A numerical simulation of boundary-layer flows near shelterbelts

We have developed a shelterbelt boundary-layer numerical model to study the patterns and dynamic processes relating to flow interaction with shelterbelts. The model simulates characteristics of all three zones of airflow passing over and through shelterbelts: the windward windspeed-reduction zone, the overspeeding zone above the shelterbelt, and the leeward windspeed-reduction zone. Locations of the maximum windspeed reduction and recirculation zone, as well as the leeward windspeed-recovery rate are well simulated by the model. Where comparisons with field measurements and wind-tunnel experiments were possible, the model demonstrated good performance for flows over and through shelters ranging from almost completely open to almost solid. The dynamic pressure resulting from the convergence and divergence of the flow field alters the perturbation pressure field. The disturbed pressure controls not only the formation of the separated flow but also the location of maximum windspeed reduction, streamline curvature, speed-up over the shelterbelt, and leeward windspeed recovery rate. The interaction of pressure with the flow produces complex flow patterns, the characteristics of which are determined, to a great extent, by shelterbelt structure.     

Evidence of long lee waves in Southern Alberta

A brief review of past studies of the of the order of 50—100 km or longer, chinook and a summary of some recent although resonance may be present satellite and visual observations of the in some cases. Both satellite and visual chinook arch cloud are presented as observations indicate the frequent ocevidence of the occurence of long currence of the wave during the colder gravity wave phenomena in the lee of period of the year. Taken together, the Rockies of Southern Alberta and these data imply that the wave pheno‐Northern Montana. A quasi‐stationary menon is potentially important in cloud band that extends several de‐ terms of local surface weather and grees of latitude along the lee of the vertical momentum transport and that mountains appears to be associated it should be subjected to more detailed with a single long wave with a length theoretical and observational studies.     

Variations of early autumn rainfall in the lee side of the Tibetan Plateau

The variability of climate in the lee side of the Tibetan Plateau (TP) often exhibits unique features. In this study, the authors investigate the variations of early autumn rainfall in the lee side of the TP (LSTP). The rainfall amount and number of rain days in LSTP increase from August to September. The center of heavy and torrential rains during September is located in LSTP as well. These unique features are attributed to later withdrawal of the East Asian summer monsoon in the middle and higher troposphere compared to the lower troposphere and stronger South Asian summer monsoon and thermal forcing of the TP. The September rainfall in LSTP experienced strong interdecadal fluctuation, with overall below normal rainfall from mid-1980s to late 2000s, while there was no apparent long-term trend in the August and October rainfall amount. The above normal September rainfall is associated with an anomalous anticyclone over the western North Pacific, a weaker-than-normal ridge around the Lake Baikal, a stronger-than-normal East Asian jet stream, and warmer air over the southeastern TP. The interdecadal decrease in rainfall in September may be caused by the central equatorial Pacific warming, which induces an anomalous cyclone over the western North Pacific. The September anomalous cyclone is stronger and more westward compared to that in August and October, and thus favors the below normal rainfall in LSTP.     

Waves to the lee of a meso-scale elliptic orographic barrier

Summary ?The paper considers a meso-scale, adiabatic, inviscid and Boussisnesq flow of a stably stratified fluid over a three-dimensional (3-D) meso-scale orographic barrier with elliptic contour, with special reference to a part of the Western ghats mountain along west coast of India and on the Khasi-Jayantia hill in the northeast India. The airstream characteristics are simplified by assuming that the upstream wind velocity (U) and buoyancy frequency (N) are constant with height. Solutions for perturbation vertical velocity (w′) and streamline displacement (η′) are expressed in terms of double integrals. These integrals cannot be evaluated exactly, hence they have been approximated by asymptotic expansion method. Side by side solutions using numerical method have also been obtained. The results of the study indicate that the updraft regions in the asymptotic solution are crescent shaped, symmetrical about the axis y = 0, tilting upwind and spreading laterally with height. The study also shows that in both asymptotic solution and numerical solution w′ and η′ fall off down wind of the barrier in the central plane (y = 0), further more in the asymptotic solution w′ and η′ fall off as x ⁻¹. The study also indicates that the discrete updraft regions obtained in the numerical solution, when joined, take a crescent shape. Received November 26, 2001; accepted February 27, 2002     

