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1.
The Regional Atmospheric Modeling System （RAMS）, which is a non-hydrostatic numerical model, has been used to investigate the impact of terrain shape and large-scale forcing on the Antarctic surface-wind regime, focusing on their roles in establishing favorable flow conditions for the formation of katabatic flow jumps. A series of quasi-2D numerical simulations were conducted over idealized slopes representing the slopes of Antarctica during austral winter conditions. Results indicate that the steepness and variations of the underlying slope play a role in the evolution of near-surface flows and thus the formation of katabatic flow jumps. However, large-scale forcing has a more noticeable effect on the occurrence of this small-scale phenomenon by establishing essential upstream and downstream flow conditions, including the upstream supercritical flow, the less stably stratified or unstable layer above the cold katabatic layer, as well as the cold-air pool located near the foot of the slope through an interaction with the underlying topography. Thus, the areas with steep and abrupt change in slopes, e.g. near the coastal areas of the eastern Antarctic, are preferred locations for the occurrence of katabatic flow jumps, especially under supporting synoptic conditions.  相似文献   

2.
Summary In this study an attempt is made to examine and analyse the mean characteristics of the katabatic flows at the western slope foot of a 1024 m high knife edge mountain using a meteorological tower and three surface meteorological stations. In addition, the frequency distribution of the occurrence of the katabatic flow over one year period is studied along the characteristics of the flow arriving in the neighbouring urban area at a distance of 1.5 km. It was found that the katabatic flow occurs mainly in autumn and spring with the highest frequency in April. The flow is generally characterised by small depth as it is affected substantially by the background flow. The expected direction of the katabatic wind dominates mainly at the level of 7 m, where the influence of the background flow is minimised. At the level of 18 m the wind direction shifts, due to the interaction of the katabatic wind with the background flow. The katabatic flow can penetrate at a distance of 1.5 km being substantially weakened. Received September 18, 1996 Revised August 4, 1997  相似文献   

3.
A realistic simulation of katabatic flows is not a straightforward task for numerical models. One complicating factor is that katabatic flows develop within a stably stratified boundary layer, which is poorly resolved and described in many numerical models. To capture the jet-shaped shallow flow a model set-up with high vertical resolution is also required. In this study, ‘a state of the art’ mesoscale numerical model is applied in a simulation of katabatic flow over a melting glacier. A basic agreement between observations and model results is found. From scale analysis, it is concluded that the simulated flow can be classified as katabatic. Although the background flow varies in strength and direction, the simulated katabatic flow over Breidamerkurjökull is persistent. Two factors vital for this persistence are identified. First, the melting snow maintains the surface temperature close to 0 °C while the air temperature warms adiabatically as it descends the slope. This provides a ‘self enhanced’ negative buoyancy that drives the flow to a balance with local friction. Second, the jet-like shape of the resulting flow gives rise to a large ‘curvature term’ in the Scorer parameter, which becomes negative in the upper jet. This prevents vertical wave propagation and isolates the katabatic layer of the influence from the free troposphere aloft. Our results suggest that the formation of local microclimates dominated by katabatic flow is a general feature over melting glaciers. The modelled turbulence structure illustrates the importance of non-local processes. Neglecting the vertical transport of turbulence in katabatic flows is not a valid assumption. It is also found that the local friction velocity remains larger than zero through the katabatic jet, due to directional shear where the scalar wind speed approaches its maximum.  相似文献   

4.
The flow and turbulence quantities governing dispersion in katabatic flows vary with both height and downslope distance. This variation cannot be accounted for in conventional plume dispersion models. In this study, three random-walk models of varying complexity are formulated to simulate dispersion in katabatic flows, and their strengths and weaknesses are discussed. The flow and turbulence parameters required by these models are determined from a high-resolution two-dimensional katabatic flow model based on a turbulent kinetic energy closure. Random-walk model calculations have been performed for several values of source height and slope angle to examine the influence of these parameters on dispersion. Finally, we simulated the perfluorocarbon and heavy methane tracer releases for Night 4 of the 1980 ASCOT field study over a nearly two-dimensional slope in Anderson Creek Valley, California. The observed peak concentrations are generally well-predicted. The effects of the pooling of the drainage air could not be taken into account in our katabatic flow model and, consequently, the predicted concentrations decay much more rapidly with time than the observed values.  相似文献   

