Cross-mountain Flow Research Articles

Vertical Motions Forced by Small-Scale Terrain and Cloud Microphysical Response in Extratropical Precipitation Systems

Abstract Airborne vertically profiling Doppler radar data and output from a ∼1-km-grid-resolution numerical simulation are used to examine how relatively small-scale terrain ridges (∼10–25 km apart and ∼0.5–1.0 km above the surrounding valleys) impact cross-mountain flow, cloud processes, and surface precipitation in deep stratiform precipitation systems. The radar data were collected along fixed flight tracks aligned with the wind, about 100 km long between the Snake River Plain and the Idaho Central Mountains, as part of the 2017 Seeded and Natural Orographic Wintertime clouds: the Idaho Experiment (SNOWIE). Data from repeat flight legs are composited in order to suppress transient features and retain the effect of the underlying terrain. Simulations closely match observed series of terrain-driven deep gravity waves, although the simulated wave amplitude is slightly exaggerated. The deep waves produce pockets of supercooled liquid water in the otherwise ice-dominated clouds (confirmed by flight-level observations and the model) and distort radar-derived hydrometeor trajectories. Snow particles aloft encounter several wave updrafts and downdrafts before reaching the ground. No significant wavelike modulation of radar reflectivity or model ice water content occurs. The model does indicate substantial localized precipitation enhancement (1.8–3.0 times higher than the mean) peaking just downwind of individual ridges, especially those ridges with the most intense wave updrafts, on account of shallow pockets of high liquid water content on the upwind side, leading to the growth of snow and graupel, falling out mostly downwind of the crest. Radar reflectivity values near the surface are complicated by snowmelt, but suggest a more modest enhancement downwind of individual ridges. Significance Statement Mountains in the midlatitude belt and elsewhere receive more precipitation than the surrounding lowlands. The mountain terrain often is complex, and it remains unclear exactly where this precipitation enhancement occurs, because weather radars are challenged by beam blockage and the gauge network is too sparse to capture the precipitation heterogeneity over complex terrain. This study uses airborne profiling radar and high-resolution numerical simulations for four winter storms over a series of ridges in Idaho. One key finding is that while instantaneous airborne radar transects of the cross-mountain flow, vertical drafts, and reflectivity contain much transient small-scale information, time-averaged transects look very much like the model transects. The model indicates substantial surface precipitation enhancement over terrain, peaking over and just downwind of individual ridges. Radar observations suggest less enhancement, but the radar-based assessment is uncertain. The second key conclusion is that, even though orographic gravity waves are felt all the way up into the upper troposphere, the orographic precipitation enhancement is due to processes very close to the terrain.

The Influence of Vertical Wind Shear on the Evolution of Mountain-Wave Momentum Flux

Abstract The influence of vertical shear on the evolution of mountain-wave momentum fluxes in time-varying cross-mountain flows is investigated by numerical simulation and analyzed using ray tracing and the WKB approximation. The previously documented tendency of momentum fluxes to be strongest during periods of large-scale cross-mountain flow acceleration can be eliminated when the cross-mountain wind increases strongly with height. In particular, the wave packet accumulation mechanism responsible for the enhancement of the momentum flux during periods of cross-mountain flow acceleration is eliminated by the tendency of the vertical group velocity to increase with height in a mean flow with strong forward shear, thereby promoting vertical separation rather than concentration of vertically propagating wave packets.

On Typhoon Track Deflections near the East Coast of Taiwan

Typhoons with “deflection tracks” (DTs) within a 200-km distance of the mountainous island of Taiwan are examined. We analyze 84 landfalling typhoons that compose 49 DT cases turning to the left-hand side, including 18 with very large deflection angles (DA > 20°) and another 7 having looped tracks (LTs). Most of the large DA and LT cases are “northern landfall” type, reaching Taiwan’s east coast poleward of 24°N and originally possessing relatively slow translation speeds (~4 m s−1). Their average translation speeds, however, increase by 50% in the 3 h prior to landfall. The WRF Model is used to simulate DT cases, and potential vorticity (PV) tendency diagnosis is used to interpret the contributions of the horizontal advection (HA), vertical advection (VA), and diabatic heating (DH) terms. The northern landfall tropical cyclones (TCs) possess significant cross-mountain flow to the south of the storm near the coast, resulting in vorticity stretching (the VA effect) and subsidence warming. The subsidence suppresses storm convection and produces heating asymmetries (the DH effect) that can induce significant southwestward deflections. The cross-mountain VA and DH effects are weaker for the “southern landfall” storms. The results explain well the observed increase of translation speed prior to landfall in DT cases and show that the HA effect, in general, does not contribute to the track deflection. Our results highlight the impact of topography on TC track by the vorticity stretching effect and by asymmetric diabatic heating.

