the tropopause has been studied extensively for its contribution to downslope windstorms but its impact on orographic precipitation has not been addressed here linear theory and numerical simulations are used to investigate how the tropopause affects the vertical structure of mountain waves and in turn orographic precipitation relative to the no tropopause case wave induced ascent above the windward slope of a two dimensional ridge is found to be enhanced or diminished depending on the ratio of the tropopause height to the vertical wavelength of the mountain waves defined here as the nondimensional tropopause height in idealized simulations of flow over both two dimensional and three dimensional ridges variations in are found to modulate the precipitation rate by roughly a factor of 2 under typical atmospheric conditions the sensitivity of precipitation to is related primarily to the depth of windward ascent but also to the location and stren

origin evolution and finescale structure of the st valentine s day mesoscale gravity wave observed during storm fest part i origin and evolution abstract on 14 15 february 1992 a long lived mesoscale gravity wave was observed over the storm scale operational and research meteorology fronts experiment systems test observational network a precipitation band formed and intensified as the wave moved across kansas and missouri the disturbance was tracked for 14 h surface dual doppler radar and wind profiler analyses as well as isentropic analyses derived from a special rawinsonde network are used to examine the origin and evolution of this wave the wave originated at the leading edge of a dry air mass associated with downslope flow in the lee of the rockies the earliest surface pressure signatures of wave motion began as a dry air mass associated with the downslope flow ascended a warm front east of a lee cyclone a weak rainband developed simultaneously with the wave at the leading edge of the dry air mass the mesoscale gravity wave and convection remained tied to the leading edge of the advancing dry air mass during the first 8 10 h of

the interaction of katabatic flow and mountain waves part i observations and idealized simulations abstract the mutual interaction of katabatic flow in the nocturnal boundary layer nbl and topographically forced gravity waves is investigated due to the nonlinear nature of these phenomena analysis focuses on information obtained from the 1993 atmospheric studies in complex terrain field program held at the mountain canyon plains interface near eldorado canyon colorado and idealized simulations perturbations to katabatic flow by mountain waves relative to their more steady form in quiescent conditions are found to be caused by dynamic pressure effects based on a local froude number climatology case study analysis and the simulations the dynamic pressure effect is theorized to occur as gravity wave pressure perturbations are transmitted through the atmospheric column to the surface and through altered horizontal pressure gradient forcing to the surface based katabatic flows it is proposed that these perturbations are a routine feature in the atmospheric record and represent a significant p

blocking and frontogenesis by two dimensional terrain in baroclinic flow part i numerical experiments the shallow atmospheric fronts that develop in the early winter along the east coast of north america have been attributed in various modeling and observational studies to the land sea contrasts in both surface heating and friction however typical synoptic conditions are such that these coastal fronts could also be a type of upstream influence by the appalachian mountain chain generalized models have suggested that relatively cold air can become trapped on the windward side of a mountain range during episodes of warm advection without a local contribution from differential surface fluxes such a process was proposed decades ago in a study of observations along the coast of norway could coastal frontogenesis be primarily a consequence of a mountain circulation acting on the large scale temperature gradient a two dimensional terrain following numerical model is used to find conditions under which orography may be sufficient to cause blocking and upstream frontogenesis in a baroclinic environment the idealized basic flow is taken to have constant vertical shear parallel to a topographic ridge and a constant perpendicular wind that advects warm or cold temperatures toward the ridge land sea contrasts are omitted in the observed cases the mountain is narrow in the sense that the rossby number is large this by itself increases the barrier effect but the experiments show that large scale warm advection is still crucial for blocking for realistic choices of ambient static stability and baroclinicity the flow can be blocked by a range like the northern appalachians if the undisturbed incident wind speed is around 10 m s21 cold advection weakens the barrier effect the long term behavior of the front in strongly blocked cases is described and compared to observations because of the background rotation and large scale temperature advection blocked solutions cannot become steady in the assumed environment however the interface between blocked and unblocked fluid can settle into a balanced configuration in some cases a simple argument suggests that in the absence of dissipation the frontal slope should be similar to that of the ambient absolute momentum surfaces

mesoscale simulation of supercritical subcritical and transcritical flow along coastal topography abstract a mesoscale atmospheric model is used to address the characteristics of stratified flow bounded by a side wall along a varying coastline initial froude number values are varied through alteration of marine inversion strength permitting examination of supercritical subcritical and transcritical flow regimes encountering several coastal configurations consistent with shallow water models sharp drops in boundary layer depth and flow acceleration occur in flow rounding convex bends however significant flow response occurs in the stratified layer aloft which is unexplained by conventional shallow water theory the strongest flow acceleration occurs in the transcritical case while regardless of inversion strength the deformation of the isentropes aloft shows general structural similarity advection of horizontal momentum is an important component of the horizontal force balance a simulation having several coastline bends exhibits a detached oblique hydraulic jump upwind of a concave bend t

a dynamical investigation of the may 2004 mcmurdo antarctica severe wind event using amps on 15 16 may 2004 a severe windstorm struck mcmurdo antarctica the antarctic mesoscale prediction system amps is used along with available observations to analyze the storm a synoptic scale cyclone weakens as it propagates across the ross ice shelf toward mcmurdo flow associated with the cyclone initiates a barrier jet along the transantarctic mountains forcing terms from the horizontal equations of motion are computed in the barrier wind to show that the local time tendency and momentum advection terms are key components of the force balance the barrier jet interacts with a preexisting near surface radiation inversion over the ross ice shelf to set up conditions favorable for the development of large amplitude mountain waves leading to a downslope windstorm in the ross island area hydraulic theory can explain the structure of the downslope windstorms with amplification of the mountain waves possibly caused by wave breaking events the underestimation of amps wind speed at mcmurdo is caused by the misplacement of a hydraulic jump downstream of the downslope windstorms the dynamics associated with the cyclone barrier jet and downslope windstorms are analyzed to determine the role of each in development of the severe winds