5.
The flow structure on a gentle slope at Vallon dOl in the northern suburbs of Marseille in southern France has been documented by means of surface wind and temperature measurements collected from 7 June to 14 July 2001 during the ESCOMPTE experiment. The analysis of the time series reveals temperature and wind speed oscillations during several nights (about 60--90 min oscillation period) and several days (about 120–180 min oscillation period) during the whole observing period. Oscillating katabatic winds have been reported in the literature from theoretical, experimental and numerical studies. In the present study, the dynamics of the observed oscillating katabatic winds are in good agreement with the theory.In contrast to katabatic winds, no daytime observations of oscillating anabatic upslope flows have ever been published to our knowledge, probably because of temperature inversion break-up that inhibits upslope winds. The present paper shows that cold air advection by a sea breeze generates a mesoscale horizontal temperature gradient, and hence baroclinicity in the atmosphere, which then allows low-frequency oscillations, similar to a katabatic flow. An expression for the oscillation period is derived that accounts for the contribution of the sea-breeze induced mesoscale horizontal temperature gradient. The theoretical prediction of the oscillation period is compared to the measurements, and good agreement is found. The statistical analysis of the wind flow at Vallon dOl shows a dominant north-easterly to easterly flow pattern for nighttime oscillations and a dominant south-westerly flow pattern for daytime oscillations. These results are consistent with published numerical simulation results that show that the air drains off the mountain along the maximum slope direction, which in the studied case is oriented south-west to north-east.  相似文献   

6.
Impacts of different terrain configurations on the general behaviour of idealised katabatic flows are investigated in a numerical model study. Various simplified terrain models are applied to unveil modifications of the dynamics of nocturnal cold drainage of air as a result of predefined topographical structures. The generated idealised terrain models encompass all major topographical elements of an area in the tropical eastern Andes of southern Ecuador and northern Peru, and the adjacent Amazon. The idealised simulations corroborate that (i) katabatic flows develop over topographical elements (slopes and valleys), that (ii) confluence of katabatic flows in a lowland basin with a concave terrainline occur, and (iii) a complex drainage flow system regime directed into such a basin can sustain the confluence despite varying slope angles and slope distances.  相似文献   

7.
We use the Wind Farm Parameterization(WFP) scheme coupled with the Weather Research and Forecasting model under multiple resolution regimes to simulate turbulent wake dynamics generated by a real onshore wind farm and their influence at the local meteorological scale. The model outputs are compared with earlier modeling and observation studies. It is found that higher vertical and horizontal resolutions have great impacts on the simulated wake flow dynamics. The corresponding wind speed deficit and turbulent kinetic energy results match well with previous studies. In addition, the effect of horizontal resolution on near-surface meteorology is significantly higher than that of vertical resolution. The wake flow field extends from the start of the wind farm to downstream within 10 km, where the wind speed deficit may exceed 4%. For a height of 150 m or at a distance of about 25 km downstream, the wind speed deficit is around 2%. This indicates that, at a distance of more than 25 km downstream, the impact of the wind turbines can be ignored. Analysis of near-surface meteorology indicates a night and early morning warming near the surface, and increase in near-surface water vapor mixing ratio with decreasing surface sensible and latent heat fluxes. During daytime, a slight cooling near the surface and decrease in the near-surface water vapor mixing ratio with increasing surface sensible and latent heat fluxes is noticed over the wind farm area.  相似文献   

8.
The analysis of katabatic flows is often complicated by heterogeneity in surface characteristics. This study focuses on an idealized type of katabatic flow driven by a simple form of inhomogeneous surface forcing: a buoyancy or buoyancy flux that varies down the slope as a top-hat profile (cold strip). We consider the two-dimensional Boussinesq system of governing flow equations with the slope angle, Brunt–Väisälä frequency, and coefficients of eddy viscosity and diffusivity treated as constants. The steady-state problem is solved analytically in a linearized boundary-layer framework. Key flow structures are a primary katabatic jet (essentially the classical one-dimensional Prandtl jet), a rotor-like feature straddling the upslope end of the strip, and two nearly horizontal jets: an inward jet of environmental air feeding into the primary jet on the upslope end of the strip and an outward jet resulting from the intrusion of the primary katabatic jet into the environment on the downslope end of the strip. Next, the corresponding nonlinear initial value problem is solved numerically until a steady state is reached at low levels. The main features of the linear solution are seen in the numerical results, but with some notable differences: (i) the primary jet in the numerical simulation requires a longer distance to attain a one-dimensional boundary-layer structure and extends further downslope off the strip before intruding into the environment; (ii) the numerically simulated outward environmental jet is narrower and more intense than the inward jet, and has a pronounced wave-like structure.  相似文献   

