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

thermally forced convection over a mountainous tropical island abstractobservations from the dominica experiment domex and cloud resolving numerical simulations are used to study a thermally forced convection event over the caribbean island of dominica on 18 april 2011 a clear diurnal cycle of island thermal forcing and cumulus convection was observed with cumuli initiating over the southwestern flank of the ridge and deepening as they drifted eastward apart from errors in cloud fraction and notably precipitation the simulations verified well against the observations provided horizontal grid spacings of 500 m or less were used the simulated flows developed an island scale solenoidal circulation with an organized and intense updraft over the ridge that focused convective initiation sensitivity tests investigated the impacts of topographic forcing subcloud winds and cloud radiative feedbacks on the island scale horizontal inflow and cloud vertical mass flux these experiments confirmed that thermal forcing drove the island convection and that the inflow an

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

influence of the wind profile on the initiation of convection in mountainous terrain a number of days with small precipitating convective cells were investigated using weather radars during the cops convective and orographically induced precipitation study field campaign in the region of the vosges and the rhine valley in central europe depending on the weather situation two distinct mechanisms could be identified for the initiation of convection on some days cells were initiated over the ridge of the vosges whereas on other days cells were initiated in the lee of the vosges the initiation of convection appeared to be concentrated in a few favourable locations using the froude number it was possible to describe the two distinct mechanisms when the froude number was low the flow was diverted around the vosges and thermally driven convergence at the ridge initiated convection whereas when the froude number was high the flow passed through mountain gaps and then converged on the lee side with the flow in the rhine valley the convergence on the lee side was enhanced at locations where the outflows through valleys converged low froude numbers were accompanied by weak winds varying with height whereas high froude numbers were observed during situations with stronger southwesterly winds increasing with height

on thermally forced circulations over heated terrain abstracta combination of analytical and numerical models is used to gain insight into the dynamics of thermally forced circulations over diurnally heated terrain solutions are obtained for two layer flows representing the boundary layer and the overlying free troposphere over an isolated mountainlike heat source a scaling based on the linearized boussinesq system of equations is developed to quantify the strength of thermally forced updrafts and to identify three flow regimes each with distinct dynamics and parameter sensitivities this scaling closely matches corresponding numerical simulations in two of these regimes the first characterized by a weakly stable boundary layer and significant background winds and the second by a strongly stable boundary layer in the third regime characterized by weak winds and weak boundary layer stability this scaling is outperformed by a fundamentally different scaling based on thermodynamic heat engines within this regime the inability of wind ventilation or

cloud trails past the lesser antilles abstractobservations and cloud resolving simulations of elongated cloud plumes or cloud trails past the lesser antilles islands in the caribbean sea are presented analysis of one year of visible satellite images reveals that each island forms cloud trails on 30 40 of days typically in the afternoon in response to diurnal island heating on around 10 of days the cloud bands are very well organized with lengths of and durations of min radiosonde analysis suggests that the well organized events are favored by moderate to strong trade winds 6 10 m s 1 and stronger trade inversions the simulated cloud trails which are consistent with observations in their morphology and diurnal cycle are organized by quasi linear bands of thermally forced convergence within the heated island wake they are sensitive to overland surface fluxes inversion strength and height terrain height and trade wind speed while surface fluxes control the strength of the wake thermal circulations the inversion controls pr

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

the evolution of convective storms initiated by an isolated mountain ridge abstractthe evolution of convective storms over the black hills an isolated mountain ridge in south dakota and wyoming and a regional convection hotspot is investigated using a 10 yr observational climatology and quasi idealized numerical simulations radar observed diurnally forced mountain convection events are classified according to their maximum cell track length and duration which are quantified using an automated cell tracking algorithm environmental conditions during these events are obtained from operational radiosonde and model analysis data these data suggest that mountain forced convective cells generally struggle to survive in the convectively inhibited flow downwind of the black hills those cells that do survive downwind prefer environments with strong bulk vertical shear over the 0 6 km layer which favors organized multicellular or supercellular convection under slightly weaker shear the cells tend to dissipate rapidly as they propagate downwind relatively weak winds aloft when c

