条件不稳定大气中二维小尺度双脊地形上空对流及降水特征

在条件不稳定大气条件下,二维小尺度双脊钟形地形上空对流触发、传播和降水分布特征主要决定于地形上游基流强度、双脊地形配置形式、地形高度及其山谷宽度。双脊地形在沿基流方向上有两种配置:高脊地形位于上游和低脊地形位于上游。对于高脊地形位于上游的双脊地形,上游高地形将起主导作用,山地上空对流及降水特征与单脊地形类似。对于低脊地形位于上游的双脊地形,上游低地形可明显地改变下游高地形的前方来流,同时,下游高地形也能够对上游低地形背风侧流动产生影响,从而导致出现地形上空复杂的对流传播、降水分布特征。对于低脊地形位于上游的双脊地形,其山谷宽度主要决定了双脊地形与单脊地形之间在对流、降水分布等的差异;当山谷宽度较小时,双脊地形可以近似为一个包络地形,此时地形上空的对流、降水特征与单脊地形类似;当山谷宽度较大时,双脊上空流动相互影响较小,此时双脊地形可以分成两个单脊地形;当山谷宽度在一定范围内,其上空的对流及其降水分布与单脊地形有明显差异。对于低脊地形位于上游、中等山谷宽度的双脊地形上空降水主要呈现4种类型:(1)山谷与低脊迎风坡降水;(2)高脊迎风坡降水;(3)低脊山峰与高脊迎风坡降水;(4)低脊背风侧、双脊山峰准静止降水。     

地形对于气流运动影响的数值研究

建立了二维、非静力平衡的数值模式，研究地形对上游气流的阻挡以及大振幅背风波谷与下坡风的形成。结果表明：地形的阻挡效应受地形高度、大气层结及地形非对称性等因子的影响。数值试验与理论分析都证明地形越高、层结越稳定时阻挡作用越强；同样条件下，迎风坡坡度大的地形容易对气流形成阻挡。此外，分析了地形高度、大气层结、地形非对称性以及基本入流大小对背风波谷及下坡风强度影响的规律，并通过一次实际观测对数值模拟结果进行了检验。     

条件不稳定湿大气中三维理想地形上空对流的动力学特征

条件不稳定湿大气情况下,气流经过三维地形可以形成不同性质的对流系统以及不同特征的地形流结构,其对流系统、地形流的性质主要取决于地形上空的对流触发、对流-地形流-重力波三者之间的相互作用,同样这些过程对于地形降水的性质、分布起重要的作用.根据不同湿Fr数(Froude number),湿条件不稳定大气经过三维小尺度山地上空时其对流和地形流动存在4种不同的流域(flow regirnes):(1)下游传播对流模态;(2)上游传播和下游传播共存对流模态;(3)山峰附近准静止和下游传播共存对流模态;(4)下坡稳定和下游传播对流共存模态.地形上空对流系统主要可以通过两种不同机制形成:(1)地形直接的抬升或减速作用;(2)在地形流形成后,由于地形流本身特性(如上游分离、背风涡旋和下坡重力波破碎)触发.在较大的Fr数情况下,地形上空对流生成后反过来可以破坏上、下游的地形流结构,但对背风坡的重力波破碎影响较小.不同初始对流有效位能(CAPE)不仅可以影响对流系统的传播、发展,而且可以影响整体地形流性质.较低的初始CAPE有利于地形流的形成,此时对流对地形流结构特征的影响相对较小,其流场性质与低Fr数流域性质相似.     

Formulation of a Semi-Geostrophic Model of Frontal Interaction with Isolated Orography

Summary  A semi-geostrophic model of frontal passage over topography was developed to examine the effects of the interaction of a well developed front with an isolated mountain, and the subsequent orographically induced flow evolution. The analytic representation of the primary wave and its frontal structure gives us the ability to control the environmental and geometrical constraints and perform an exploration of parameter space. A number of problems appearing because of this approach are discussed. The results shown relate the characteristics of the orographic perturbation to varying amplitude of the primary wave and varying frontal intensity, but suggest that the representation of flow blocking by the mountain,which this model lacks, is crucial for the representation of frontal intensification in the lee of orography. Received March 2, 1999/Revised May 7, 1999     

Deep and shallow south foehn in the region of Innsbruck: Typical features and semi-idelized numerical simulations