Observed versus simulated mountain waves over Scandinavia – improvement of vertical winds, energy and momentum fluxes by enhanced model resolution?

Abstract. Two mountain wave events, which occurred over northern Scandinavia in December 2013 are analysed by means of airborne observations and global and mesoscale numerical simulations with horizontal mesh sizes of 16, 7.2, 2.4 and 0.8 km. During both events westerly cross-mountain flow induced upward-propagating mountain waves with different wave characteristics due to differing atmospheric background conditions. While wave breaking occurred at altitudes between 25 and 30 km during the first event due to weak stratospheric winds, waves propagated to altitudes above 30 km and interfacial waves formed in the troposphere at a stratospheric intrusion layer during the second event. Global and mesoscale simulations with 16 and 7.2 km grid sizes were not able to simulate the amplitudes and wavelengths of the mountain waves correctly due to unresolved mountain peaks. In simulations with 2.4 and 0.8 km horizontal resolution, mountain waves with horizontal wavelengths larger than 15 km were resolved, but exhibited too small amplitudes and too high energy and momentum fluxes. Simulated fluxes could be reduced by either increasing the vertical model grid resolution or by enhancing turbulent diffusion in the model, which is comparable to an improved representation of small-scale nonlinear wave effects.

Profiling Radar Observations and Numerical Simulations of a Downslope Wind Storm and Rotor on the Lee of the Medicine Bow Mountains in Wyoming

This study describes a downslope wind storm event observed over the Medicine Bow range (Wyoming, USA) on 11 January 2013. The University of Wyoming King Air (UWKA) made four along-wind passes over a five-hour period over the mountain of interest. These passes were recognized as among the most turbulent ones encountered in many years by crew members. The MacCready turbulence meter aboard the UWKA measured moderate to severe turbulence conditions on each pass in the lee of the mountain range, with eddy dissipation rate values over 0.5 m2/3 s−1. Three rawinsondes were released from an upstream location at different times. This event is simulated using the non-hydrostatic Weather Research and Forecast (WRF) model at an inner- domain resolution of 1 km. The model produces a downslope wind storm, notwithstanding some discrepancies between model and rawinsonde data in terms of upstream atmospheric conditions. Airborne Wyoming Cloud Radar (WCR) vertical-plane Doppler velocity data from two beams, one pointing to the nadir and one pointing slant forward, are synthesized to obtain a two-dimensional velocity field in the vertical plane below flight level. This synthesis reveals the fine-scale details of an orographic wave breaking event, including strong, persistent downslope acceleration, a strong leeside updraft (up to 10 m·s−1) flanked by counter-rotating vortices, and deep turbulence, extending well above flight level. The analysis of WCR-derived cross-mountain flow in 19 winter storms over the same mountain reveals that cross-mountain flow acceleration and downslope wind formation are difficult to predict from upstream wind and stability profiles.

Estimating the Response of Extreme Precipitation over Midlatitude Mountains to Global Warming

AbstractGlobal warming–induced changes in extreme orographic precipitation are investigated using a hierarchy of models: a global climate model, a limited-area weather forecast model, and a linear mountain wave model. The authors consider precipitation changes over an idealized north–south midlatitude mountain barrier at the western margin of an otherwise flat continent. The intensities of the extreme events on the western slopes increase by approximately 4% K−1 of surface warming, close to the “thermodynamic” sensitivity of vertically integrated condensation in those events due to temperature variations when vertical motions stay constant. In contrast, the intensities of extreme events on the eastern mountain slopes increase at about 6% K−1. This higher sensitivity is due to enhanced ascent during the eastern-slope events, which can be explained in terms of linear mountain wave theory as arising from global warming–induced changes in the upper-tropospheric static stability and the tropopause level. Similar changes to these two parameters also occur for the western-slope events, but the cross-mountain flow is much stronger in those events; as a consequence, linear theory predicts no increase in the western-slope vertical velocities. Extreme western-slope events tend to occur in winter, whereas those on the eastern side are most common in summer. Doubling CO2 not only increases the precipitation, but during extreme western slope events it shifts much of the precipitation from snow to rain, potentially increasing the risk of heavy runoff and flooding.