an intercomparison of model predicted wave breaking for the 11 january 1972 boulder windstorm abstract two dimensional simulations of the 11 january 1972 boulder colorado windstorm obtained from 11 diverse nonhydrostatic models are intercompared with special emphasis on the turbulent breakdown of topographically forced gravity waves as part of the preparation for the mesoscale alpine programme field phase the sounding used to initialize the models is more representative of the actual lower stratosphere than those applied in previous simulations upper level breaking is predicted by all models in comparable horizontal locations and vertical layers which suggests that gravity wave breaking may be quite predictable in some circumstances characteristics of the breaking include the following pronounced turbulence in the 13 16 km and 18 20 km layers positioned beneath a critical level near 21 km a well defined upstream tilt with height and enhancement of upper level breaking superpositioned above the low level hydraulic jump sensitivity experiments indicate that the structure of the wave bre

the role of wave dynamics and small scale topography for downslope wind events in southeast greenland abstractin ammassalik in southeast greenland downslope winds can reach hurricane intensity and represent a hazard for the local population and environment they advect cold air down the ice sheet and over the irminger sea where they drive large ocean atmosphere heat fluxes over an important ocean convection region earlier studies have found them to be associated with a strong katabatic acceleration over the steep coastal slopes flow convergence inside the valley of ammassalik and in one instance mountain wave breaking yet for the general occurrence of strong downslope wind events the importance of mesoscale processes is largely unknown here two wind events one weak and one strong are simulated with the atmospheric weather research and forecasting wrf model with different model and topography resolutions ranging from 1 67 to 60 km for both events but especially for the strong one it is found that lower resolutions underestimate the wind speed because they misrepresent the steepness of the

dynamics of the foehn mechanism in the mcmurdo dry valleys of antarctica from polar wrf foehn events over the mcmurdo dry valleys mdvs the largest ice free region of antarctica promote glacial melt that supports biological activity in the lakes streams rocks and soils although mdvs foehn events are known to depend upon the synoptic scale circulation the physical processes responsible for foehn events are unknown a polar optimized version of the weather research and forecasting model polar wrf is used for a case study of a representative summer foehn event from 29 december 2006 to 1 january 2007 in order to identify and explain the mdvs foehn mechanism pressure differences across an elevated mountain gap upstream of the mdvs provide forcing for southerly flow into the western upvalley entrance of the mdvs complex terrain over the elevated gap and the mdvs leads to mountain wave effects such as leeside acceleration hydraulic jumps wave breaking and critical layers these mountain wave effects depend on the ambient geostrophic wind direction pressure driven channelling then brings the warm dry foehn air downvalley to eastern mdv sites brief easterly intrusions of maritime air into the eastern mdvs during foehn events previously have been attributed to either a sea breeze effect in summer or local cold pooling effects in winter in this particular case the easterly intrusions result from blocking effects of nearby ross island and the adjacent antarctic coast temperature variability during the summer foehn event which is important for meltwater production and biological activity when it exceeds 0 c primarily depends on the source airmass rather than differences in foehn dynamics

controlled experiments on the diffusion rate of stable isotopes of water in artificial firn we studied the diffusion rate of stable isotopes of water ice in a controlled laboratory experiment where isotopically different layers of fabricated firn were stacked this experiment was done in order to study the diffusion rate between the isotopically different layers and to compare the measurements with forward calculated diffusion rates of the fabricated firn stack using three different analytical and numerical methods the layers varied in thickness within the stack and the studied period includes variations in firn temperature we sampled the firn stack on five different dates along the 144 day long experiment we obtained a quantitatively good match between the calculated and measured diffusion rates with a root mean square similarity of 67 80 in the depth averaged diffusion rates despite the good average match we found some qualitative discrepancy between our calculations and our measurements the calculated diffusion rates predict 5 faster isotopic decay of thinner layers and 13 slower decay of thicker layers than our measurements show we speculate that grain size and grain fabric may influence the tortuosity of the firn pack and that the current generalization of tortuosity being a function of firn density as the only free parameter is an oversimplification

alpine gravity waves lessons from map regarding mountain wave generation and breaking the two month special observing period of the mesoscale alpine programme map in autumn 1999 included a variety of complex mountain wave events seven wave events were carefully analyzed compared with numerical models and described in published papers these detailed investigations revealed some common dynamical elements i e the importance of low level processes involving the slow moving boundary layer low level wind shear causing either wave absorption or decoupling spilling upstream blocking and latent heat release based on these studies it is clear that any quantitative prediction of mountain wave generation must take full account of these lower troposphere processes the newest numerical models show significant skill in simulating these effects using these models the climatology of waves over the alps can be studied copyright 2007 royal meteorological society

airflow over a two dimensional escarpment i observations observations of forced flow over the highly asymmetric topography of the darling scarp western australia are presented these are typified by shallow surface gradient winds of 10 20 m s 1 with an inherent critical level aloft and a stable layer produced through nocturnal radiational cooling the observations highlight a topographically induced hydraulic jump lee waves or vertically propagating hydrostatic waves depending on the upwind stability and wind profile the vertically propagating waves and hydraulic jump are observed to produce accelerated flow down the escarpment