Summary ?Numerical simulations of the south foehn in the region of Innsbruck are presented. They are semi-idealized in the sense that realistic orography but idealized initial and boundary conditions are used. The focus of this study is on typical features of the fully developed foehn, the breakthrough phase of the foehn and the diurnal cycle of the foehn. In addition, the impact of the large-scale wind direction is examined, including conditions leading to shallow foehn. The simulated flow fields have been found to be in very good agreement with observations except for a few minor details. In the lower part of the Sill Valley (the valley going from the Brenner pass down to Innsbruck), the wind speed is significantly higher than in the upper part. The acceleration can be traced back to the three-dimensional propagation of gravity waves excited over the adjacent mountain ridges. The amplitude of the gravity waves over the various mountain ridges depends sensitively on the wind direction, large wave amplitudes occurring only when the angle between the wind direction and the ridge line is not too small. For southwesterly or south–southwesterly large-scale flow, wave amplitudes are significantly larger to the east of Innsbruck than to the west. Foehn breakthrough at Innsbruck is usually preceded by a moderate westerly (downvalley) wind that is restricted to a rather small area around Innsbruck. The simulations reveal that this so-called pre-foehn is mainly a consequence of the gravity wave asymmetry, producing an asymmetric pressure perturbation with lower pressure to the east of Innsbruck. Shallow foehn, defined as a foehn occurring when the large-scale flow at crest height (700 hPa) is approximately westerly, is associated with relatively weak wave activity along the Sill Valley. It is found that at least a weak southerly wind component below crest height is necessary to maintain a significant shallow foehn over a longer time. Received October 10, 2001; accepted June 20, 2002 Published online: February 20, 2003     

A Study of Orographic Blocking and Barrier Wind Development Upstream of the Southern Alps, New Zealand

Summary Upper level and surface wind data for 1994 are used to provide an initial identification of the orographic effect on regional airflow patterns upwind of the mountain barrier. A case study of the development of upstream blocking and barrier jets is also provided. The predominance of gradient airflow from between northwest and southwest through this region results in frequent trans-mountain winds. The mountains are seen to have a major effect on airflow in the lowest 2000 m above sea level, with clear evidence of orographic blocking and barrier wind development. Some variability in the extent of this blocking was noted during 1994, which appeared to be associated with changes in the synoptic circulation and air mass characteristics. The frequent occurrence of southwesterly winds between 300 m and 2000 m indicates significant deflection of the predominant winds to follow the southwest-northeast orientation of the mountains. These southwesterly barrier winds occur in opposition to the apparent pressure gradient. Northeasterly barrier winds occur mainly below 300 m, and represent a down-gradient, localised flow that is frequently separated from overlying northwesterly gradient winds by a transitional layer, within which the wind backs with height. The controls of the extent of orographic blocking are only assessed superficially, due to the lack of good thermodynamic data upstream of the mountains, although a combination of wind speed and atmospheric stability is obviously important. These initial results provide a useful insight into the extent of orographic effects on regional windfields, which will serve as the basis for future observational and modelling studies. Received June 11, 1998 Revised April 16, 1999     

Idealised Numerical Experiments of Alpine Flow Regimes and Southside Precipitation Events

Summary  Heavy precipitation events to the south of the Alps are usually associated with a southerly pre-frontal low-level jet advecting moisture toward the southern slopes of the Alps. Here we use idealised numerical simulations to assess the nature of the associated flow regimes and the mechanisms leading to vertical lifting and precipitation. The idealisations comprise: a simplified arc-shaped barrier-like orographic obstacle of Alpine scale; neglection of the tropopause; a stationary two-dimensional upstream flow configuration that includes a frontal structure and a low-level jet; hydrostatic dynamics with free-slip lower boundary conditions; and a simplified set of parameterizations to address dry, moist absolutely stable, and moist conditionally unstable upstream flow configurations. Within the dry dynamics, typical settings lead to Alpine-scale flow splitting with pronounced left/right asymmetries with respect to the incident southerly flow. Strong vertical lifting occurs over the western portion of the upstream slopes, within the stream of air that tries to circum go the elongated obstacle on the western flank. Thus, despite belonging to the “flow-around” regime, these flow configurations can exhibit vertical lifting over the whole height of the obstacle. The responsible asymmetry is primarily induced by the Coriolis effect in the presence of an elongated mountain, but it can further be intensified by the impinging low-level jet and the arc-shape of the Alpine topography. With a conditionally unstable moist upstream profile, the flow is able to surmount the obstacle without pronounced horizontal deflections. Maximum precipitation rates of are obtained. When moist convection is suppressed by using a moist absolutely stable upstream profile, the flow is again substantially deflected and shows the typical characteristics of the dry flow regime discussed above, with somewhat reduced precipitation rates as compared to the convective case. Overall there is evidence that the asymmetry introduced by the Coriolis effect, a pronounced low-level jet, and a moist upstream profile, all facilitate vertical lifting and thereby provide a suitable environment for heavy condensation and precipitation. Received March 22, 1999/Revised August 18, 1999     