Snow Growth and Transport Patterns in Orographic Storms as Estimated from Airborne Vertical-Plane Dual-Doppler Radar Data

Abstract Airborne vertical-plane dual-Doppler cloud radar data, collected on wind-parallel flight legs over a mountain in Wyoming during 16 winter storms, are used to analyze the growth, transport, and sedimentation of snow. In all storms the wind is rather strong, such that the flow is unblocked. The sampled clouds are mixed phase, shallow, and generally produce snowfall over the mountain only. The 2D scatterers’ mean motion in the vertical along-track plane below flight level is synthesized using one radar antenna pointing to nadir, and one 30° forward of nadir. This yields instantaneous cross-mountain hydrometeor streamlines. The dynamics of the orographic flow dominate the precipitation patterns across the mountain. Three patterns are distinguished: the first two contain small convective cells, either boundary layer (BL) convection or elevated convection, the latter likely due to the release of potential instability in orographically lifted air. In these patterns the cross-mountain flow is relatively undisturbed. Precipitation from BL convection falls mostly on the windward side but precipitation from elevated convection may fall mostly in the lee. The third pattern is marked by more stratified flow, often with vertically propagating mountain waves, and with strong, plunging flow in the lee, resulting in rapid clearing of the storm across the crest and occasionally a hydraulic jump. In this case, most snow tends to fall upwind of the crest, although a shallow, sublimating snow “foot” is often seen over the leeward slopes.

Numerical Simulations of Sheared Conditionally Unstable Flows over a Mountain Ridge

Abstract In 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 systematically analyzed herein. Large rainfall rates are obtained when the cold pool, caused by the evaporative cooling of rain from precipitating convective clouds, remains quasi stationary upstream of the mountain peak. This condition occurs when the cold-pool propagation is approximately countered by the environmental wind. The large precipitation amounts can be attributed to weak upper-level flow, which favors stronger updrafts and upright convective cells, and to the ground-relative stationarity of the cells. This solution feature is produced with ambient wind shear within a narrow region of the parameter space explored here and does not occur in the numerical solutions obtained in the authors’ previous studies with uniform wind profiles.

Mesoscale modelling of two 'drying events': governing processes and implications for fire danger

This study uses mesoscale modelling to examine the processes underlying two 'drying events' and their implications for fire danger. Both events occurred in Gippsland, Victoria, Australia, in the lee of the Great Dividing Range, and the role of terrain-induced flows in causing the events is one underlying focus. The Weather Research and Forecasting (WRF) model is used in a nested configuration; the highest resolution domain has horizontal grid spacing equal to 1.5 km. The WRF model simulations identify that the first event (29 December 2001) is caused by a regime transition from blocked to unblocked flow, with the cross-mountain flow bringing warmer and drier conditions. The second event (29 May 2007) is related to enhanced downslope flow. Simulations of both events also elucidate important mesoscale processes, namely 'streamers' and lee waves, that cause significant perturbations in the fire danger in the lee of the mountains.

The Overamplification of Gravity Waves in Numerical Solutions to Flow over Topography

Abstract The tendency of high-resolution numerical weather prediction (NWP) models to overpredict the strength of vertically propagating mountain waves is explored. Discrete analytic mountain-wave solutions are presented for the classical problem of cross-mountain flow in an atmosphere with constant wind speed and stability. Time-dependent linear numerical solutions are also obtained for more realistic atmospheric structures. On one hand, using second-order-accurate finite differences on an Arakawa C grid to model nonhydrostatic flow over what might be supposed to be an adequately resolved 8Δx-wide mountain can lead to an overamplification of the standing mountain wave by 30%–40%. On the other hand, the same finite-difference scheme underestimates the wave amplitude in hydrostatic flow over an 8Δx-wide mountain. Increasing the accuracy of the advection scheme to the fourth order significantly reduces the numerical errors associated with both the hydrostatic and nonhydrostatic discrete solutions. The Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model is used to generate two 70-member ensemble simulations of a mountain-wave event during the Terrain-Induced Rotor Experiment. It is shown that switching from second-order advection to fourth-order advection leads to as much as a 20 m s−1 decrease in vertical velocity on the lee side of the Sierra Nevada, and that the weaker fourth-order solutions are more consistent with observations.