wind forces and related saltation transport the effect of several wind characteristics on sand transport was studied in three experiments in north burkina faso west africa the first experiment is used to analyse the relation between wind speed and shear stress fluctuations across height the second experiment is used to study the relation of these wind characteristics with saltation transport for fourteen convective storms registered during the rainy seasons of 2002 and 2003 the effect of sampling time is studied for two of these convective storms the third experiment relates the turbulent structures of four convective storms to saltation transport wind speed measurements were undertaken with two sonic anemometers and sediment transport was measured by two saltiphones the sampling frequency was either 8 or 16 hz the sonic frame of reference was rotated according to a triple rotation horizontal fluctuations showed a fairly good correlation with height because the wind speed at both sensors was affected by the same vortices the correlation coefficients ranged from 0 42 when the distance between the sensors was 1 75 m to 0 92 when the distance was 0 25 m the instantaneous reynolds stress had the weakest correlation correlation coefficient of 0 05 at 1 75 m between the sensors and 0 56 at 0 25 m between the sensors because the momentum at 2 m above the soil surface is transported by different eddies than those close to the ground this also explains the fairly good correlation coefficients between the horizontal components of the wind and saltation compared to the poor correlations between instantaneous reynolds stress and saltation an increase in sampling time did not have much impact on these correlation coefficients up to sampling periods of about 30 s however this sampling interval would be too coarse to describe the vertical wind component adequately the classification of the moments of shear stress into the turbulent structures sweeps ejections inward and outward interactions showed that the mean saltation flux is higher at sweeps and outward interactions than at ejections and inward interactions also saltation occurred more often during sweeps and outward interactions than during ejections and inward interactions

convective circulations induced by surface heterogeneities a simple theory for convective circulations induced by surface heterogeneities is proposed the theory is based on the thermodynamics of heat engines and provides a simple physical explanation for the general characteristics of circulations forced by surface inhomogeneities in sloping terrains in particular the theory is applied to a mesoscale circulation induced by deforestation it predicts that the intensity of the mesoscale convective circulation forced by deforestation depends on the difference of the near surface nonadiabatic temperature and humidity between the forest and cleared regions and on the depth of the convective boundary layer the theory was successfully tested against observations made during a field experiment in the amazon forest and a nearby clearing

topographically forced convergence in western washington state abstract several times a year when the low level winds from off the pacific ocean are within a narrow range of speed and direction air passes both north and south of the olympic mountains of washington state and is forced to converge in puget sound by the north south oriented cascade range this phenomenon termed the puget sound convergence zone often results in a band of cloudiness and precipitation in northern and central puget sound with clear subsiding air to the north and south this paper presents the results of a series of case studies in which the structure of the zone its meteorological manifestations and the environmental conditions necessary for its formation are explored it is shown that the convergence zone is skillfully forecast by surface coastal winds and undergoes a strong annual and diurnal cycle being most frequent during the late spring and early summer months and during the afternoon and early evening it is also found that the zone is structurally similar to a shallow cold fr

paleomagnetic evidence for proterozoic tectonism in the kapuskasing structural zone ontario the kapuskasing structural zone ksz in the superior province of north central ontario is thought to be an oblique cross section through middle archean crust samples from 30 sites in granulite gneisses and anorthosites of the ksz have been analyzed using alternating field and thermal demagnetization methods three remanent magnetization components were found with mean directions of d 23 i 50 k 52 95 11 n 5 sites for the a component d 73 i 68 k 50 95 5 n 18 for the b component and d 332 i 59 k 53 95 17 n 3 for the c component v constanzo alvarez and d j dunlop found similar a and b components in less metamorphosed rocks from the same units to the immediate west they attributed the a component to uplift at 2 55 ga with subsequent 15 to 30 wnw tilt and the b component to thermochemical overprinting at 1 1 ga as the ksz was reactivated during keweenawan rifting and volcanism we also attribute the a component to uplift during the 2 55 ga kenoran orogeny with concomitant 10 5 wnw downward tilting before intrusion of the 2 45 ga matachewan dikes to give its present paleopole of 33 e 67 dp 6 dm 9 n 13 using data from both studies we attribute acquisition of the b component to uplift at about 1 88 ga as suggested by recent thermal modeling and tectonic syntheses to give a paleopole of 124 w 30 n dp 7 dm 8 n 20 the c component is found in only three sites and may be either an hybrid of a 1 1 ga keweenawan crm with the b component or a fault block tilted b component near the ivanhoe lake cataclastic zone

entrainment rayleigh friction and boundary layer winds over the tropical pacific abstract winds over the tropical pacific are interpreted using mixed layer theory the theory which posits that the surface winds can be derived in terms of a force balance among surface drag pressure gradients coriolis forces and the vertical mixing of momentum into the boundary layer entrainment is very successful in predicting the seasonal climatology of the surface winds the model is also used as a basis for interpreting previous results in particular the model illustrates why studies that model the momentum flux divergence as a rayleigh damping find optimal damping coefficients that are anisotropic a linear variant of the model which also incorporates entrainment but neglects the quadratic relation between the wind speed and the surface stress is also found to predict the surface winds skillfully in addition to improving the representation of the winds it leads to realistic representations of the divergence of the vector wind if the key parameters of the model the entrainment rate and t