气流过山运动的非线性浅水理论

用一个两层密度不同的流体所组成的非线性浅水模型讨论了过山气流的变化特征。结果表明：在背风坡下游,如果弗罗德数Ｆｒ大于１,则在不同的条件下将会出现波状的、水跃的以及“破裂”的气流,如果Ｆｒ小于１,将只会出现“破裂”气流;在山地上空,地形波关于地形分布是非对称的。     

A multi-scale simulation of an extreme downslope windstorm over complex topography

Summary A severe localized windstorm, with near-surface winds > 60 ms⁻¹, occurred in an isolated valley within the Alpine mountains (> 1800 m) of central Norway on 31 January 1995. A multi-scale numerical simulation of the event was performed with the Naval Research Laboratory (NRL)’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS), configured with four nested grids telescoping down to 1-km horizontal resolution. The windstorm occurred in response to topographic blocking and deformation of a lower-tropospheric warm front and attendant jet (> 35 ms⁻¹ at 2 km). The key findings are: i) mountain wave resonance and amplification arising from the interaction of the surface-based front and jet with complex orography, ii) sensitivity of the wave response to differential diabatic heating (vertical) gradients above the front, and iii) trapped response within the layer of large frontal stratification in the lower troposphere and subsequent amplification consistent with the theoretically-established two-layer windstorm analogue of Durran (1986). Received September 29, 1999 Revised December 30, 1999     

Flow Over Hills with Variable Roughness

The effect of variable roughness length upon the flow characteristics over hills is investigated. The changes considered herein cover a range of flow configurations such as the change from a forested (rough) valley with a moderately smooth hilltop to a grassy valley (smooth) with a “spiky” (rough) mountain top. The effect of moving the roughness with respect to the hill is also considered. Although many of the flow features change when the position of the roughness change is varied with respect to the hill these changes have very little impact upon the global properties used within orographic drag parametrization schemes.     

1998年长江流域夏季降水异常的一个物理机制

2005年春末夏初云南出现了55年来最严重的高温干旱天气。通过诊断分析发现,造成此异常气候主要原因是中高层大气环流季节转换滞后,冷空气活动偏北;南海夏季风爆发偏晚;热带对流偏弱;西太平洋副高偏南、偏强、偏西。     

An Observational Study of the Mesoscale Mistral Dynamics

We investigate the mesoscale dynamics of the mistral through the wind profiler observations of the MAP (autumn 1999) and ESCOMPTE (summer 2001) field campaigns. We show that the mistral wind field can dramatically change on a time scale less than 3 hours. Transitions from a deep to a shallow mistral are often observed at any season when the lower layers are stable. The variability, mainly attributed in summer to the mistral/land–sea breeze interactions on a 10-km scale, is highlighted by observations from the wind profiler network set up during ESCOMPTE. The interpretations of the dynamical mistral structure are performed through comparisons with existing basic theories. The linear theory of R. B. Smith [Advances in Geophysics, Vol. 31, 1989, Academic Press, 1–41] and the shallow water theory [Schär, C. and Smith, R. B.: 1993a, J. Atmos. Sci. 50, 1373–1400] give some complementary explanations for the deep-to-shallow transition especially for the MAP mistral event. The wave breaking process induces a low-level jet (LLJ) downstream of the Alps that degenerates into a mountain wake, which in turn provokes the cessation of the mistral downstream of the Alps. Both theories indicate that the flow splits around the Alps and results in a persistent LLJ at the exit of the Rhône valley. The LLJ is strengthened by the channelling effect of the Rhône valley that is more efficient for north-easterly than northerly upstream winds despite the north–south valley axis. Summer moderate and weak mistral episodes are influenced by land–sea breezes and convection over land that induce a very complex interaction that cannot be accurately described by the previous theories.     