Influence of lee waves on the near‐surface flow downwind of the Pennines

AbstractThe results of a recent field experiment focusing on the near‐surface pressure and flow fields downstream of the Pennines in northern England are presented. The main aim of the experiment is the improvement of wind forecasts downstream of orography. Trapped lee waves commonly occur in westerly flow in this region, and during the experiment there were numerous instances of apparent flow separation, indicating the formation of lee‐wave rotors. The spatial variability of the near‐surface flow in these circumstances is closely linked to the positions of lee‐wave crests and troughs aloft, and appears to be a response to pressure gradients induced by the lee waves. For large‐amplitude waves, it has been possible to demonstrate a correlation between the fractional change of the flow speed across the measurement array (which if large enough may lead to flow separation) and a normalized pressure‐perturbation amplitude.For a group of lee‐wave cases during which the cross‐mountain flow is strong, a rapid decrease in the Scorer parameter within the lower portion of the troposphere appears to be a prerequisite for rotors to form. However, this does not guarantee their occurrence. For a fixed Scorer‐parameter profile, idealized two‐dimensional simulations indicate that the lee‐wave‐induced pressure‐perturbation amplitude, and hence the occurrence of rotors, is controlled largely by the strength of the wind upstream close to the mountain‐top level. It seems that the combination of a favourable Scorer‐parameter profile and sufficiently strong low‐level winds is required for rotors to develop. © Crown Copyright 2007. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

Wintertime Precipitation Episodes in Central Chile: Associated Meteorological Conditions and Orographic Influences

Abstract Central Chile (32°–35°S) is a mountainous and densely populated strip of land between the South American Pacific coast and the main divide of the Andes, 5000 m in height. In this study, wintertime precipitation episodes in central Chile are characterized using precipitation gauge, river discharge, radiosonde, and Special Sensor Microwave Imager (SSM/I) passive microwave radiometer observations over a 10-yr period (1993–2002). Precipitation episodes that typically occur as cold frontal rainstorms move over the region from west to east, within which the cross-mountain flow is blocked at lower levels. The influence of the Andes on the climatological precipitation pattern extends several hundred kilometers upstream of the coast. Over the mainland, the wintertime precipitation is most strongly related to the height of the mean topography surrounding the rain gauge sites, rather than the actual altitudes of the instruments, although higher-elevation locations are not well sampled by available rainfall observations. Between the coast and foothills of the Andes, the precipitation pattern is relatively uniform despite the complex coastal topography. On the western face of the Andes climatological enhancement factors of between 1 and 3 are inferred. Regression analysis against radiosonde data at a coastal site reveals that the precipitation is strongly related to the zonal (cross mountain) moisture flux. The strongest relationship is found when the moisture flux is multiplied by the relative humidity. This variable explains 50% of the variance in daily area average precipitation in central Chile and up to 60% of the variance in the daily precipitation recorded at individual stations. The factors contributing to events of heavy precipitation enhancement in the Andes were examined. Events of heavy, but isolated, precipitation in the Andes tend to occur in the warmer, prefrontal sector of approaching storms and are associated with unusually high moisture fluxes near to and above the crest of the mountain range. Strongly frontal episodes, characterized by widespread rainfall throughout central Chile, lead to variable, but on average rather weak, enhancement in the Andes.

Mountain-Wave Momentum Flux in an Evolving Synoptic-Scale Flow

Abstract The evolution of mountain-wave-induced momentum flux is examined through idealized numerical simulations during the passage of a time-evolving synoptic-scale flow over an isolated 3D mountain of height h. The dynamically consistent synoptic-scale flow U accelerates and decelerates with a period of 50 h; the maximum wind arrives over the mountain at 25 h. The synoptic-scale static stability N is constant, so the time dependence of the nonlinearity parameter, ɛ(t) = Nh/U(t), is symmetric about a minimum value at 25 h. The evolution of the vertical profile of momentum flux shows substantial asymmetry about the midpoint of the cycle even though the nonlinearity parameter is symmetric. Larger downward momentum fluxes are found during the accelerating phase, and the largest momentum fluxes occur in the mid- and upper troposphere before the maximum background flow arrives at the mountain. For a period of roughly 15 h, this vertical distribution of momentum flux accelerates the lower-tropospheric zonal-mean winds due to low-level momentum flux convergence. Conservation of wave action and Wentzel–Kramers–Brillouin (WKB) ray tracing are used to reconstruct the time–altitude dependence of the mountain-wave momentum flux in a semianalytic procedure that is completely independent of the full numerical simulations. For quasi-linear cases, the reconstructions show good agreement with the numerical simulations, implying that the basic asymmetry obtained in the full numerical simulations may be interpreted using WKB theory. These results demonstrate that even slow variations in the mean flow, with a time scale of 2 days, play a dominant role in regulating the vertical profile of mountain-wave-induced momentum flux. The time evolution of cross-mountain pressure drag is also examined in this study. For almost-linear cases, the pressure drag is well predicted under steady-state linear theory by using the instantaneous incident flow. Nevertheless, for mountains high enough to preserve a moderate degree of nonlinearity when the synoptic-scale incident flow is strongest, the evolution of cross-mountain pressure drag is no longer symmetric about the time of maximum wind. A higher drag state is found when the cross-mountain flow is accelerating. These results suggest that the local character of the topographically induced disturbance cannot be solely determined by the instantaneous value of the nonlinearity parameter ɛ.