response of a stably stratified flow to cooling abstract the spreading of the low level cold pool produced by evaporation of precipitation is generally acknowledged to be an important mechanism for the regeneration of moist convection we show that cooling a stably stratified nocturnal boundary layer produces very different results from the corresponding daytime case in which the boundary layer is neutral in particular dynamic behavior is sometimes closer to that of a gravity wave than to a density current the gravity wave speed defines a minimum propagation speed for a self regenerating convective system

thermally forced low froude number flow past three dimensional obstacles abstract the present study extends the discussion of the flow of a density stratified fluid past three dimensional obstacles for froude number o 1 to flows past an isolated obstacle with heated cooled surface the study focuses on a response of thermally forced stratified flows representative of mesoscale flows past mountainous islands such as hawaii or taiwan in order to minimize the span of the parameter space uniform ambient wind and stratification axially symmetric bell shaped hills with moderate slopes representative of mesoscale mountains and a simple thermal forcing function that mimics natural effects over mountainous islands are assumed the earth s rotation surface friction viscosity dissipation and moisture are neglected with these simplifications flows can be characterized with two parameters the froude number and a characteristic scale of thermal forcing defined later in this paper the principal question addressed in this study is under what circumstances will a transition occur

diurnal variation of surface airflow and rainfall frequencies on the island of hawaii abstract the high resolution portable automated mesonet data from the hawaiian rainband project are used to document the circulation over the entire island of hawaii for the first time it is shown that the surface airflow and rainfall occurrences over the island are strongly modulated by the diurnal heating cycle most areas over the island show daytime upslope and nighttime downslope components separated by morning and evening transitions on the windward slope the onset of the downslope upslope wind during the evening morning transition starts on the slopes and progresses downward the effects of island blocking are also evident on the windward slopes of mauna kea and mauna loa the mean winds are weak 1 m s 1 due to flow deceleration flow splitting occurs in the hilo bay area as the trade winds are forced to move around the island on the lee side the trade winds are absent for stronger trade wind days the island blocking is more significant with a higher surface pressure 0 2 0 5 hpa on

boundary layer updrafts driven by airflow over heated terrain abstractthis study presents linear and nonlinear scalings for boundary layer ascent forced by airflow over heated terrain and compares them to results from corresponding high resolution numerical simulations close agreement between theory and simulation is found over most of the parameter space considered including variations in background winds boundary layer stability mountain height and diabatic heating rate as expected the linear and nonlinear scalings perform best for linear and nonlinear flows respectively for a convective boundary layer the scalings accurately predict vertical motion for all flows considered including those that extend well into the nonlinear regime thus these scalings may ultimately help to improve the parameterization of subgrid orographic ascent in large scale models the vertical velocity scalings are less accurate for mechanically blocked flows in stable boundary layers for which a simple vertical displacement scaling is superior although the scalings do not tre

cloud resolving simulations of deep convection over a heated mountain abstract cloud resolving numerical simulations of airflow over a diurnally heated mountain ridge are conducted to explore the mechanisms and sensitivities of convective initiation under high pressure conditions the simulations are based on a well observed convection event from the convective and orographically induced precipitation study cops during summer 2007 where an isolated afternoon thunderstorm developed over the black forest mountains of central europe but they are idealized to facilitate understanding and reduce computational expense in the conditionally unstable but strongly inhibited flow under consideration sharp horizontal convergence over the mountain acts to locally weaken the inhibition and moisten the dry midtroposphere through shallow cumulus detrainment the onset of deep convection occurs not through the deep ascent of a single updraft but rather through a rapid succession of thermals that are vented through the mountain convergence zone into the deepening cloud mass emerging t

a modeling study and scaling analysis of orographic effects on boundary layer shallow convection effects of orography on boundary layer shallow convection under various background winds are studied using a two dimensional model together with scaling analysis under a motionless background state the flow response over a heated hill is a vortex pair with one horizontal vortex over each slope additional lifting of the inversion by this vortex pair is estimated by wt n with wt being the intensity of the terrain induced thermal circulation and n the brunt vaisalafrequency for which a simple expression is given based on heat engine framework modification of the cbl top by terrain induced waves is quantified using linear gravity wave theory in a two layer atmosphere configuration these simple estimates of the cbl top perturbation are found to be consistent with the model results when waves and convection dominate the flow pattern a convective core is found to occur over high hills when the winds are light developing in the late afternoon when the cbl depth is high a scaling is proposed to determine the existence of the convective core in terms of hill slope background winds boundary layer depth and tropospheric stability the downstream modification of convection by terrain related processes is also examined under light wind conditions the reversed thermal circulation in the lee and corresponding downward motions may suppress thermal eddies in the vicinity of the lee of a hill

