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1.
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
2.
Eric D. Skyllingstad 《Boundary-Layer Meteorology》2003,106(2):217-243
A large-eddy simulation model with rotated coordinates and an open boundary is used to simulate the characteristics of katabatic flows over simple terrain. Experiments examine the effects of cross winds on the development of the slope-flow boundary layer for a steep (20°) slope and the role of drainage winds in preventing turbulence collapse on a gentle slope (1°). For the steep flow cases, comparisons between model average boundary-layer velocity, temperature deficit, and turbulence kinetic energy budget terms and tower observations show reasonable agreement. Results for different cross slope winds show that as the cross slope winds increase, the slope flow deepens faster and behaves more like a weakly stratified, sheared boundary layer. Analysis of the momentum budget shows that near the surface the flow is maintained by a balance between downslope buoyancy forcing and vertical turbulence flux from surface drag. Above the downslope jet, the turbulence vertical momentum flux reverses sign and acceleration of the flow by buoyancy is controlled by horizontal advection of slower moving ambient air. The turbulence budget is dominated by a balance between shear production and eddy dissipation, however, buoyancy and pressure transport both are significant in reducing the strength of turbulence above the jet. Results from the gentle slope case show that even a slight terrain variation can lead to significant drainage winds. Comparison of the gentle slope case with a flat terrain simulation indicates that drainage winds can effectively prevent the formation of very stable boundary layers, at least near the top of sloping terrain. 相似文献
3.
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
4.
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
5.
G. T. Amanatidis K. H. Papadopoulos J. G. Bartzis C. G. Helmis 《Boundary-Layer Meteorology》1992,58(1-2):117-132
The identification of katabatic flows and their characteristics observed on a simple slope (the western side of Hymettos mountain) for a six-month period (January to June, 1990) are presented. This is the first application of data obtained from an 84 m high meteorological research tower recently erected at the National Center for Scientific Research Demokritos in Athens, Greece. The tower is described with respect to construction and instrumentation. The topography of the region and the observational site are also described. Criteria for the identification of katabatic flows are presented, while the frequency of occurrence of such flows is estimated. Finally, two case studies are analysed in order to reveal the special characteristics of the katabatic flows. 相似文献
6.
A numerical investigation of mechanisms affecting drainage flows in highly Complex Terrain 总被引：1，自引：0，他引：1
Summary Numerical simulations of increasing complexity are conducted to investigate topographic controls and ambient wind effects upon drainage flows along a portion of the Colorado Front Range in the central Rocky Mountains. A series of two-dimensional simulations show the effects upon the drainage flow of changing slope gradient at the mountain-plain interface. For a given mountain slope, a decrease in the slope of the plain decelerates the mountain drainage jet as it approaches the plain and causes the jet to elevate. The integrated effects of slope and valley drainage are presented with particle trajectories for a particular drainage basin along the Front Range. A nested grid simulation of drainage flow from multiple basins along the Front Range shows that basin area is an important factor in the strength of the drainage flow and that canyon topography variations greatly affect the behavior of the drainage jet as it flows through the canyon mouth onto the plain. Strong drainage winds developed on each of four case night simulations due to the presence of only weak ambient wind below mountaintop. The weak winds represent a decoupling of the near-surface from stronger winds above mountaintop. The canyon drainage exhibited substantial temporal variability in wind speed with the inclusion of ambient winds, due to interactions between ambient and drainage winds.With 11 Figures 相似文献
7.
Evening and Morning Transition of Katabatic Flows 总被引：1，自引：1，他引：0
An experimental investigation of the evening and morning transition phases of katabatic slope flows has been conducted to identify the mechanisms for their development and destruction over an isolated slope. The momentum and energy equations of the flow have been used to describe these mechanisms for the particular topographic features of the studied slope, and to outline the differences from the dynamics of well-developed simple slope flows. In the lowest portion of the slope, frontal characteristics have been identified in early evening periods when the local pre-existing near-surface thermal structure does not impose a katabatic acceleration. The frontal shape is determined by the near-surface thermal stability and ambient wind. The flow initiation is distinctly different when it is linked to the local surface cooling, in which case it develops gradually and produces a slight local warming.The erosion of the katabatic layer at mid-slope precedes that at the foot and is closely linked to dilution of the local surface inversion. The flow erosion at the foot is often delayed, as the warming of air proceeds uniformly at all heights near the ground, so maintaining the inversion due to warming produced by mixing and advective processes linked to the upslope flow development. The latter initiates first at mid-slope and then at the foot, where for a non-negligible time period it flows over the persistent katabatic flow. The prerequisite for the development of this structure is the maintenance of a shallow inversion in the first 2–3 m above the ground surface.The morning dilution of the katabatic flow is apparently different from common experience over simple slopes and may be attributed to the steep upper portion of the slope in association with its easterly orientation, which results in strong non-uniformity of the solar heating along the slope. 相似文献
8.