foehn jets over the larsen c ice shelf antarctica previously unknown foehn jets have been identified to the east of the antarctic peninsula ap above the larsen c ice shelf these jets have major implications for the east coast of the ap a region of rapid climatic warming and where two large sections of ice shelf have collapsed in recent years during three foehn events across the ap leeside warming and drying is seen in new aircraft observations and simulated well by the met office unified model metum at 1 5 km grid spacing in case a weak southwesterly flow and an elevated upwind inversion characterise a highly nonlinear flow regime with upwind flow blocking in case c strong northwesterly winds characterise a relatively linear case with little upwind flow blocking case b resides somewhere between the two in flow regime linearity the foehn jets apparent in aircraft observations where available and metum simulations of all three cases are mesoscale features up to 60 km in width originating from the mouths of leeside inlets through back trajectory analysis they are identified as a type of gap flow in cases a and b the jets are distinct being strongly accelerated relative to the background flow and confined to low levels above the larsen c ice shelf they resemble the shallow foehn of the alps case c resembles a case of deep foehn with the jets less distinct the foehn jets are considerably cooler and moister relative to adjacent regions of calmer foehn air this is due to a dampened foehn effect in the jet regions in case a the jets have lower upwind source regions and in the more linear case c there is less diabatic warming and precipitation along jet trajectories due to the reduced orographic uplift across the mountain passes

episodes of strong flow down the western slope of the subtropical andes nocturnal flows down the narrow andean valleys within the western slope of the subtropical andes central chile are episodically enhanced by easterly downslope winds that flow into the santiago basin over the radiatively cooled air above the surface local regional and large scale data have been used here to characterize the mean features of these episodes about 80 of easterly downslope flow episodes in austral winter are forced by a reversal in the sea level pressure gradient along the coast of south central chile when a midlatitude cold high migrates from southern chile eastward across the andes under midtroposphere sw winds associated with a warm ridge aloft under these circumstances low level easterly offshore flow sets in producing a compensating downslope flow that subsides over central chile the remaining cases are associated with prefrontal conditions under a midlatitude trough with nw winds aloft since in most of these cases the easterly low level flow occurs beneath westerly flow higher above these episodes classify as strong windward downslope flows within the andean valleys and canyons the near surface air experiences a sensible warming and drying at night and early morning during these episodes as the strong downvalley winds tend to destroy the surfacebased radiative inversion and mix down warmer air from aloft at the exit region of these valleys into the central basin these downslope flows in austral winter are not able to flush the cold air pool there hence dawn surface temperatures over the basin tend to be lower than average as clear skies and dry subsiding air aloft favor surface radiative cooling the resulting enhancement of the near surface static stability hampers the subsequent development of the mixed layer leading to severe air pollution events in santiago and other cities in central chile a comparative discussion on governing mechanisms with respect to apparently similar phenomena as gap flow and shallow foehn is included

investigation of upstream boundary layer influence on mountain wave breaking and lee wave rotors using a large eddy simulation abstract interactions between a turbulent boundary layer and nonlinear mountain waves are explored using a large eddy simulation model simulations of a self induced critical layer which develop a stagnation layer and a strong leeside surface jet are considered over time wave breaking in the stagnation region forces strong turbulence that influences the formation and structure of downstream leeside rotors shear production is an important source of turbulence in the stagnation zone and along the interface between the stagnation zone and surface jet as well as along the rotor edges buoyancy perturbations act as a source of turbulence in the stagnation zone but are shown to inhibit turbulence generation on the edges of the stagnation zone surface heating is shown to have a strong influence on the strength of downslope winds and the formation of leeside rotors in cases with no heating a series of rotor circulations develops capped by a region of increased winds weak heating disrupts this system an

applicability of reduced gravity shallow water theory to atmospheric flow over topography abstractapplicability of the reduced gravity shallow water rgsw theory to a shallow atmospheric layer capped by an inversion underneath a deep stratified atmosphere over a two dimensional ridge has been investigated using linear analysis and nonlinear numerical simulations two key nondimensional parameters are identified namely and where g is the reduced gravity acceleration h0 is the rgsw layer depth and n and u are the buoyancy frequency and wind speed respectively in the layer above the inversion if j and are around unity or larger the response of the rgsw flow over the ridge can be significantly modified by pressure perturbations aloft any jumplike perturbations in the rgsw layer rapidly decay while propagating away from the ridge as the perturbation energy radiates into the upper layer with j and much less than unity rgsw theory is more adequate for describing rgsw flows in addition inversion splitting occurs downstream of a jump when where ni is the buoyancy frequency in the

on the orographically generated low level easterly jet and severe downslope storms of march 2006 over the tacheng basin of northwest china abstracta high latitude low level easterly jet llej and downslope winds causing severe dust storms over the tacheng basin of northwestern china in march 2006 when the dust source regions were previously covered by snow with frozen soil are studied in order to understand the associated meteorological conditions and the impact of complex topography on the generation of the llej observational analyses show the development of a large scale geostrophically balanced easterly flow associated with a northeastern high pressure and a southeastern low pressure system accompanied by a westward moving cold front with an intense inversion layer near the altitudes of mountain ridges a high resolution model simulation shows the generation of an llej of near typhoon strength which peaked at about 500 m above the ground as well as downslope windstorms with marked wave breakings and subsidence warming in the leeside surface layer as the large scale cold easterly flow moves through a constricting saddle pass and

supercritical flow interaction within the cape blanco cape mendocino orographic complex abstract supercritical flow interaction occurring in the marine boundary layer between closely spaced coastal capes is investigated with a mesoscale numerical prediction model as an extension of previous work the u s navy s coupled ocean atmosphere mesoscale prediction system coamps is used to perform idealized model simulations with marine layers of varying upstream froude number to elucidate the different flow responses for a single convex bend the impact upon the supercritical flow of introducing a series of closely spaced coastal bends is then investigated the expansion fan is significantly reduced in magnitude and size by the formation of a compression wave at a blocking concave bend approximately 150 km downstream building upon the idealized marine layer response real data forecasts are then examined for several time periods of supercritical flow interaction between cape blanco oregon and cape mendocino california observations from the coastal waves 1996 cw96 field program were colle