9.
A non-hydrostatic numerical model, the Regional Atmospheric Modeling System (RAMS), has been used to investigate the development of katabatic jumps in Coats Land, Antarctica. In the control run with a 5 m s-1downslope directed initial wind, a katabatic jump develops near the foot of the idealized slope. The jump is manifested as a rapid deceleration of the downslope flow and a change from supercritical to subcritical flow, in a hydraulic sense, i.e., the Froude number (Fr) of the flow changes from Fr > 1 to Fr> 1. Results from sensitivity experiments show that an increase in the upstream flow rate strengthens the jump, while an increase in the downstream inversion-layer depth results in a retreat of the jump. Hydraulic theory and Bernoulli's theorem have been used to explain the surface pressure change across the jump. It is found that hydraulic theory always underestimates the surface pressure change, while Bernoulli's theorem provides a satisfactory estimation. An analysis of the downs balance for the katabatic jump indicates that the important forces are those related to the pressure gradient, advection and, to a lesser extent, the turbulent momentum divergence. The development of katabatic jumps can be divided into two phases. In phase I, the t gradient force is nearly balanced by advection, while in phase II, the pressure gradient force is counterbalanced by turbulent momentum divergence. The upslope pressure gradient force associated with a pool of cold air over the ice shelf facilitates the formation of the katabatic jump.  相似文献   

10.
彭莉莉  邓剑波  谢傲 《湖北气象》2020,39(2):201-206
利用南岳山南坡不同海拔高度上的3个气象观测站2015年9月1日-2018年8月31日逐时降水资料,分析了南岳山降水日演变特征。结果表明:从山底到山顶总降水量逐渐增加,存在3个降水峰值时段,分别在清晨、午后和傍晚,清晨雨量峰值主要由该时段降水频次较高所致,午后与傍晚雨量峰值主要与该时段降水强度较大有关,山顶高山站与山底站降水量差异主要体现在午后与傍晚时段;小时最大降水量主要出现在午后至傍晚,山底站短时强降水出现时段较分散,山腰和山顶高山站短时强降水主要集中在午后至傍晚时段;持续时间小于等于6 h的短持续降水频次多于持续时间大于6 h长持续降水频次,其主要出现在午后至傍晚,长持续降水过程多出现在凌晨至中午,其对总降水量的贡献大于短持续降水。  相似文献   

11.
The 2018 Winter Olympic and Paralympic Games will be held in Pyeongchang, Korea, during February and March. We examined the near surface winds and wind gusts along the sloping surface at two outdoor venues in Pyeongchang during February and March using surface wind data. The outdoor venues are located in a complex, mountainous terrain, and hence the near-surface winds form intricate patterns due to the interplay between large-scale and locally forced winds. During February and March, the dominant wind at the ridge level is westerly; however, a significant wind direction change is observed along the sloping surface at the venues. The winds on the sloping surface are also influenced by thermal forcing,showing increased upslope flow during daytime. When neutral air flows over the hill, the windward and leeward flows show a significantly different behavior. A higher correlation of the wind speed between upper-and lower-level stations is shown in the windward region compared with the leeward region. The strong synoptic wind, small width of the ridge, and steep leeward ridge slope angle provide favorable conditions for flow separation at the leeward foot of the ridge. The gust factor increases with decreasing surface elevation and is larger during daytime than nighttime. A significantly large gust factor is also observed in the leeward region.  相似文献   

12.
Amplified Arctic warming is one of the key features of climate change. It is evident in observations as well as in climate model simulations. Usually referred to as Arctic amplification, it is generally recognized that the surface albedo feedback governs the response. However, a number of feedback mechanisms play a role in AA, of which those related to the prevalent near-surface inversion have received relatively little attention. Here we investigate the role of the near-surface thermal inversion, which is caused by radiative surface cooling in autumn and winter, on Arctic warming. We employ idealized climate change experiments using the climate model EC-Earth together with ERA-Interim reanalysis data to show that boundary-layer mixing governs the efficiency by which the surface warming signal is ‘diluted’ to higher levels. Reduced vertical mixing, as in the stably stratified inversion layer in Arctic winter, thus amplifies surface warming. Modelling results suggest that both shortwave—through the (seasonal) interaction with the sea ice feedback—and longwave feedbacks are affected by boundary-layer mixing, both in the Arctic and globally, with the effect on the shortwave feedback dominating. The amplifying effect will decrease, however, with climate warming because the surface inversion becomes progressively weaker. We estimate that the reduced Arctic inversion has slowed down global warming by about 5% over the past 2 decades, and we anticipate that it will continue to do so with ongoing Arctic warming.  相似文献   