Summary The Adélie Land coastal section of East Antarctica is known for strong katabatic winds. Although the primary forcing of these persistent drainage flows has been attributed to the radiative cooling of the sloping ice topography, effects of ambient horizontal pressure gradients can play a central role in shaping the Antarctic surface wind regime as well. Oberrvations of the katabatic wind at the near-coastal Adélie Land station D-10 have been sorted into strong and weak wind classes. Concurrent radiosonde ascents at nearby Dumont D'Urville have been used to depict the timeaveraged large scale conditions accompanying the katabatic wind classes. Results suggest that strong katabatic wind cases are associated with low pressure over the coastal margin and easterly upper level motions. Numerical simulations have been conducted to examine the effect of prescribed large scale forcing on the evolution of the katabatic wind. The model runs indicate that the ambient environment plays a key role in the development and intensity of the katabatic wind regime.With 7 Figures 相似文献
9.
The integral model for katabatic flows proposed by Manins and Sawford (1979) is solved numerically. It is shown that numerical solutions can be approximated by Ball's (1956) model in the upper part of the slope, while they tend toward Manins and Sawford's simplified solution farther downslope. The importance of entrainment and ambiant stable stratification is shown. Some limitations of Manins and Sawford's model are discussed. 相似文献
10.
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
11.
Direct Numerical Simulation of Turbulent Katabatic Slope Flows with an Immersed-Boundary Method 总被引：1，自引：1，他引：0
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
12.
Spectral Maxima In A Perturbed Stable Boundary Layer 总被引：4，自引：1，他引：4
Wind velocity data have been collected on Nansen Ice Sheet, Antarctica, close to the base of a steeply sloping glacier along which frequently flow katabatic winds. The aim of this study is to investigate how turbulent energy and momentum flux are perturbed by the flow interaction with topography and by the strong mechanical mixing produced by downslope flows. Spectral and cospectral analyses, performed on the wind velocity components, provide evidence that such a perturbation, at any stability, is restricted to frequencies lower than the inertial subrange. Longitudinal spectra display an energy increment, due to turbulence generated by topography and by mechanical forcing related to the katabatic wind structure. The energy, supplied by the topographic forcing, displaces the turbulent energy maximum toward lower frequencies. In near-neutral stratification the spectral maximum occurs at a reduced frequency, which seems to be consistent with the height of the steepest part of the slope, and seems to shift toward higher frequencies as a linear ,function of the local stability parameter,Ll. The parameterisation of the orographic perturbation by means of a similarity relationship allows us to scale u spectra in the same way as over uniform terrain. The scaled, perturbed spectra collapse onto a unique curve in the mid-frequency as well in the inertial subrange, while maxima are grouped in a cluster. Lateral and vertical velocity spectra exhibit shapes independent of stability, suggesting a topographic perturbation that is predominantly over stability effects. 相似文献
13.
H. A. Flocas C. G. Helmis S. N. Blikas D. N. Asimakopoulos J. G. Bartzis D. G. Deligiorgi 《Theoretical and Applied Climatology》1998,59(3-4):237-249
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
14.
K. Shankar Rao 《Boundary-Layer Meteorology》1999,90(3):541-549
Lagrangian stochastic models are well-suited for modeling dispersion in the stable boundary layer, especially in complex terrain. This note briefly describes the formulations and application of a Lagrangian stochastic model to predict dispersion of tracers released within nocturnal drainage flows. 相似文献
15.
由于坡度、坡向和地形之间相互遮蔽等局地地形因子的影响,确定实际复杂地形下太阳散射辐射是比较困难的.本文在前人研究的基础上,对以前的模型进行了一些改进,考虑了坡度、坡向和地形相互遮蔽作用对复杂地形下天文辐射的影响,基于数字高程模型(DEM)数据,研制了以复杂地形下天文辐射为起始数据的复杂地形下太阳散射辐射的分布式模型,在模型中还考虑了散射辐射的各向异性.以地形复杂的贵州高原为例,应用100 m×100 m分辨率的DEM数据及气象站常规观测气象资料,计算了贵州高原复杂地形下各月及年的太阳散射辐射精细空间分布.结果表明:(1)局地地形因子(如坡度、坡向和地形遮蔽)对贵州高原复杂地形下太阳散射辐射的空间分布影响较大,随着地形的起伏变化,太阳散射辐射的空间分布明显不同,纬向分布特征不明显.(2)对于太阳散射辐射而言,地形对其的影响仍然很大,在太阳散射辐射计算时也是不容忽视的. 相似文献
16.