non hydrostatic effects on mountain wave breaking in directional shear flows mountain waves excited by narrow 3d orography are investigated using idealized numerical simulations of atmospheric flows with directional wind shear the stability of these waves is compared with the stability of hydrostatic mountain waves the focus is on understanding how wave breaking is modified via gravity wave critical level interaction when non hydrostatic dispersive effects arise the influence of non hydrostatic effects on wave breaking appears to be a function of the intensity of the background shear increasing the stability of the flow inhibiting wave breaking for weak wind shear but decreasing it instead enhancing wave breaking for stronger wind shear

mountain waves over the hohe tauern influence of upstream diabatic effects the structure and intensity of large amplitude trapped lee waves over the hohe tauern range of the alps on 20 september 1999 are investigated through the analysis of in situ aircraft observations airborne lidar global positioning system dropsondes rapid scan satellite imagery and a suite of linear and nonlinear model simulations observations indicate that the lee waves attained a maximum vertical velocity of more than 9 m s 1 potential temperature perturbations greater than 10 k and a horizontal wavelength of approximately 12 15 km high resolution nonlinear simulations accurately capture the trapped wave evolution and characteristics blocking of the southerly flow upstream of the hohe tauern decreases the depth of the layer that ascends the mountain crest and modulates the wave response vertically propagating waves are partially ducted into a train of lee waves as a result of weak stability aloft which forms due to diabatic processes associated with precipitation upstream of the hohe tauern crest linear analytic solutions confirm the importance of the weak stability layer in the upper troposphere for the development of the non hydrostatic evanescent waves results from idealized two dimensional nonlinear simulations suggest that the vertical depth of diabatic heating associated with the upstream precipitation is important in this case latent heating over a relatively shallow depth tunes the atmosphere for a nonlinear resonance that leads to strong descent in the lee and reinforcement of the lee waves copyright 2003 royal meteorological society

measurements of form and frictional drags over a rough topographic bank pressuredifferencesacrosstopographygenerateaform dragthatopposestheflow in the watercolumn and viscous and pressureforcesacting on roughnesselements of the topographic surface generatea frictionaldrag on the bottom form drag and bottom roughness lengths were estimated over the east flower garden bank efgb in the gulf of mexico by combining an array of bottom pressure measurements and profiles of velocity and turbulent kinetic dissipation rates the efgb is a coral bank about 6km wide and 10km long located at the shelf edge that rises from 100 m water depth to about 18m below the sea surface the average frictional drag coefficient over the entire bank was estimated as 0 006 using roughness lengths that ranged from 0 001cm for relatively smooth portions of the bank to 1 10cm for very rough portions over the corals the measured form drag over the bank showed multiple time scale variability diurnal tides and lowfrequency motions with periods ranging from 4 to 17 days generated form drags of about 2000nm 21 with average drag coefficients ranging between 0 03 and 0 22 which are a factor of 5 35 times larger than the average frictional drag coefficient both linear wave and quadratic drag laws have similarities with the observed form drag the form dragis an important flow retardation mechanismevenin the presence of the large frictional drag associated with coral reefs and requires parameterization

internal hydraulic jumps and overturning generated by tidal flow over a tall steep ridge abstract recent observations from the hawaiian ridge indicate episodes of overturning and strong dissipation coupled with the tidal cycle near the top of the ridge simulations with realistic topography and stratification suggest that this overturning has its origins in transient internal hydraulic jumps that occur below the shelf break at maximum ebb tide and then propagate up the slope as internal bores when the flow reverses a series of numerical simulations explores the parameter space of topographic slope barotropic velocity stratification and forcing frequency to identify the parameter regime in which these internal jumps are possible theoretical analysis predicts that the tidally driven jumps may occur when the vertical tidal excursion is large which is shown to imply steep topographic slopes such that dh dxn 1 the vertical length scale of the jumps is predicted to depend on the flow speed such that the jump froude number is of order unity the numerical results agree with the theoreti

an intercomparison of t rex mountain wave simulations and implications for mesoscale predictability abstractnumerical simulations of flow over steep terrain using 11 different nonhydrostatic numerical models are compared and analyzed a basic benchmark and five other test cases are simulated in a two dimensional framework using the same initial state which is based on conditions during intensive observation period iop 6 of the terrain induced rotor experiment t rex in which intense mountain wave activity was observed all of the models use an identical horizontal resolution of 1 km and the same vertical resolution the six simulated test cases use various terrain heights a 100 m bell shaped hill a 1000 m idealized ridge that is steeper on the lee slope a 2500 m ridge with the same terrain shape and a cross sierra terrain profile the models are tested with both free slip and no slip lower boundary conditions the results indicate a surprisingly diverse spectrum of simulated mountain wave characteristics including lee waves hydraulic like jump features and gravity wave breaking the vertical v

the response of bora type flow to sea surface temperature a non linear two dimensional hydrostatic incompressible numerical model with a higher order turbulence closure scheme is used to study the effect of sea surface temperature on the severe downslope wind called bora at the adriatic coast a non linear large amplitude mountain wave is generated and is broken beneath and within its critical layer due to resonant tuning between the initially single layer atmosphere and the terrain the tuning is governed by the froude number a qualitative and sometimes quantitative analogy exists between the wave breaking unsteady stratified flow and the hydraulic jump steady two layer flow it is also known that the strongest adriatic bora appears during the winter season when the sea surface temperature is typically larger than the ground surface temperature firstly a relatively higher lower sea surface temperature means an additional distortion moderation of the mountain wave and consequently a larger smaller area with bora wind maxima for a relatively higher sea surface temperature a propagating hydraulic jump occurs typically bora maxima are about three to four times larger than the related geostrophic wind 8 m s 1 secondly the presence and importance of the inertial oscillation are indicated since the wave breaking is the vital component of the strongest bora cases there is a relatively large elevated area i e the critical layer with substantial flow decelerations and generally low wind speeds the wave breaking area has a rossby number o 1 hence the earth s rotation appears to be an important part of bora evolution the simulations presented consider generalized bora cases which may pertain to other similar orographic flows