13.
A 10-months long monitoring experiment to investigate the diurnal and seasonal variation of aerosol size distribution at Nagarkot (1,900 m asl) in the Kathmadu Valley was carried out as part of a study on katabatic and anabatic influence on pollution dispersion mechanisms. Seasonal means show total aerosol number concentration was highest during post-monsoon season (775 ± 417 cm?3) followed by pre-monsoon (644 ± 429 cm?3) and monsoon (293 ± 205 cm?3) periods. Fine particle concentration (0.25 μm ≤ DP ≤ 2.5 μm) dominated in all seasons, however, contribution by coarse particles (3.0 μm ≤ DP ≤ 10.0 μm) is more significant in the monsoon season with contributions from particles larger than 10.0 μm being negligible. Our results show a regular diurnal pattern of aerosol concentration in the valley with a morning and an evening peak. The daily twin peaks are attributed to calm conditions followed by transitional growth and break down of the valley boundary layer below. The peaks are generally associated with enhancement of the coarse particle fraction. The evening peak is generally higher than the morning peak, and is caused by fresh evening pollution from the valley associated with increased local activities coupled with recirculation of these trapped pollutants. Relatively clean air masses from neighbouring valleys contribute to the smaller morning peak. Gap flows through the western passes of the Kathmandu Valley, which sweep away the valley pollutants towards the eastern passes modulated by the mountain - valley wind system, are mainly responsible for the dominant pollutant circulation patterns exhibited within the valley.  相似文献   

14.
Summary Simulations of katabatic flow with a two-dimensional dynamic numerical model with a soil parameterization indicate that downslope flow developed over a moist slope is weaker than that over a dry slope. This agrees with earlier findings that daytime anabatic (upslope) flow is weaker over a moister slope. But, whereas the weaker anabatic flow is produced because surface evaporation prevents the moist slope from heating as much as a dry slope, the weaker katabatic flow is produced over moist slopes because (1) the soil thermal conductivity is greater in moist soil, and (2) downward longwave radiation flux from the atmosphere to the surface is greater because of higher humidity in the air near the surface from evaporation. The higher thermal conductivity allows warmer soil temperatures (heat) to diffuse upward to the soil surface and prevents the surface temperature from becoming as cold in the moist run as in the dry run.With 6 Figures  相似文献   

15.
We investigate a Cartesian-mesh immersed-boundary formulation within an incompressible flow solver to simulate laminar and turbulent katabatic slope flows. As a proof-of-concept study, we consider four different immersed-boundary reconstruction schemes for imposing a Neumann-type boundary condition on the buoyancy field. Prandtl’s laminar solution is used to demonstrate the second-order accuracy of the numerical solutions globally. Direct numerical simulation of a turbulent katabatic flow is then performed to investigate the applicability of the proposed schemes in the turbulent regime by analyzing both first- and second-order statistics of turbulence. First-order statistics show that turbulent katabatic flow simulations are noticeably sensitive to the specifics of the immersed-boundary formulation. We find that reconstruction schemes that work well in the laminar regime may not perform as well when applied to a turbulent regime. Our proposed immersed-boundary reconstruction scheme agrees closely with the terrain-fitted reference solutions in both flow regimes.  相似文献   

16.
The present and twenty-first century near-surface wind climate of Greenland is presented using output from the regional atmospheric climate model RACMO2. The modelled wind variability and wind distribution compare favourably to observations from three automatic weather stations in the ablation zone of southwest Greenland. The Weibull shape parameter is used to classify the wind climate. High values (κ > 4) are found in northern Greenland, indicative of uniform winds and a dominant katabatic forcing, while lower values (κ < 3) are found over the ocean and southern Greenland, where the synoptic forcing dominates. Very high values of the shape parameter are found over concave topography where confluence strengthens the katabatic circulation, while very low values are found in a narrow band along the coast due to barrier winds. To simulate the future (2081–2098) wind climate RACMO2 was forced with the HadGEM2-ES general circulation model using a scenario of mid-range radiative forcing of +4.5 W m?2 by 2100. For the future simulated climate, the near-surface potential temperature deficit reduces in all seasons in regions where the surface temperature is below the freezing point, indicating a reduction in strength of the near-surface temperature inversion layer. This leads to a wind speed reduction over the central ice sheet where katabatic forcing dominates, and a wind speed increase over steep coastal topography due to counteracting effects of thermal and katabatic forcing. Thermally forced winds over the seasonally sea ice covered region of the Greenland Sea are reduced by up to 2.5 m s?1.  相似文献   