A study of the environmental influence on the amplitude of lee waves

A three-layer theoretical model is used to calculate the lee wave of a real example occurring over Blue Ridge in Pittsburgh, in which the maximum vertical velocity is 0.11 m s^-1. Based on this, the influence of changes in the thickness and values of the Scorer parameter in each layer are analyzed. It is shown that the influence of each layer parameters on the lee-wave amplitude is different, and the amplitude is more sensitive to the changes in the lower layer. Since the environment changes can affect the Scorer parameter profile, the influence of the environment on the amplitude is studied. The results show that the amplitude will decrease in the daytime because of solar heating, and increase at night because of radiational cooling, according to observational data. The case is also simulated by the Advanced Regional Prediction System （ARPS） model. The simulated amplitude is 0.089 m s^-1, which is close to the calculated result. Numerical sensitivity experiments are performed to test the former calculated experiments. The simulated results are consistent with the analytically calculated results.     

三层模式中风场对背风波的影响

为了研究风场对背风波的影响,针对边界层附近为弱稳定层结的背风波,建立了一个三维三层的理论模型和线性计算模式,分析了各层中风速和风向的变化对背风波特征的影响,揭示了气流过孤立山脉产生背风波的有利风场条件。结果表明：背风波的波长、振幅等特征对各层风速和风向的变化具有相当的敏感性,波长随着低、高层风速的增大而增大,随着中层风速的增大先减小后增大;振幅随着低、中层风速的增大先增大后减小,随着高层风速的增大而增大。此外,风速和上下层风向切变的增大均使背风波的形态逐渐由横波型转为辐散型,但是上下层风向的切变对背风波形态的影响比风速更为显著。     

Diagnosis of mesoscale structures in cases of lee cyclogenesis during ALPEX

Summary Several cases of lee cyclogenesis that occurred during ALPEX-SOP have been analyzed, with the aim of separating the large scale structures from the subsynoptic/meso-a scale features that are characteristic of this meteorological phenomenon. The results presented here are mainly based on composites of the analyzed cases. We assume that the deformation caused by the orography can be, at least to a reasonable extent, isolated from the undisturbed state using scale separation. The analysis technique we employ provides the scale separation as built-in in the interpolation algorithm. The scale separation error due to the large inhomogeneities of the data density distribution is partly corrected using a method described in the text. The orographic disturbance appears in different mesoscale fields as a quasiantisymmetric dipolar structure. For example, high/low pressure, cold/warm temperature and anticyclonic/cyclonic couplets characterize the mesoscale fields near the Alps. A qualitative agreement is found with the structure of the orographically induced perturbations predicted by the normal mode theory of lee cyclogenesis.With 7 Figures     

Numerical case study of the Altai-Sayan lee cyclogenesis over East Asia

Summary Climatological studies show that the Altai-Sayan lee side is one of the major cyclogenesis areas in the Northern Hemisphere. In case of the Altai-Sayan lee cyclogenesis, the surface cyclone is generated when a primary cyclone is swept north of the mountains. In the mid-troposphere, a trough develops and finally turns into a cutoff low within 48 h. The main synoptic features are similar to those of Alpine cyclogenesis. Numerical simulations are performed to assess the effect of different representation of orography on the Altai-Sayan cyclogenesis. Two experiments are performed, a step-mountain (ETA) and an envelope orography (SGM) experiment. The ETA experiment produced the cyclogenesis in a way similar to that in the analysis both at the surface and at mid-troposphere. The SGM experiment failed in the simulation of the upper cutoff low. The difference in predicted pressure between the ETA and the SGM experiment shows a dipolar structure suggesting that the blocking effect of the mountains is essential in the development of the Altai-Sayan lee cyclogenesis.With 7 Figures     

Normal mode theory of lee cyclogenesis and the initial value problem

Summary An initial value problem is proposed to study the influence of mountains upon atmospheric flows. The normal mode, quasigeostrophic theory of lee cyclogenesis is revisited; in order to model Alpine cyclogenesis, the flow, upon which the mountain acts, should represent a cyclone wave propagating eastward in a sheared zonal current. Two different models are considered in order to describe cyclone waves: an unstable Eady normal mode and a finite amplitude, neutral Rossby mode in the two layer model. It is shown that the topographic modifications proper of normal modes emerge very rapidly from the integration of the initial value problem. It is also shown that an enhanced deepening of the upper level trough can occur when finite amplitude effects are taken into account.With 8 Figures     

Observations of a lee wave and rotor episode in central California

Acoustic sounders and in-situ meteorological sensors were used to record the boundary-layer flow structure along the eastern slopes of the Altamont Hills in central California during a wind energy field program. Oscillating speeds in this area suggest the development of quasi-stationary lee waves and rotors under certain atmospheric conditions. Data averaging of 15 minutes or less is needed to study these phenomena.     