topographically generated gravity waves in the brooks range alaska amplified wind events are identified over the brooks range in alaska with observed wind speed increasing from calm to 5 ms 1 over 24 to 36 hours in simulations using an limited area climate system model the wind events are captured consistently although generally display greater strength than observed cooling associated with the wind events lead to an identification of the ne region of the north slope of alaska below the inversion as the source region for the air flow analysis of the wind event of 7th january 1992 supports the nonlinear hydraulic model of gravity waves

lee wave resonances over double bell shaped obstacles lee wave resonance over double bell shaped obstacles is investigated through a series of idealized highresolution numerical simulations with the nonhydrostatic coupled ocean atmosphere mesoscale prediction system coamps model using a free slip lower boundary condition the profiles of wind speed and stability as well as terrain derive from observations of lee wave events over the sierra nevada and inyo mountains from the recently completed terrain induced rotor experiment t rex numerical experiments show that double bell shaped obstacles promote trapped lee waves that are in general shorter than those excited by an isolated ridge while the permissible trapped lee wave modes are determined by the upstream atmospheric structure primarily vertical wind shear the selected lee wave wavelengths for two obstacles that are close or equal in height are dictated by the discrete terrain spectrum and correspond to higher harmonics of the primary orographic wavelength which is equal to the ridge separation distance the exception is the smallest ridge separation distance examined one that corresponds to the owens valley width and is closest to the wavelength determined by the given upstream atmospheric structure for which the primary lee wave and orographic wavelengths were found to nearly coincide the influence two mountains exert on the overall lee wave field is found to persist at very large ridge separation distances for the nonlinear nonhydrostatic waves examined the ridge separation distance is found to exert a much stronger control over the lee wave wavelengths than the mountain half width positive and negative interferences of lee waves which can be detected through their imprint on wave drag and wave amplitudes were found to produce appreciable differences in the flow structure mainly over the downstream peak with negative interference characterized by a highly symmetric flow pattern leading to a low drag state

nonpropagating form drag and turbulence due to stratified flow over large scale abyssal hill topography abstractdrag and turbulence in steady stratified flows over abyssal hills have been parameterized using linear theory and rates of energy cascade due to wave wave interactions linear theory has

radar observations and numerical modeling of a precipitating line during map iop 5 abstract during the special observing period of the mesoscale alpine programme a narrow north south oriented frontal precipitation band was observed by two airborne and one operational doppler radar on 4 october 1999 over the friuli target area in northeast italy convective precipitation was mostly concentrated to the south of the eastern alps in regions of smooth orography and warm southerly flow ahead of the cold front while stratiform rain formed above the mountainous chain where a marked cold northerly flow wrapping around the eastern alps progressed southwestward doppler data collected in both convective and stratiform regions of the precipitation line as well as high resolution numerical simulations performed with the nonhydrostatic meso nh model are used to investigate the associated flow structure in connection with the underlying topography the mutually consistent observed and simulated precipitation and wind fields clearly identify low level flow convergence relative to the cold front

strong downslope winds and application of hydraulic like theory an experimental campaign was conducted at the lee side of a 1 km high steep mountain in order to study the development of strong downslope winds under favorable conditions using combined remote and insitu instrumentation the examination of the upstream atmospheric conditions reveals that the development of strong downslope winds is favored by a mean state critical layer or a significant decrease in static stability such as at the top of a temperature inversion at the proper height above the mountain top strong downslope winds could occur even for wind directions with a deviation of 60 off the axis perpendicular to the ridgeline as long as the cross mountain wind has a significant value at least 7 ms 1 the developed disturbances are associated with intense downdrafts of the order of 4 5 ms 1 within the first 600 m above ground and characteristic vertical turbulent structures that were observed by sodar the same phenomenon is observed not only during nighttime but also under unstable and neutral conditions of the atmospheric boundary layer abl albeit with weaker intensity fourier analysis of the vertical velocity field demonstrated that the typical time period of intense disturbances was about 5 min further evidence is also provided for the application of the hydraulic like theory under real atmospheric conditions

foehn warming distributions in nonlinear and linear flow regimes a focus on the antarctic peninsula the structure of lee side warming during foehn events is investigated as a function of cross barrier flow regime linearity two contrasting cases of westerly flow over the antarctic peninsula ap are considered one highly nonlinear the other relatively linear westerly flow impinging on the ap provides one of the best natural laboratories in the world for the study of foehn owing to its maritime setting and the larsen c ice shelf lcis providing an expansive homogeneous and smooth surface on its east side numerical simulations with the met office unified model at 1 5 km grid size and aircraft observations are utilized in case a relatively weak southwesterly cross peninsula flow and an elevated upwind inversion dictate a highly nonlinear foehn event with mountain wave breaking observed the consequent strongly accelerated downslope flow leads to high amplitude warming and ice shelf melt in the immediate lee of the ap however this foehn warming diminishes rapidly downwind due to upward ascent of the foehn flow via a hydraulic jump in case c strong northwesterly winds dictate a relatively linear flow regime there is no hydraulic jump and strong foehn winds are able to flow at low levels across the entire ice shelf mechanically mixing the near surface flow preventing the development of a strong surface inversion and delivering large fluxes of sensible heat to the ice shelf consequently in case c ice melt rates are considerably greater over the lcis as a whole than in case a our results imply that although nonlinear foehn events cause intense warming in the immediate lee of mountains linear foehn events will commonly cause more extensive lee side warming and over an ice surface higher melt rates this has major implications for the ap where recent east coast warming has led to the collapse of two ice shelves immediately north of the lcis