17.
Local Winds In A Valley City   总被引：1，自引：0，他引：1  
Local winds were studied around a valley city, by using a high resolution two-dimensional mesoscale model forced by surface temperatures from a measurement campaign around Lanzhou City, China, during stagnant conditions. In the simulations nighttime winds are purely katabatic downslope winds without urban effects, despite the fact that the city is 6–7°C warmer than its surroundings all night. In contrast, daytime near-surface winds result from upslope flow resisted by an opposing simultaneous urban heat-island circulation (UHIC). Hence winds remain weak and variable around a city in a narrow valley during daytime. These conditions may lead to severe air quality problems day and night.The local circulations are sensitive to the widths of the valley and/or city,and also latitude, as is demonstrated by model experiments. Interestingly, in a flat and calm environment an extratropical daytime UHIC cell may turn into a weak `anti-UHIC' by the morning, due to frictional decoupling after sunset and subsequent inertial oscillation during the night, analogously tothe land breeze and nocturnal low-level jet formation.  相似文献   

18.
Katabatic flow is a dynamical process occurring on relatively calm, clear nights above sloping terrain. Its existence is dependent on long-wave radiative transfer, particularly radiative flux divergence within the air itself, for both its generation and (it is concluded here), along with advective warming, for much of its retardation.Utilising sounding data closely spaced in time, a discussion is presented of the importance of surface shear, interfacial shear, advective warming and radiative divergence in a strong katabatic flow. It is concluded that radiative divergence is important in generating static and dynamic instabilities in the flow. The role of radiative cooling in mixing of momentum has largely been ignored so far, and might explain why higher-order models tend to overestimate katabatic speeds on smooth slopes.  相似文献   

19.
The results of an observational and modeling study of the nocturnal slope winds in a simple valley are presented. The valley was approximately 225 m deep in the region of the measurements, and featured a uniform slope angle of approximately 23 ° on one of its sidewalls. The wind and temperature structure of the katabatic flows on the valley sidewalls were measured with tower-mounted instruments, and a Doppler sodar and instruments on a tethered balloon and a 61-m tower were used to determine the atmospheric conditions near the center of the valley. The temperature structure of the slope flows was summarized by characteristic scale parameters h and T for the inversion depth and strength, respectively. On the sidewalls 50 m above the valley floor, the inversion depths were generally smaller and the inversion strengths were weaker than they were on the sidewalls 100 m higher. These results differ significantly from those obtained over a simple slope of an isolated mountain or ridge. The down-valley winds are shown to be important in limiting the strength of the sidewall inversions. The formation of an inversion in the valley also has a pronounced effect on the structure of the slope flows. Numerical simulations suggest that the presence of adiabatic layers in the valley atmosphere is associated with decreases in the slope-flow inversion depth with increasing downslope distance. The simulations also indicate that the length scales that characterize the momentum and inversion depths behave similarly in flows down simple slopes but not in flows down the sidewalls of a valley.Work supported by the U.S. Army Research Office under Contract DA-AG29-K-0231 and the U.S. Department of Energy under Contract DE-AC06-76RLO 1830.  相似文献   

20.
作者的目的是分析地形对由于热力差异所引起的局地锋生现象和地转适应过程的影响．为此，分别用零位涡流和均匀位涡流来近似代替实际大气，并借助位涡、绝对动量、位温的守恒关系，就地形对适应锋生和适应过程中能量转换等问题进行了简单讨论。初始位温扰动的水平分布及其相对于地形的位置对适应锋生有重要影响。当初始热力扰动主要位于地形迎风坡山脚时，地形不利于锋面形成，这时要形成锋面不连续，初始位温梯度的水平变化必须非常显著；当初始热力扰动主要位于山顶附近时，地形促进流体运动的辐合，有利于锋面不连续的形成，这时即使初始位温梯度的水平变化不是很大，也可能会形成锋面；当初始热力扰动主要位于背风坡时，地形是否有利于锋面不连续的形成取决于流体的层结情况，如果流体的基本层结很弱，地形促进锋面不连续的形成，反之，地形不利于锋面不连续的形成，这种差异主要是由于在这两种情况下，地形引起的下沉增温效应的强弱是不同的。另外，地形对地转适应过程中动能和位能之间的能量转换率也有影响，与没有地形的情况相比，当初始热力差异显著区位于迎风坡时，能量转换率减小，当不平衡场位于背风坡时，能量转换率增加，原因是在迎风坡流体要克服重力作功，在背风坡重力对流体作正功。  相似文献   

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