W. D. Neff 《Boundary-Layer Meteorology》1988,42(3):207-228
We examine methods for the interpretation of sodar facsimile records obtained in the study of complex terrain flows. Acoustic scattering theory is presented first and then interpreted using a simpolified second-order turbulence closure scheme. The use of this theory suggests the strong sensitivity of acoustic scatter to changes in the wind shear. With this introduction, detailed sodar facsimile records, temperature and wind profiles, and model calculations follow. Characteristic scattering patterns are described for simple drainage jets, complex basin flows, convection with a capping inversion, stratus, and dynamical instabilities. Examples are also shown of bistatic facsimile records detailing the strong temporal and spatial variability in small-scale turbulence. 相似文献
17.
基于数字地形模型的山区太阳辐射的时空分布模拟 总被引：2，自引：2，他引：0
在数字地形模型(DTM)的基础上,利用地理信息系统软件ArcGIS确定阴影、提供的地图代数语言功能,模拟了甘肃定西安家沟小流域任意时段内天文辐射的空间分布。该模型借助于ArcGIS的地形分析功能,解决了常规方法不能解决的地形遮蔽对天文辐射的影响。该模型是一个物理模型,对天文辐射能的时空分布可做出较精确描述,提供在常规条件下的重要参数。时空分布分析表明:地形对天文辐射的影响很大,尤其是坡向的影响;天文辐射随着季节变化很大,从3月底开始直到6月上旬一直处于上升阶段,然后下降;地形对天文辐射的影响程度随着季节不同有所不同,但是没有表现出明显的规律。 相似文献
18.
起伏地形下浙江省散射辐射时空分异规律模拟 总被引：2，自引：0，他引：2
结合影响起伏地形下太阳散射辐射的天空因素与地面因素,通过基于数字高程模型（DEM）数据的起伏地形下天文辐射模型和地形开阔度模型,综合考虑地面因素对散射辐射的影响;基于常规地面气象站观测资料建立的水平面散射辐射模型,考虑天空因素对散射辐射的影响;建立了起伏地形下浙江省散射辐射分布式估算模型;逐月计算了浙江省散射辐射（100m×100m）的空间分布。结果表明：散射分量分布与地理地形因子、季风影响、大气透明程度有关,由高纬向低纬逐渐增加;季节分布特点为,夏季〉春季〉秋季〉冬季;坡度、坡向对散射辐射的分布影响小,但辐射值与开阔度呈正相关,各季辐射最大值分布在开阔度大处,最小值在开阔度最小处,不同季节有所伸缩。计算结果可以为气候变化和环境资源研究提供基础数据。 相似文献
19.
使用贵州省1961—2004年84个气象台站44年历史冰雹记录及1:1000000全国数字高程模型(DEM)资料, 采用基于GIS的数字地形分析、分区统计和图像分类方法, 研究了冰雹分布与地形高程、坡向、坡度及地形切割深度的关系。研究表明:地形高程是影响贵州省降雹分布的最主要地形影响因子; 微观地形因子如坡向和坡度对降雹日数的变异并没有显著性影响, 但大范围的地势抬升及暖湿空气的迎风坡有利于降雹; 地形切割深度并不是年平均降雹日数差异的显著影响因子; 纬度位置的不同, 使其受暖湿空气影响程度不同, 热力条件也存在差异, 也是影响平均降雹日数差异的因子之一; 根据3个影响因子建模获得的方程及贵州省冰雹风险分区图, 经统计检验和与历史乡镇降雹资料比较, 具有较好的一致性。 相似文献
20.
以起伏地形下天文辐射的分布模型为基础,借助地理信息系统(GIS)处理数据,将深圳市1∶250,000DEM数据作为地形的综合反映,模拟计算了起伏地形下(坡度、坡向和地形遮蔽)深圳市天文辐射,分析了起伏地形下深圳市天文辐射的分布规律,完成了深圳市100m×100m分辨率的各月及全年的天文辐射的空间制图。结果表明:对于局部地形起伏引起的天文辐射的变化,秋、冬季最为显著,向阳坡和背阴坡的极值差异较大,这和太阳高度角随着季节变化而冬半年相对较低、夏半年相对较高有关。坡度对天文辐射的影响在冬半年较大,随着坡度的增大,辐射差值增大的幅度呈递减趋势。 相似文献