水汽和潜热释放对背风波影响的数值试验

利用中尺度数值模式ARPS进行了理想场的数值模拟,分析研究了水汽和潜热释放对大气层结稳定度的影响以及其在背风波的发展和演变过程中的作用,研究发现,潜热释放对大气层结分布的影响要远大于水汽对大气层结分布的直接影响,如果没有潜热的释放,水汽对背风波的发展和演变的作用非常小,而潜热释放可以使湿层结稳定度急剧下降,迅速破坏原有的层结分布,使这个区域出现非拦截的强烈的垂直运动,波动的崩溃更加迅速和明显。但需要说明的是在试验中,将数值模式里控制潜热释放的参数设为:0、1/2、2的假定情况,则在实际的大气运动过程中是不可能存在的。     

Atmospheric lee waves in the Aegean Sea and their possible influence on the sea surface

Unusual satellite images of the Aegean Sea, in both the visible and infrared ranges, are discussed. Alternating bright and dark features downwind of islands suggest the presence of atmospheric lee waves. However, close examination of these features indicates that the observed signal is likely the signature of the influence of the lee waves on the sea surface rather than the signature of the lee waves themselves through atmospheric effects.     

两方程湍流模型对内背风波数值模拟的影响

为了深入理解非静力近似下的波-湍相互作用问题,本研究在σ坐标的海洋环境研究和预报模型(MERF)中引入常用的Mellor-Yamada两方程湍混合参数化方案(MY2.5),评估垂向湍混合对小尺度背风波传播过程的影响.瞬时状态场的模拟结果表明,无论是否为非静力近似条件,上述湍参数化方案的引入都会减弱背风波传播的模拟效果.从时间平均场的试验结果来看,垂向湍混合过程会显著减小非静力近似和静力近似之间的差异.此外,能量收支分析的诊断结果表明,MY2.5方案会显著抑制陆坡地形下的背风波传播过程,进而将更多的潮能转化到不可逆的湍混合过程中.     

Water exchange in fjords induced by tidally generated internal lee waves

Tidal processes are examined that control the water exchange between two basins of the Trondheimsfjord through a narrow channel with sills. For this purpose, a non-hydrostatic numerical model based on the laterally averaged Reynolds equations in the Boussinesq approximation was developed. The model takes into account the real vertical fluid stratification, variable bottom topography and variable cross-section of the fjord. Numerical experiments were performed to investigate tidally generated internal waves and their influence on the water exchange.The model produces both baroclinic tides and tidally generated lee waves. It was found that, for the Skarnsund strait which connects the Middle Fjord and the Beitstadfjord, the internal tides generated over the Skarnsund sills are very weak. Their amplitudes do not exceed 1 m.The intense short internal waves, which are identified as unsteady lee waves, comprise the basic input of the total internal wave field. These waves are generated by tidal currents at sill breaks, are trapped by topography in the generation area and grow by continuing feedback into large-amplitude waves. As the tidal flow slackens, they move upstream as freely propagating waves.As essentially nonlinear responses, the lee waves cause a nonlinear water transport. The detailed analysis of the residual currents produced by unsteady lee waves (which are propagating in both directions from the Scarnsund sills) has shown, in particular, that the residual currents can reach values as high as 0.27 m s⁻¹.It was also found that such currents exert a considerable effect on the water exchange through the Skarnsund strait between the adjacent basins. This mechanism can play an important role in water renewal and formation of the Beitasdfjord waters.     

过山波动对积云生成启动机制作用的数值试验

本文用数值试验方法,讨论了对于较小尺度山脉过山波和背风坡积云形成的关系.证明了在中层大气层结稳定、水平风速较强的条件下,过山波可能成为背风坡积云形成的启动机制.