Effects of Inversion Height and Surface Heat Flux on Downslope Windstorms

2011 ◽  
Vol 139 (12) ◽  
pp. 3750-3764 ◽  
Author(s):  
Craig M. Smith ◽  
Eric D. Skyllingstad
Keyword(s):  
Vertical Displacement ◽  
Falkland Islands ◽  
Similar Response ◽  
Surface Cooling ◽  
Low Level ◽  
Jet Length ◽  
Downslope Winds ◽  
Downslope Wind ◽  
Downslope Windstorms ◽  
Level Inversion

Abstract Simulations 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 without synoptic-scale changes in atmospheric structure. Most windstorms on the Falkland Islands generally have a short jet length; rare, longer jet length storms typically occur in conjunction with a strong low-level inversion. Idealized numerical experiments tend to produce a similar response depending on the presence of strong low-level inversion and surface cooling. Results suggest that surface heating can have significant control on the flow response by reducing the low-level inversion strength, or by changing the stratification and wind velocity below the inversion, thereby preventing a strong downslope windstorm.

scholarly journals Initial-Condition Sensitivities and the Predictability of Downslope Winds

2009 ◽  
Vol 66 (11) ◽  
pp. 3401-3418 ◽  
Author(s):  
Patrick A. Reinecke ◽  
Dale R. Durran
Keyword(s):  
Wave Breaking ◽  
Initial Conditions ◽  
Static Stability ◽  
Synoptic Scale ◽  
Wind Speeds ◽  
Downslope Winds ◽  
First Case ◽  
Downslope Wind ◽  
The Difference ◽  
Strong Winds

Abstract The sensitivity of downslope wind forecasts to small changes in initial conditions is explored by using 70-member ensemble simulations of two prototypical windstorms observed during the Terrain-Induced Rotor Experiment (T-REX). The 10 weakest and 10 strongest ensemble members are composited and 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−1 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, the sensitivity to initial conditions is weaker but still significant. The difference in downslope wind speeds between the weak- and strong-member composites grows to 22 m s−1 over 12 h. During and one hour before the windstorm, the synoptic-scale flow exhibits appreciable differences between the strong- and weak-member 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.

The Breakdown of a Low-level Inversion Studied by Means of Detailed Soundings with a Modified Radiosonde

1952 ◽  
Vol 33 (9) ◽  
pp. 373-379 ◽  
Author(s):  
Frank Gifford
Keyword(s):  
Minimum Temperature ◽  
Low Level ◽  
Short Series ◽  
Pilot Balloon ◽  
Level Inversion

A short series of unusually detailed temperature, humidity, and wind soundings to a height of about 750 mb., made at Silver Hill, Md. on the night of October 30th and 31st, 1950, and consisting of eight flights of specially modified radiosondes and 26 double-theodolite pilot balloon runs, is presented. The nocturnal breakdown of the ground inversion with steep wind gradients, a phenomenon first remarked by Durst in 1933, is here observed apparently to be associated with the sudden lowering of an upper (turbulence or subsidence) inversion. Alternative explanations for this are advanced, and implications for minimum temperature and stratus forecasting noted. The accuracy of the observations is discussed.

scholarly journals Evaluating the Experimental High-Resolution Rapid Refresh–Alaska Modeling System Using USArray Pressure Observations

2018 ◽  
Vol 33 (4) ◽  
pp. 933-953 ◽  
Author(s):  
Taylor A. McCorkle ◽  
John D. Horel ◽  
Alexander A. Jacques ◽  
Trevor Alcott
Keyword(s):  
High Resolution ◽  
Surface Pressure ◽  
Weather Conditions ◽  
Horizontal Resolution ◽  
Production Cycle ◽  
Downslope Winds ◽  
Downslope Wind ◽  
Wind Events ◽  
Environmental Prediction ◽  
Mean Square Errors

Abstract The High-Resolution Rapid Refresh–Alaska (HRRR-AK) modeling system provides 3-km horizontal resolution and 0–36-h forecast guidance for weather conditions over Alaska. This study evaluated the experimental version of the HRRR-AK system available from December 2016 to June 2017, prior to its operational deployment by the National Centers for Environmental Prediction in July 2018. Surface pressure observations from 158 National Weather Service (NWS) stations assimilated during the model’s production cycle and pressure observations from 101 USArray Transportable Array (TA) stations that were not assimilated were used to evaluate 265 complete 0–36-h forecasts of the altimeter setting (surface pressure reduced to sea level). The TA network is the largest recent expansion of Alaskan weather observations and provides an independent evaluation of the model’s performance during this period. Throughout the study period, systematic differences in altimeter setting between the HRRR-AK 0-h forecasts were larger relative to the unassimilated TA observations than relative to the assimilated NWS observations. Upon removal of these initial biases from each of the subsequent 1–36-h altimeter setting forecasts, the model’s 36-h forecast root-mean-square errors at the NWS and TA locations were comparable. The model’s treatment of rapid warming and downslope winds that developed in the lee of the Alaska Range during 12–15 February is examined. The HRRR-AK 0-h forecasts were used to diagnose the synoptic and mesoscale conditions during this period. The model forecasts underestimated the abrupt increases in the temperature and intensity of the downslope winds with smaller errors as the downslope wind events evolved.