Flow splitting at the bifurcation between two valleys: idealized simulations in comparison with Mesoscale Alpine Programme observations

Summary This paper investigates the characteristics of channelled airflow in the vicinity of a junction of three idealized valleys (one valley carrying the incoming flow and two tributaries carrying the outflow), using a two-dimensional single-layer shallow water model. Particular attention is given to the flow splitting occurring at the junction. Nondimensionalized, the model depends on the valley geometry, the Reynolds number, which is related to the eddy viscosity, and on the difference of the hydrostatic pressure imposed at the exit of the tributaries. At the spatial scale considered in this study, the Rossby number relating the inertial and Coriolis forces is always larger than 1, implying that the effect of earth rotation can be neglected to a first approximation. The analysis of the flow structure within the three valleys as well as the calculation of the split ratio (fraction of the air flow diverted into one of the two downstream valleys with respect to the total mass flux in the upstream valley) show that (i) the flow pattern depends strongly on the Reynolds number while the split ratio is comparatively insensitive; (ii) the valley geometry and the difference between the upstream and downstream hydrostatic pressures affect the flow pattern, the location of the split point and the split ratio; (iii) the relative contribution of flow deflection by the sidewalls and the blocking/splitting mechanism differs between the settings of a “Y-shape” valley and a “T-shape” valley. Quantitative comparison of the present results with numerical simulations of realistic cases and with observations collected in the region of the Rhine and Seez valleys (Switzerland) (“Y-shape” valley) and in the region of the Inn and Wipp valleys (Austria) (“T-shape” valley) during the Mesoscale Alpine Programme (MAP) field experiment shows good agreement provided that the normalized valley depth NΔH/U_u significantly exceeds 1, i.e., when “flow around” is expected. A structural disagreement between the idealized simulations and the observed wind field is found only when NΔH/U_u ≃ 1, that is, in the “flow over” regime. This shows that the dimensionless valley depth is indeed a good indicator for flow splitting, implying that the stratification is a key player in reality.     

Remote Sensing Observations of Effects of Mountain Blocking on Travelling Gravity-Shear Waves and Associated Clouds

Trapped Kelvin–Helmholtz (K–H) waves and vortices were monitored as they were generated immediately upwind of a mountain and driven into the barrier by a low-level jet. A stratus cloud visually revealed the embedded, propagating, gravity-shear waves. Interactions of the waves with the mountain were deciphered using remote sensing measurements of the structure, motions, and microphysics within the cloud and conceptual models based on existing theories. The observations show that the mountain acted as a dam to the flow that was primed for, but did not spontaneously induce, the waves. In response to the blocking, the waves spatially developed a pattern of formation, amplification, and breakdown between the upstream flow and the barrier, and altered the associated clouds in the process. Notably, radar signatures of velocity variance depicted organized, intertwined ribbons of relatively large vorticity within the wave layer. These provided measured evidence of the vortex sheet and streamwise vortex tubes predicted by advanced K–Hinstability theory, the three-dimensional version of Scorer's `stripe', the layer of rotational fluid between opposed flows that led to the wave generation. A theory of resonant interaction of wave trains, but with blocking imposed, appears to explain the internal structure of the pile-up of the flow and wave amplification approaching the barrier. Evolution of the supporting atmospheric thermal structure and introduction of a boundary-layer flow reversal follow a current model of blocking, although some features may have developed more directly from wave-driven mixing. The remote sensors also measured the influence of the waves on the cloud liquid water, including a cumulative enlargement of droplets as they were carried through a series of waves.     

Mountain–valley precipitation differences in the northern Alps: an exemplary high-resolution modeling study