Numerical and observational case-study of a deep Adriatic bora

A detailed analysis of bora winds is presented that were observed on 28 March 2002 in the north-eastern part of the Adriatic Sea. Very high-resolution numerical simulations are compared with airborne, surface, and balloon observations. The key instrument for the verification of the numerical results is an aerosol backscatter lidar on board the DLR Falcon research aircraft. The high spatial resolution of the model and of the observational dataset allows several small-scale aspects of the bora winds to be explored. The study emphasises the great impact of boundary-layer effects including convective mixing and surface friction on the structure of the bora flow. The numerical model reveals that the diurnal cycle of the planetary boundary layer causes a diurnal variation of the gravity-wave amplitude and consequently a variation in the bora strength. The downslope flow is strongest during the night-time when the impinging air mass is stably stratified and exhibits a low-level jet. The bora strength weakens during the daytime due to the evolution of a neutrally stratified convective boundary layer. Lidar observations and simulations both suggest that the observed bora winds are driven by the dynamics of orographic gravity waves. In the deep north-easterly cross-mountain flow steeply amplified but non-breaking gravity waves as well as trapped lee waves are found throughout the troposphere. Vertical separation of the boundary layer over the steep leeward terrain slope prevents the bora flow from reaching the coast. As a result, the flow is kept aloft in a series of at least three unsteady trapped lee waves before it reattaches to the surface over the sea downstream of the mountains. No clear evidence is found for the existence of low-level rotors underneath these lee waves. The ability of the model to capture the trapped waves is found to depend on the horizontal model resolution. Bora winds are identified as jets emanating from several mountain gaps. The physical mechanism for the gap flow formation is boundary-layer separation controlled by surface friction. Flow separation is more effective over the highest terrain where a broad wake forms downstream with weak winds compared to the mountain gaps where the strong bora flow eventually reattaches to the surface. The strongest gap jet belongs to the Vratnik Pass upstream of the town Senj and forms the primary shear line in the northern Adriatic with a corresponding potential-vorticity (PV) banner. The numerical model reveals several secondary orographic PV banners and embedded PV filaments. The vertical PV distribution exhibits a two-layer structure with PV anomalies at lower (higher) elevations originating from mountain gaps (isolated mountain peaks). Surface friction has a minor direct effect on PV generation but has a major indirect effect in controlling the mechanism for the generation of PV. In a simulation without surface friction the primary PV banner forms as a result of gravity-wave breaking. In the more realistic simulation, however, the primary mechanism for PV generation is flow separation controlled by surface friction. Copyright © 2005 Royal Meteorological Society

Bora‐driven potential vorticity banners over the Adriatic

AbstractA case study is presented of the secondary potential vorticity (PV) banners over the northern Adriatic that occurred in an early stage of a bora on 7 November 1999 during the Mesoscale Alpine Programme (MAP) Special Observation Period. The dynamics and structure of the lee‐side and cross‐mountain flow past the Dinaric Alps were investigated using data collected in a dual‐aircraft (NCAR Electra and NOAA P‐3)MAP Intensive Observing Period 15 mission over the Adriatic and high‐resolution numerical simulations. The observational study employs flight‐level, dropsonde, and Scanning Aerosol Backscatter Lidar data. The observed flow structure is compared with simulations results of the COAMPS model run at a horizontal resolution of 3 km.The Dinaric Alps, the north‐west/south‐east oriented coastal mountain range of Croatia, has an irregular ridge line with a number of peaks in the range of 1.5–2 km with several prominent mountain passes. The identified jet and wake structure within the east‐north‐easterly bora over the Adriatic was found to be well correlated with the upwind distribution of mountain passes and peaks. The wake flow structure was found also to be in excellent agreement with the climatological profile of the bora strength along the Croatian coast. The attendant secondary PV banners separating individual jets and wakes, diagnosed by computing PV from the flight‐level data, were found to have a characteristic horizontal scale of 10–25 km, and a maximum amplitude of up to ∼6 pvu within the boundary layer. Over the open sea, the thickness of the boundary layer, within which the east‐north‐easterly bora also achieved its maximum strength, was approximately 1 km.Detailed comparison with the numerical model results shows that, at the horizontal resolution of 3 km, the COAMPS model reproduces well the overall flow structure. The COAMPS‐simulated PV field was found to be in good agreement with the PV derived from observations. The differences in substructure between simulated and experimentally derived PV profiles derive from minor differences between modelled and observed velocity and potential temperature profiles, which are subsequently accentuated by computing differentiated quantities such as vorticity and potential temperature gradients. The high predictability and steadiness of the PV banners, and a good correlation with the geometry of the upwind topography, support the orographic generation mechanism of PV with dissipation in hydraulic jumps and gravity‐wave breaking regions as the likely main source of PV. Copyright © 2004 Royal Meteorological Society