numerical simulations of the foehn in the rhine valley on 24 october 1999 map iop 10 abstract this paper presents numerical simulations of the map iop 10 foehn case 24 october 1999 for the lower alpine rhine valley which was one of the two target areas selected for foehn observations the simulations have been performed with a modified version of the fifth generation pennsylvania state university ncar mesoscale model mm5 most importantly the horizontal diffusion of temperature and the mixing ratios of water vapor and cloud water is computed truly horizontally rather than along the model surfaces as is the case in the original mm5 in the rhine valley the foehn case under consideration was characterized by a complex and rapidly changing flow field in the early morning the foehn was restricted to the upper parts of the valley while a cold air pool was present in the lower parts and around the exit toward lake constance during the following hours the cold air pool was gradually eroded and the foehn reached lake constance around noon only 3 h later a shallow cold front passing f

evolution of a cold front encountering steep quasi 2d terrain coordinated aircraft observations on 8 9 december 2001 during improve 2 abstract research aircraft observations from the 8 9 december 2001 case of the second phase of the improvement of microphysical parameterization through operational verification experiment improve 2 describe the evolution of a wide cold frontal rainband wcfr during its eastward advance from the pacific coastline to a point 200 km inland over the cascade mountains of oregon this analysis has two objectives first to illustrate the rapid weakening of the circulation associated with a landfalling wcfr and the relationship of these changes to terrain induced airflow modifications and second to quantify the degree to which this weakening impacted cloud microphysical properties such as liquid water content ice particle concentrations and precipitation rate the kinematic structure of the wcfr is detailed using doppler radar observations from a noaa p 3 aircraft while some concomitant cloud microphysical properties are documented using flight level measurements from the university of washington convai

complex bora flow in the lee of southern velebit the complexity of bora flow in the lee of southern velebit croatia is investigated by means of high resolution numerical simulations carried out with the us naval research laboratory s nrl coupled ocean atmosphere mesoscale prediction system coamps model the aim of this study is to identify reasons for the strong spatial variability of bora in the wider zadar region and the uncharacteristically weak bora in the city of zadar the primary focus is put on the severe bora episode of 20 december 2004 during which a sodar sonic detection and ranging system was operated at zemunik airport numerical results are verified against the available surface and upper air observations upstream conditions governing dynamics of this particular bora event are characterized by the presence of a mean state critical level at 3 4 km above mean sea level amsl defining the upstream bora layer low level airflow over zadar peninsula during this event displays marked temporal variability during the morning a highly three dimensional and unsteady wake i e region of reduced wind speed is established over zadar peninsula downstream of a hydraulic jump that forms in the lee of but not collocated with highest portions of the southern velebit terrain during its most extensive phase the wake scales the entire length of zadar peninsula in the horizontal and extends up to the critical level in the vertical a wake vortex develops as the flow reverses along the wake centreline in the evening hours an undular bore forms in the same location with large amplitude waves on top of it inducing boundary layer separation and rotors wake flow is affected both by changes in the upstream atmospheric structure and diurnal boundary layer evolution sensitivity experiments show that the influence of velebit s height is particularly strong governing the onset and strength of bora the terrain of zadar peninsula exerts a strong influence on the characteristics of the developed bora flow particularly when non hydrostatic effects are significant boundary layer separation is particularly sensitive to downstream orography copyright 2012 royal meteorological society

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

initial condition sensitivities and the predictability of downslope winds the sensitivity of downslope wind forecasts to small changes in initial conditions is explored by using 70 memberensemblesimulationsoftwoprototypicalwindstormsobservedduringtheterrain inducedrotor experiment t rex the10 weakestand 10 strongest ensemblemembersare compositedand compared for each event in the first case the 6 h ensemble mean forecast shows a large amplitude breaking mountain wave and severe downslope winds nevertheless the forecasts are very sensitive to the initial conditions because the difference in the downslope wind speeds predicted by the strong and weak member composites grows to larger than 28 m s 21 over the 6 h forecast the structure of the synoptic scale flow one hour prior to the windstorm and during the windstorm is very similar in both the weak and strong member composites wave breaking is not a significant factor in the second case in which the strong winds are generated by a layer of high static stability flowing beneath a layer of weaker mid and upper tropospheric stability in this case thesensitivitytoinitialconditionsisweakerbutstillsignificant thedifferenceindownslopewindspeeds between the weak and strong member composites grows to 22 m s 21 over 12 h during and one hour before the windstorm the synoptic scale flow exhibits appreciable differences between the strong and weakmember composites although this case appears to be more predictable than the wave breaking event neither case suggests that much confidence should be placed in the intensity of downslope winds forecast 12 or more hours in advance

supercritical channel flow in the coastal atmospheric boundary layer idealized numerical simulations numerical simulations of supercritical coastal flows were performed with a three dimensional nonlinear hydrostatic meso scale model with a higher order turbulence closure the purpose was to resolve questions from previous studies related to terrain forcing idealized terrain representing northern california was used this facilitated testing of hypotheses in a simple fashion it was found that the angle by which the coast turns away from the flow regulates the acceleration in the expansion fan and determines how far downstream the maximum in the wind speed is located simulations with curved coastlines confirmed that the gradual curvature of the coastal mountains north of cape mendocino is sufficient to excite an expansion fan a decreasing height of the terrain along the coast lead to an increased acceleration of the flow largest when the slope of the terrain was confined to the change in coastline orientation a cape perpendicular to the main coastal mountains significantly blocked the flow even when as low as half the upstream marine atmospheric boundary layer mbl depth moreover a realistic cape caused gravity wave breaking these simulations confirmed that the local terrain at cape mendocino is responsible for the collapse of the mbl in shelter cove one striking feature in this study was that the flow started to accelerate far upstream of the change in coastline orientation even in supercritical conditions this phenomenon was most pronounced in the cases with the highest wind speeds using normalized wind speed and the bernoulli function from calculated back trajectories it was concluded that this feature is not due to deviations from linearity or from the shallow water concept

large amplitude mountain wave breaking over greenland abstract a large amplitude mountain wave generated by strong southwesterly flow over southern greenland was observed during the fronts and atlantic storm track experiment fastex on 29 january 1997 by the noaa g iv research aircraft dropwindsondes deployed every 50 km and flight level data depict a vertically propagating large amplitude wave with deep convectively unstable layers potential temperature perturbations of 25 k that deformed the tropopause and lower stratosphere and a vertical velocity maximum of nearly 10 m s 1 in the stratosphere the wave breaking was associated with a large vertical flux of horizontal momentum and dominated by quasi isotropic turbulence the coupled ocean atmosphere mesoscale prediction system coamps nonhydrostatic model with four nested grid meshes with a minimum resolution of 1 7 km accurately simulates the amplitude location and timing of the mountain wave and turbulent breakdown finescale low velocity plumes that resemble wakelike structures emanate from highl