the mississippi valley convection minimum on summer afternoons observations and numerical simulations abstractanalysis of 15 years of composite radar images over the continental united states reveals a distinct minimum of deep convection occurrence over the interior lower mississippi valley on summer afternoons relative to surrounding areas to understand the mechanisms behind this convection signature quasi idealized numerical simulations with the weather research and forecasting wrf model are performed the simulations which broadly reproduce the valley convection minimum suggest that convective inhibition is maximized and low level ascent minimized over the flat valley terrain by contrast weaker inhibition and stronger mechanically forced ascent over the hills flanking the valley combine to initiate convection more readily although the orography of the region is unremarkable it has a stronger influence on the regional convection pattern than do variations in land use

a numerical study of particle motion and two phase interaction in aeolian sand transport using a coupled large eddy simulation discrete element method wind blown sand movement considered as a particle laden two phase flow was simulated by a new numerical code developed in the present study the discrete element method was employed to model the contact force between sand particles large eddy simulation was used to solve the turbulent atmospheric boundary layer motions of sand particles were traced in the lagrangian frame within the near surface region of the atmospheric boundary layer interparticle collisions will significantly alter the velocity of sand the sand phase is quite dense in this region and its feedback force on fluid motion cannot be ignored by considering the interparticle collision and two phase interaction four way coupling was achieved in the numerical code profiles of sand velocity from the simulations were in good agreement with experimental measurements the mass flux shows an exponential decay and is comparable to reported experimental and field measurements the turbulence intensities and shear stress of sand particles were estimated from particle root mean square velocities distributions of slip velocity and feedback force were analysed to reveal the interactions between sand particles and the continuous fluid phase

the addition of heat to a stratified airstream with application to the dynamics of orographic rain summary the response of a stratified airstream to combined thermal and orographic forcing is investigated theoretically using the linearized hydrostatic equations of motion the magnitude of the heating aloft is computed from observed rainfall rates the elevated heating is shown to produce vertically propagating waves whose amplitude relative to the mountain waves is determined by the parameter gqb cptuanh symbols appendix i for typical wind speeds and rainfall rates the thermally generated waves will equal or exceed the orographically generated waves these waves produce large vertical variations in the wave momentum flux at the levels where heat is being added and alter or even reverse the mountain drag in hydrostatic flow the phase relationship between the heating rate and the induced vertical displacements is found unexpectedly to be negative in the vicinity of the heating air parcels are displaced downwards this result can be explained by wave propagation ideas but not by parcel arguments this helps to explain why mountain wave amplitudes are sometimes reduced in moist atmospheres the broad heat induced descent may act to limit the amount of condensation precipitation occurring in a stable middle level orographic cloud other types of orographic precipitation involving embedded convection in stratus layers low level feeder clouds or deep cumulus convection may not be adversely effected by the heat induced descent

modeling saltation intermittency 1 in order to investigate the intermittency of the aeolian saltation a saltation model forced with instantaneous velocity fields has been introduced in a large eddy simulation airflow model the coupled model is evaluated on a flat erodible surface under various wind conditions and soil particle size distribution it is first shown that the model is able to simulate a well developed saltation layer in equilibrium with the turbulent flow the main characteristics of the saltation layer and their sensitivity to wind conditions are in good agreement with previous data set then the saltation intermittency is visualized through the presence of blowing sand structures near the surface known as aeolian streamers this is the first time that such structures are reproduced numerically from a correlation analysis we confirm previous thoughts that these sand structures are a visual footprint of past turbulent eddies propagating in the surface boundary layer the streamers appear to be embedded in larger saltation structures with increasing wind conditions the spatial scales of these streamers change with wind conditions and soil particle size distribution this is explained by two mechanisms 1 the modification of eddy structures with the main characteristics of the saltation layer and 2 the reduction of saltating particle sensitivity to the near surface eddies with increasing wind condition and soil median particle diameter as the eddy lifetime decreases within the saltation layer and the particle response time increases respectively the standard deviation of the saltation flux associated to these saltation patterns represents about 10 to 20 of the mean saltation flux

the role of small scale orographic features in the spatial distribution of precipitation radar images from the cevennes experiment which took place during the autumns of 1986 88 on the southeastern flank of the massif central in france reveal a characteristic and repetitive structure of the precipitation distribution in southerly mediterranean flows which are frequent there in autumn precipitation is often organized into narrow bands or plumes oriented south north the period 14 and 15 november 1986 has been selected as an example on which to perform numerical simulations and analyse the mechanism yielding this specific precipitation organization the numerical model correctly reproduces the bands especially at high horizontal resolution 1 km these rainfall bands are focused on the small scale topography oriented east west composed of a succession of ridges and penetrating valleys the bands are generated upstream of these ridges and enhanced on the lee side by convergence created by deflection around the obstacle and penetration of the flow into the valleys idealized simulations and sensitivity experiments have been carried out to verify these hypotheses these simulations suggest that small scale orography plays a decisive role in forcing the precipitation into bands while upstream moisture conditions mainly affect the precipitation intensity and its south north extent copyright 2002 royal meteorological society