A Theoretical Study of Cold Air Damming with Upstream Cold Air Inflow

Abstract The previously developed two-layer model of cold air damming is extended to include upstream cold air inflow. The upper layer is an isentropic cross-mountain flow. The lower layer is a cold boundary layer flow partially blocked by a two-dimensional mountain with a cold dome formed on the windward side of the mountain. The interface represents a sloping inversion layer coupling the two layers. The shape of the interface can be approximated by a cubic polynomial, and the interfacial coupling condition yields a set of algebraic equations that quantify the scale and intensity of the dammed flow as functions of the external parameters characterizing the environmental conditions. It is found that the cold dome shrinks as the Froude number increases or, to a minor degree, as the Ekman number decreases or/and the upstream inflow veers from northeasterly to southeasterly (with respect to a longitudinal mountain to the west). The mountain-parallel jet speed increases as the Ekman number decreases or/and th...

An Analytic Model of Cold Air Damming and Its Applications

Abstract It is shown that the geometric shape of the cold dome in the two-layer model of cold air damming of Xu can be described approximately by a cubic polynomial and thus a set of coupled algebraic equations can be derived to quantify the scale and intensity of cold air damming as functions of the external parameters that characterize the environmental flow. In particular, these functions are easily computed and plotted in the parameter space, showing quantitatively how the cold dome width (scaled by the Rossby radius of deformation) and mountain-parallel jet speed change with the Froude number, surface roughness, inertial aspect ratio, and incident angle of the upstream inflow. The results also show that the cold dome width and mountain-parallel jet speed are insensitive to the depth of the upper-layer cross-mountain flow if the cross-mountain flow is sufficiently deep. The surface roughness and inverse inertial aspect ratio are found to have nearly the same control on the cold dome width and mountain...

A Theoretical Study of Cold Air Damming

Abstract The dynamics of cold air damming are examined analytically with a two-layer steady state model. The upper layer is a warm and saturated cross-mountain (easterly or southeasterly onshore) flow. The lower layer is a cold mountain-parallel (northerly) jet trapped on the windward (eastern) side of the mountain. The interface between the two layers represents a coastal front—a sloping inversion layer coupling the trapped cold dome with the warm onshore flow above through pressure continuity. An analytical expression is obtained for the inviscid upper-layer flow with hydrostatic and moist adiabatic approximations. Blackadar's PBL parameterization of eddy viscosity is used in the lower-layer equations. Solutions for the mountain-parallel jet and its associated secondary transverse circulation are obtained by expanding asymptotically upon a small parameter proportional to the square root of the inertial aspect ratio—the ratio between the mountain height and the radius of inertial oscillation. The geometr...

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...

Linear Results on the Barrier Effects of Mesoscale Mountains

Abstract We examine the factors determining whether a mountain acts as a strong barrier to an impinging flow. A primary concern is the extent to which the barrier effect is reduced when smoothed orography is used in a numerical model. These questions are addressed within the model of linear, rotating, stratified flow over topography first studied by Queney. The ground level Green's functions for this model are derived and their near- and far-field asymptotic behavior is discussed. Using the asymptotic results, the upstream deceleration is estimated as a function of Ro=U/fL and hm=Nhm/U, where U is the far upstream speed of the cross-mountain flow, f the Coriolis parameter, L the mountain width, N the Brunt-Vaisala frequency, and hm is the maximum mountain height. Effects of terrain shape are also considered. The Green's functions are evaluated numerically and used to calculate the response to a family of mountain profiles; a comparison with asymptotic results shows the latter to be very useful. It is conc...