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

implications of regime transitions for mountain wave breaking predictability abstract a suite of high resolution two dimensional ensemble simulations are used to investigate the predictability of mountain waves wave breaking and downslope windstorms for relatively low hills and mountains perturbation growth is weak and ensemble spread is small gravity waves and wave breaking associated with higher mountains exhibit rapid perturbation growth and large ensemble variance near the regime boundary between mountain waves and wave breaking a bimodal response is apparent with large ensemble variance several ensemble members exhibit a trapped wave response and others reveal a hydraulic jump and large amplitude breaking in the stratosphere the bimodality of the wave response brings into question the appropriateness of commonly used ensemble statistics such as the ensemble mean in these situations small uncertainties in the initial state within observational error limits result in significant ensemble spread in the strength of the downslope wind speed wave breaking and wave mom

observations of nonclassical frontal propagation and frontally forced gravity waves adjacent to steep topography through the integrated analysis of remote sensing and in situ data taken along the front range of colorado this study describes the interactions that occurred between a leeside arctic front and topographically modulated flows these interactions resulted in nonclassical frontal behavior and structure across northeastern colorado the shallow arctic front initially advanced southwestward toward the front range foothills before retreating eastward then a secondary surge of arctic air migrated westward into the foothills during its initial southwestward advance the front exhibited obstacle like density current characteristics its initial advance was interrupted by strong downslope northwesterly flow associated with a high amplitude mountain wave downstream of the continental divide and by a temporal decrease in the density contrast across the front due to diurnal heating in the cold air and weak cold advection in the warm air the direction and depth of flow within the arctic air also influenced the frontal propagation the shallow obstacle like front actively generated both vertically propagating and vertically trapped gravity waves as it advanced into the downslope northwesterly flow resulting in midtropospheric lenticular wave clouds aloft that tracked with the front because the front entered a region where strong downslope winds and mountain waves extended downstream over the high plains the wave field in northeastern colorado included both frontally forced and true mountain forced gravity waves a sequence of scorer parameter profiles calculated from hourly observations reveals a sharp contrast between the prefrontal and postfrontal wave environments consequently analytic resonant wave mode calculations based on the scorer parameter profiles reveal that the waves supported in the postfrontal regime differed markedly from those supported in the prefrontal environment this result is consistent with wind profiler observations that showed the amplitude of vertical motions decreasing substantially through 16 km above mean sea level msl after the shallow frontal passage

effects of changing stress states on the development of caldera bounding faults geological evidence from kumano caldera japan abstract collapse of a large caldera can cause spatial and temporal perturbations of stress and formation of caldera faults the stress variations influence the direction of slip vectors on the fault planes hence stress estimation is important for the study of caldera forming processes in our paleostress estimation the stress variations in the collapse of the ca 14 ma kumano caldera in japan have been revealed a stress inversion method based on the wallace bott hypothesis was used to compute the orientation of the principal stress axes 1 2 3 and the stress ratio 2 3 1 3 where 0 1 the caldera faults formed simultaneously with the caldera forming ash flow tuff eruption therefore paleostress solutions obtained from slip data measured on such faults show the spatial and temporal changes of the stress at the time of the caldera collapse the computed stress ratio characterizes a pair of stress fields in the early stage the stress field with 1 0 shows a semi radial trajectory of stress 2 and an eastern concentric trajectory of stress 3 this stress regime resulting from pre collapse tumescence counteracts the gravitational force and thus produces smaller net vertical stress the regional tumescence above an inflated magma chamber is the most plausible source of the stress field and it is consistent with the timing of the caldera formation in the late stage the stress field with 0 5 shows the semi radial trajectory of stress 2 and the west convex and concentric trajectory of stress 3 change of the stress ratio from 1 0 to 0 5 implies that increase in the relative magnitude of the stress 1 caused the deeper subsidence of the caldera floor stress variations may be of significant value for reconstructing the structural history of the caldera

the role of upstream waves and a downstream density pool in the growth of lee waves stratified flow over the knight inlet sill abstract observations and modeling simulations are presented that illustrate the importance of a density contrast and the upstream response to the time dependence of stratified flow over the knight inlet sill repeated sections of velocity and density show that the flow during ebb and flood tides is quite different a large lee wave develops early in flood tide whereas lee wave growth is suppressed until the second half of ebb tide there is a large upstream response that displaces as much water as accumulates in the lee wave one that is large enough to also block the flow at a depth roughly consistent with simple dynamics there is a large density contrast between the seaward and landward sides of the sill and a salty pool of water is found in the seaward basin that is not found landward the interface with this salty pool demarks the point of flow separation during ebb initially suppressing the lee wave and then acting as its lower boundary a simple two dimensional numerical model of the inlet wa

erratum to effects of changing stress states on the development of caldera bounding faults geological evidence from kumano caldera japan j volcanol geotherm res 144 2005 89 103 abstract a paper published in the journal of volcanology and geothermal research miura d 2005 effects of changing stress states on the development of caldera bounding faults geological evidence from kumano caldera japan j volcanol geotherm res 144 89 103 contains an erroneous result in the stress tensor calculation for fault slip data and thus is incorrect more adequate and reliable solutions of stress tensor have been provided the revised analysis shows some changes in the orientations of stresses 2 and 3 and the stress ratio being smaller than the original one no other contents of the original paper however will be modified by this correction the important conclusions of the paper remain the same