Downslope Flows on a Low-Angle Slope and Their Interactions with Valley Inversions. Part II: Numerical Modeling

2008 ◽  
Vol 47 (7) ◽  
pp. 2039-2057 ◽  
Author(s):  
Shiyuan Zhong ◽  
C. David Whiteman
Keyword(s):  
Wind Speed ◽  
Pressure Gradient ◽  
Buoyancy Force ◽  
Atmospheric Modeling ◽  
Gradient Force ◽  
Ambient Atmosphere ◽  
Pressure Gradient Force ◽  
Small Contribution ◽  
Downslope Winds ◽  
Downslope Wind

Abstract The characteristics of well-developed downslope winds observed by tethered balloon soundings at multiple locations over a low-angle slope in the Salt Lake Valley are studied using the Regional Atmospheric Modeling System (RAMS). The model successfully simulated the key properties of the observed wind and temperature structure and evolution and provided insight into the forcing mechanisms. The results show that, although the slope angle is only 1.6°, the buoyancy force associated with the local temperature perturbation caused by nocturnal cooling of the slope surface is capable of producing the unusually strong and deep downslope winds observed by the tethersondes. The hypothesis that the flow is produced locally by the temperature deficit is further confirmed by analysis of the momentum budget that indicates a very small contribution from advection to the downslope mass flux. The analysis also reveals the importance of the along-slope pressure gradient force, which has been neglected by some previous investigators. On an isolated slope, the pressure gradient force, which develops as the downslope-flow layer deepens with downslope distance, is important mostly in the upper part of the downslope wind layer where it counterbalances the buoyancy force. On a slope in a valley, the pressure gradient force interacts with the valley inversion to produce intermittency in the downslope jet and may also significantly slow the flow as the inversion strengthens during the night. The simulations for two different observational nights indicate that the maximum downslope wind speed is sensitive to ambient stability, with near-neutral ambient stability yielding a stronger downslope jet than does a more stable ambient atmosphere. Sensitivity studies suggest that an increase in down-valley winds leads to a decrease in the maximum downslope wind speed and an increase in the thickness of the downslope wind layer. An increase in slope roughness, on the other hand, increases the height of the downslope jet but has little effect on other properties. The downslope wind is stronger over a gentle 1.6° slope than over a much steeper slope of 11°, mainly because of the combination of the stronger buoyancy and weaker pressure gradient over the gentle slope.

scholarly journals Diurnal Variation of Downslope Winds in Owens Valley during the Sierra Rotor Experiment

2008 ◽  
Vol 136 (10) ◽  
pp. 3760-3780 ◽  
Author(s):  
Qingfang Jiang ◽  
James D. Doyle
Keyword(s):  
Diurnal Variation ◽  
Numerical Simulations ◽  
Western Slope ◽  
Owens Valley ◽  
Mountain Waves ◽  
Downslope Winds ◽  
Wind Event ◽  
Downslope Wind ◽  
Rotor Experiment ◽  
The Impact

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 state). The westerlies plunged into the valley in the afternoon and propagated across the valley floor (in-valley westerly state). After sunset, the westerlies within the valley retreated toward the western slope, where the downslope winds persisted throughout the night.

scholarly journals On the Orographically Generated Low-Level Easterly Jet and Severe Downslope Storms of March 2006 over the Tacheng Basin of Northwest China

2018 ◽  
Vol 146 (6) ◽  
pp. 1667-1683 ◽  
Author(s):  
Guangxing Zhang ◽  
Da-Lin Zhang ◽  
Shufang Sun
Keyword(s):  
Large Scale ◽  
Inversion Layer ◽  
Dust Storms ◽  
Frozen Soil ◽  
Gradient Force ◽  
Mountain Range ◽  
Pressure Gradient Force ◽  
Low Level ◽  
Downslope Winds ◽  
The Impact