This paper investigates the dependence on environmental conditions of altitudinal precipitation differences in the northern Alps, based on high-resolution numerical simulations with the MM5 model for a selected region in the Bavarian Alps (Zugspitze mountain and surrounding valley stations). Three exemplary precipitation events representing climatological regimes with different orographic enhancement characteristics are selected. After validating the MM5 precipitation fields against the available surface observations, the model results are used to analyse the interactions of atmospheric dynamics and cloud microphysics with the local orography. The first two cases (19–22 March 1997, 05–09 February 1999) are characterized by a strong northwesterly or northerly flow, associated with large precipitation differences between the mountain and the surrounding valley stations. For these cases, the model results indicate a dominance of the classical seeder–feeder mechanism, with strong orographic lifting generating dense orographic clouds over each individual mountain ridge, which in turn intensify precipitation. The related surface precipitation maxima can be found near the mountain peaks or somewhat in the lee due to hydrometeor drifting. The third case (05–07 December 1992) represents conditions with relatively small (i.e. below climatological average) precipitation differences between the Zugspitze and the surrounding valley stations. For this event, the model results indicate that relatively weak ambient winds at and below Alpine crest level (700 hPa) were primarily responsible for the lack of substantial precipitation enhancement. Precipitation was nevertheless moderately intense because of strong frontal lifting at higher levels. In all three cases, the agreement between simulated and observed precipitation patterns is so high that there is good reason to expect that mountain–valley precipitation differences will be quantitatively predictable for nonconvective events once a sufficiently high model resolution is computationally affordable.     

Small-scale dynamics of the south foehn in the lower Wipp Valley

Summary In this paper, very-high-resolution numerical simulations are presented to analyze the small-scale dynamics of the foehn in the lower Wipp Valley and the adjacent parts of the Inn Valley. This region was one of the target areas for foehn observations during the Mesoscale Alpine Programme (MAP). Our simulations consider two MAP cases that markedly differed in the depth of the foehn flow. To isolate the dynamical effect of the key orographic features in the Wipp Valley region, we performed sensitivity experiments with different topography modifications. These involve lowering or even removing the Nordkette range, which constitutes the northern side wall of the east–west-oriented Inn Valley, and closing the Stubai Valley, which is the northernmost and largest tributary of the Wipp Valley. A comparison with surface and lidar observations indicates that our present model resolution of 467 m significantly improves the realism of the simulations compared to a resolution of 800 m, as used in a previous study. The Nordkette is found to have a twofold impact on the dynamics of foehn breakthrough into the Inn Valley. In reality, this mountain chain deflects part of the southerly foehn current coming from the Wipp Valley into the perpendicularly oriented Inn Valley. Our sensitivity tests indicate that this flow deflection tends to accelerate the foehn breakthrough into the Inn Valley, while upstream blocking effects induced by the Nordkette act to slow down the process of foehn breakthrough. The flow pattern in the Wipp Valley reveals that the upstream effects of the Nordkette are not quite far-reaching. The amplitude of the gravity waves over the lower Wipp Valley gets somewhat reduced by these upstream effects, but the overall flow pattern remains largely unaffected. Closing the Stubai Valley also has a minor effect of the wave structure and tends to reduce the cross-valley variability of the foehn flow in the lower Wipp Valley.     

Gravity wave breaking in easterly flow over Greenland and associated low level barrier- and reverse tip-jets

A first evidence of severe turbulence in the lower stratosphere during easterly tropospheric flow over Greenland is presented. A numerical simulation shows the turbulence to be associated with gravity wave breaking and that simulating with a horizontal resolution of 3 km gives substantially greater and more realistic turbulence than at a 9 km horizontal resolution. It is concluded that real-time simulations at high resolutions would improve aviation forecasts. As the atmospheric flow impinges on South-Greenland a barrier jet, a reverse tip jet and amplified mountain waves with secondary wave breaking are generated at the same time.     

Gravity Wave Momentum Fluxes and Surface Pressure Drags due to the Pyrénées: Variation with Numerical Model Horizontal Resolution

Summary. ?A hydrostatic numerical model is used to simulate the lee wave event IOP3 (0000 GMT to 1200 GMT 15^th October 1990) from the PYREX mountain experiment. Results from integrations at different horizontal resolutions are used to investigate the effect on surface pressure drag and the vertical flux of horizontal momentum due to orographically forced gravity waves. In particular, results showing the dependence on resolution of the partitioning between resolved and parametrized wave drag and fluxes are presented. With the model horizontal gridlength changing from 50 km to 10 km the majority of wave momentum flux changes from being parametrized to becoming resolved. More significantly, there is a change in the profile of flux with height. At 50 km resolution the largest inferred mean flow decelerations are at lower stratospheric level due to the parametrization scheme. At 10 km resolution this is shifted, with less deceleration high up and more wave deceleration lower down in the troposphere. Numerical weather prediction models are now beginning to take account of such low level drag with beneficial results. Received March 2, 1999/Revised July 15, 1999     