gravity wave breaking over the central alps role of complex terrain abstract the characteristics of gravity waves excited by the complex terrain of the central alps during the intensive observational period iop 8 of the mesoscale alpine programme map is studied through the analysis of aircraft in situ measurements gps dropsondes radiosondes airborne lidar data and numerical simulations mountain wave breaking occurred over the central alps on 21 october 1999 associated with wind shear wind turning and a critical level with richardson number less than unity just above the flight level 5 7 km of the research aircraft ncar electra the electra flew two repeated transverses across the otztaler alpen during which localized turbulence was sampled the observed maximum vertical motion was 9 m s 1 corresponding to a turbulent kinetic energy tke maximum of 10 5 m2 s 2 spectrum analysis indicates an inertia subrange up to 5 km wavelength and multiple energy containing spikes corresponding to a wide range of wavelengths manual analysis of gps dropsonde data indic

arcuate pyroclastic conduits ring faults and coherent floor at kumano caldera southwest honshu japan abstract at the 41 23 km miocene kumano caldera in southwestern japan a large arcuate pyroclastic breccia unit is interpreted as the dissected conduit for voluminous explosive eruptions the pyroclastic breccia and associated granite porphyry occur along the southern margin of the caldera these rocks collectively as much as 22 km long and 800 m wide intrude the arcuate fault bounding the southern caldera margin the breccia is interpreted as the vent facies of ash flow tuff adjacent to the caldera based on similar lithologies phenocryst modes of juvenile clasts and matrix character cataclasites along the major faults include both country rocks and pyroclastic breccia yet some cataclasite blocks are contained within the pyroclastic breccia these geometric and textural relations suggest that the faulting took place during caldera forming eruption prevolcanic sedimentary rocks enclosed by the arcuate faults are interpreted as the dissected coherent floor of the caldera which were subsided several hundreds to a thousand meters no large scale piecemeal disruption of the caldera floor is evident orientations of striations with the cataclasites and altitudes of the base of the tuff suggest the asymmetric trap door subsidence of this caldera the overall caldera geometry is the nested trap door piston cylinder subsidence each 21 15 and 25 23 km in diameter in contrast to previous interpretation of a funnel shape

three dimensional characteristics of stratospheric mountain waves during t rex abstract measurements from the national science foundation national center for atmospheric research nsf ncar gulfstream v g v obtained during the recent terrain induced rotor experiment t rex indicate marked differences in the character of the wave response between repeated flight tracks across the sierra nevada which were separated by a distance of approximately 50 km observations from several of the g v research flights indicate that the vertical velocities in the primary wave exhibited variations up to a factor of 2 between the southern and northern portions of the racetrack flight segments in the lower stratosphere with the largest amplitude waves most often occurring over the southern flight leg which has a terrain maximum that is 800 m lower than the northern leg multiple racetracks at 11 7 and 13 1 km altitudes indicate that these differences were repeatable which is suggestive that the deviations were likely due to vertically propagating mountain waves that varied systematically in am

impact of environmental variations on simulated squall lines interacting with terrain abstractthe complex evolution of convective systems crossing or attempting to cross mountainous terrain represents a substantial forecasting challenge this study examines the processes associated with environments of crossing squall lines which were able to redevelop strong convection in the lee of a mountain barrier and noncrossing squall lines which were not able to redevelop strong convection downstream of the barrier in particular numerical simulations of mature convective systems crossing idealized terrain roughly approximating the appalachian mountains were used to test the first order impact of variations in the vertical wind profile upon system maintenance by itself the wind profile showed no ability to uniquely discriminate between simulated crossing and noncrossing squall lines each test revealed a similar pattern of orographic enhancement suppression and lee reinvigoration in which a hydraulic jump deepened the system s cold pool and renewed the low level lifting increasing th

review mesoscale modelling in complex terrain abstract this review focuses on those aspects of mesoscale models which require special techniques and parameterizations when modelling flow over complex terrain the basic concepts and limitations of various approaches are discussed and applications of models to specific problems are presented

convective cloud bands downwind of mesoscale mountain ridges abstractelongated and quasi stationary cloud bands capable of producing heavy precipitation have recently been observed in the lee of midlatitude mountain ridges herein idealized explicit convect

the formation of internal bores in the atmosphere a laboratory model summary recently several atmospheric observations have been interpreted as internal undular bores or internal solitary waves evolving from internal bores that propagate along low level temperature inversions it has been speculated that such disturbances are generated by some type of gravity current such as cold fronts sea breeze fronts and thunderstorm outflows interacting with an existing temperature inversion in this paper we describe a systematic laboratory study in a water channel of internal bores and their generation by the movement of gravity currents through a two layer model of the atmosphere we compare the results of our laboratory experiments with previous theories and numerical simulations and with several detailed atmospheric observations of internal bores at different stages of development

numerical study of the local downslope wind hirodo kaze in japan abstract the hirodo kaze a local strong wind accompanying the downslope winds in japan is examined using a mesoscale numerical model the model successfully reproduces the major features of the observed hirodo kaze that occurred in association with typhoon pabuk during the hirodo kaze the severe downslope winds in the transitional flow develop in the lower troposphere below the mean state critical layer the hirodo kaze is closely linked to the strong wind region accompanying the severe downslope winds after the cessation of the hirodo kaze distinct mountain waves dominate in the lower troposphere where the scorer parameter l2 decreases with height the region of strong wind retreats windward as the hirodo kaze ceases temporal changes in the characteristics of mountain waves in the lee of mt nagi are primarily attributed to the changes in the large scale environmental winds due to the movement of the intense cyclone environmental conditions favorable for the occurrence of the hirodo kaze includ