A 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 across a higher mountain ridge followed by a lower parallel ridge, respectively. The two different airstreams are merged to form an intense LLEJ of cold air, driven mostly by zonal pressure gradient force, and then the LLEJ moves along a zonally oriented mountain range to the north. Results indicate the importance of the lower ridge in enhancing the downslope winds associated with the higher ridge and the importance of the saddle pass in generating the LLEJ. We conclude that the intense downslope winds account for melting snow, warming and drying soils, and raising dust into the air that is then transported by the LLEJ, generated mostly through the saddle pass, into the far west of the basin.

scholarly journals Structure and Mechanisms of the Southern Hemisphere Summertime Subtropical Anticyclones

2010 ◽  
Vol 23 (8) ◽  
pp. 2115-2130 ◽  
Author(s):  
Takafumi Miyasaka ◽  
Hisashi Nakamura
Keyword(s):  
Southern Hemisphere ◽  
Rossby Wave ◽  
Sensible Heat Flux ◽  
Wave Activity ◽  
Convective Heating ◽  
Surface Cooling ◽  
Near Surface ◽  
Low Level ◽  
Zonal Wavenumber ◽  
Upper Level

Abstract The three-dimensional structure and dynamics of the climatological-mean summertime subtropical anticyclones in the Southern Hemisphere (SH) are investigated. As in the Northern Hemisphere (NH), each of the surface subtropical anticyclones over the South Pacific, South Atlantic, and South Indian Oceans is accompanied by a meridional vorticity dipole aloft, exhibiting barotropic and baroclinic structures in its poleward and equatorward portions, respectively, in a manner that is dynamically consistent with the observed midtropospheric subsidence. Their dynamics are also similar to their NH counterpart. It is demonstrated through the numerical experiments presented here that each of the SH surface anticyclones observed over the relatively cool eastern oceans can be reproduced as a response to a local near-surface cooling–heating couplet. The cooling is mainly due to radiative cooling associated with low-level maritime clouds, and the heating to the east is due to sensible heat flux over the dry, heated continental surface. The low-level clouds act to maintain the coolness of the underlying ocean surface, which is also maintained by the alongshore surface southerlies. As in the NH, the presence of a local atmosphere–ocean–land feedback loop is thus suggested, in which the summertime subtropical anticyclones and continental cyclones to their east are involved. Both the model experiments conducted here and the diagnosed upward flux of Rossby wave activity suggest that, in addition to continental deep convective heating, the land–sea heating–cooling contrasts across the west coasts of the three continents can contribute to the formation of the summertime upper-level planetary wave pattern observed in the entire subtropical SH, characterized by the zonal wavenumber-3 component. Though rather subtle, there are some interhemispheric differences in the summertime subtropical anticyclones, including their smaller magnitudes in the SH and the stronger equatorward propagation of upper-level Rossby wave activity emanating from the SH surface anticyclones.

scholarly journals Direct and semi-direct impacts of absorbing biomass burning aerosol on the climate of southern Africa: a Geophysical Fluid Dynamics Laboratory GCM sensitivity study

2010 ◽  
Vol 10 (20) ◽  
pp. 9819-9831 ◽  
Author(s):  
C. A. Randles ◽  
V. Ramaswamy
Keyword(s):  
Water Vapor ◽  
Biomass Burning ◽  
Southern Africa ◽  
Climate Impacts ◽  
Atmospheric Water Vapor ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Atmospheric Water ◽  
Surface Cooling ◽  
Low Level ◽  
Biomass Burning Aerosol

Abstract. Tropospheric aerosols emitted from biomass burning reduce solar radiation at the surface and locally heat the atmosphere. Equilibrium simulations using an atmospheric general circulation model (GFDL AGCM) indicate that strong atmospheric absorption from these particles can cool the surface and increase upward motion and low-level convergence over southern Africa during the dry season. These changes increase sea level pressure over land in the biomass burning region and spin-up the hydrologic cycle by increasing clouds, atmospheric water vapor, and, to a lesser extent, precipitation. Cloud increases serve to reinforce the surface radiative cooling tendency of the aerosol. Conversely, if the climate over southern Africa were hypothetically forced by high loadings of scattering aerosol, then the change in the low-level circulation and increased subsidence would serve to decrease clouds, precipitation, and atmospheric water vapor. Surface cooling associated with scattering-only aerosols is mitigated by warming from cloud decreases. The direct and semi-direct climate impacts of biomass burning aerosol over southern Africa are sensitive to the total amount of aerosol absorption and how clouds change in response to the aerosol-induced heating of the atmosphere.

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