冬季大范围持续性极端低温事件与欧亚大陆大型斜脊斜槽系统研究进展

本文总结了近年来关于我国冬季大范围持续性极端低温事件（EPECE）及其对应的欧亚大陆大型斜脊斜槽系统的研究成果。EPECE和普通寒潮是冬季影响我国的两类不同时间尺度大型冷空气活动,对它们的异同点进行梳理和深入理解是非常必要的。最新研究进展可概括为如下:（1）基于极端低温站点的范围和极端低温过程的持续性特点,客观界定了我国冬季EPECE。近年来的研究表明,欧亚大陆大型斜脊斜槽系统是冬季EPECE形成和维持的主要关键环流系统。同时,鉴于大型斜脊斜槽系统的重要应用意义,建立了客观识别方法。（2）从前兆信号、环流演变、阻塞高压和反气旋式波破碎活动的角度,揭示了EPECE和普通寒潮事件之间的关键区别。全国类EPECE的发生具有一周之前的前兆信号,而普通寒潮并不存在这么早的前兆信号。EPECE以从乌拉尔山到东北亚的广阔区域的阻塞高压活动为关键特征,而普通寒潮则主要以区域性阻塞高压为其主要特征。这两类事件对应的阻塞高压活动的差异可由天气尺度波破碎活动的差异加以解释。（3）最新的研究解释了大型斜脊斜槽系统形成和维持的动力学机理。基本流场对位涡扰动的正压作用是大型斜脊斜槽系统的形成和维持最重要的动力学机制。基本流场通过变形场作功和线性平流使大型扰动维持和向下游发展。与阻塞高压不同,非线性作用并非大型斜脊斜槽系统维持的主要原因。     

Effects of shear and sharp gradients in static stability on two-dimensional flow over an isolated mountain ridge

Summary ?We have investigated the effects of shear and sharp gradients in static stability and demonstrated how a mountain wave and its associated surface winds can be strongly influenced. Linear theory for two-dimensional, nonrotating stratified flow over an isolated mountain ridge with positive shear and constant static stability shows that the horizontal wind speeds on both the lee and upslope surfaces are suppressed by positive shear. The critical F(=U/Nh where U is the basic wind speed, N the Brunt-Vaisala frequency, and h the mountain height) for the occurrence of wave breaking decreases when the strength of the positive shear increases, while the location for the wave-induced critical level is higher in cases with larger positive shear. The linear theory is then verified by a series of systematic nonlinear numerical experiments. Four different flow regimes are found for positive shear flow over a two-dimensional mountain. The values of critical F which separate the flow regimes are lower when the strength of the positive shear is larger. The location of stagnation aloft from numerical simulations is found to be quite consistent with those predicted by linear theory. We calculate the strongest horizontal wind speed on the lee surface (U _max), the smallest horizontal wind speed on the upslope surface (U _min), the reflection (Ref), and the transmission (Tran) coefficients for different combinations of the stability ratio between the upper and lower layers (i.e. and z ₁ (interface height) in a two-layer atmosphere from linear analytical solutions. Both Ref and Tran are found to be functions of log() but not the interface height (z ₁). Ref is larger when is much different from 1, no matter whether it is larger or smaller than 1. However, Tran decreases when log() increases and approaches 0 when log() is large. The magnitude of the largest U _max (smallest U _min) increases (decreases) as the absolute value of log() increases. It is found that the largest U _max occurs when the nondimensional z ₁ is near for cases with a less stable upper layer or when z ₁ is near for cases with a more stable upper layer. These results are confirmed by nonlinear numerical simulations. We find that linear theory is very useful in qualitative analysis of the possibility of high-drag state for different stability profiles. The location of stagnation aloft in a two-layer atmosphere from numerical simulations agrees very well with those predicted by linear theory. The above findings are applied to investigate the Boulder severe downslope windstorm of 11 January 1972. We find that the windstorm cannot develop if the near mountain-top inversion is located at a higher altitude (e.g.,  km). However, if there exists a less stable layer right below the tropopause, the windstorm can develop in the absence of a low-level inversion. These results indicate the importance of partial reflection due to the structured atmosphere in influencing the possibility of severe downslope windstorms, although partial reflection may not be the responsible mechanism for the generation of windstorms. Received September 25, 1999/Revised February 9, 2000