太行山东麓一次强对流降雹过程中的地形强迫

利用WRF模式对2011年5月26日发生在太行山东麓的一次强对流降雹过程进行数值模拟,探讨了太行山及周边地形在本次强对流过程的作用。结果表明,控制华北平原的偏东暖湿气流受太行山阻挡并与切变线东南侧的西南暖湿气流汇合,在太行山东侧形成水汽高值区。太行山东坡下垫面向上热通量明显高于华北平原,午后850hPa高度山坡与平原的假相当位温梯度达到0.2 K·km~(-1),850~600 hPa假相当位温垂直梯度达4 K·km~(-1),对应上坡风的垂直速度大于1 m·s~(-1),热力环流为太行山东麓对流的发生提供了动力条件。太行山东侧暖湿气层之上为偏西干冷气流,由此形成的强热力不稳定与水汽高值区、上坡风共同造成太行山东麓强对流过程的发生。局地小尺度地形抬升与重力波共同促使太原盆地有对流单体生成,该单体移经太行山西侧迎风坡受阻挡抬升而增强,越过山顶后与维持在太行山东侧的对流单体发生合并,从而导致对流云的强烈发展。     

The effects of boreal forest expansion on the summer Arctic frontal zone

Over the last 100?years, Arctic warming has resulted in a longer growing season in boreal and tundra ecosystems. This has contributed to a slow northward expansion of the boreal forest and a decrease in the surface albedo. Corresponding changes to the surface and atmospheric energy budgets have contributed to a broad region of warming over areas of boreal forest expansion. In addition, mesoscale and synoptic scale patterns have changed as a result of the excess energy at and near the surface. Previous studies have identified a relationship between the positioning of the boreal forest-tundra ecotone and the Arctic frontal zone in summer. This study examines the climate response to hypothetical boreal forest expansion and its influence on the summer Arctic frontal zone. Using the Weather Research and Forecasting model over the Northern Hemisphere, an experiment was performed to evaluate the atmospheric response to expansion of evergreen and deciduous boreal needleleaf forests into open shrubland along the northern boundary of the existing forest. Results show that the lower surface albedo with forest expansion leads to a local increase in net radiation and an average hemispheric warming of 0.6°C at and near the surface during June with some locations warming by 1–2°C. This warming contributes to changes in the meridional temperature gradient that enhances the Arctic frontal zone and strengthens the summertime jet. This experiment suggests that continued Northern Hemisphere high-latitude warming and boreal forest expansion might contribute to additional climate changes during the summer.     

热带大气中的地形强迫运动

本文研究了在基本气流无切变和有切变的情况下地形对低伟大气大尺度定常运动的影响。从赤道β平面的线性化正压原始方程出发,求得一个解析形式的地形影响函数。对它的分析表明:大气的受迫运动不仅取决于越过山脉的基本气流的方向,而且还与山脉所处的纬度以及山脉的尺度有关。当适度的西风越过山脉时,会在背风侧形成地形槽。在线性模式中,基本气流的东西风切变对地形强迫波的经向伸展范围起限制作用。利用本文的模式,对东南亚近赤道地区的气压场和流场进行模拟,其结果与观测事实定性一致。最后,本文还讨论了越赤道气流通道的形成机制。      

Parameterisation of orographic cloud dynamics in a GCM

A new parameterisation is described that predicts the temperature perturbations due to sub-grid scale orographic gravity waves in the atmosphere of the 19 level HadAM3 version of the United Kingdom Met Office Unified Model. The explicit calculation of the wave phase allows the sign of the temperature perturbation to be predicted. The scheme is used to create orographic clouds, including cirrus, that were previously absent in model simulations. A novel approach to the validation of this parameterisation makes use of both satellite observations of a case study, and a simulation in which the Unified Model is nudged towards ERA-40 assimilated winds, temperatures and humidities. It is demonstrated that this approach offers a feasible way of introducing large scale orographic cirrus clouds into GCMs.     

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 subgrid parameterization of orographic precipitation

Summary Estimates of the impact of global climate change on land surface hydrology require climate information on spatial scales far smaller than those explicitly resolved by global climate models of today and the foreseeable future. To bridge the gap between what is required and what is resolved, we propose a subgrid-scale parameterization of the influence of topography on clouds, precipitation, and land surface hydrology. The parameterization represents subgrid variations in surface elevation in terms of probability distributions of discrete elevation classes. Separate cloud, radiative, and surface processes are calculated for each elevation class. Rainshadow effects are not treated by the parameterization; they have to be explicitly resolved by the host model. The simulated surface temperature, precipitation, and snow cover for each elevation class are distributed to different geographical locations according to the spatial distribution of surface elevation within each grid cell.The subgrid parameterization has been implemented in the Pacific Northwest Laboratory's climate version of the Penn State/NCAR Mesoscale Model. The scheme is evaluated by driving the regional climate model with observed lateral boundary conditions for the Pacific Northwest and comparing simulated fields with surface observations. The method yields more realistic spatial distributions of precipitation and snow cover in mountainous areas and is considerably more computationally efficient than achieving high resolution by the use of nesting in the regional climate model.With 17 Figures     

地形暴雨的多普勒天气雷达观测分析

为了进一步揭示地形对暴雨的影响,提高强降水的预报、监测和预警能力,针对三次落区于太行山东部迎风坡的暴雨天气,通过对多普勒天气雷达的观测分析发现:三次暴雨均为积层混合型絮状回波,受太行山地形的影响,无论回波移动方向如何,在偏东风作用下,较强回波均在太行山东侧迎风坡加强(或有新生)、合并、移速减慢,对应降水加强;三次过程径向速度场均为连续的大尺度风场特征,低层均有偏东风"牛眼"出现并伴有风速辐合,"牛眼"的形成、强弱、面积的大小和维持时间的长短以及风向的变化对三次暴雨的形成至关重要。低层偏东风"牛眼"结构急流的存在为太行山迎风坡暴雨形成的判据。     

Observations of atmospheric boundary layer evolution above the Gulf Stream frontal zone

Ship borne measurements of atmospheric boundary layer (ABL) parameters, sea-surface temperature and radar signals are analyzed to reveal the effects of the ABL transformation above the Gulf Stream temperature frontal zone. It was found that local changes in vertical gradients of wind speed and air temperature are well correlated with sub-mesoscale (~ 10 km) sea surface temperature variations. These effects are accompanied by appropriate variations in surface wind stresses that were identified from microwave backscatter.For steady atmospheric conditions the same effects were observed on spatial scales of 100 km, demonstrating positive radar signal contrast of the Gulf Stream warm waters with respect to surrounding Sargasso sea and shelf water areas. A simplified model of the ABL, accounting for an effect of spatial inhomogeneity by introducing an internal boundary layer, is used to analyze field observations. The model is able to reproduce both sub-mesoscale and mesoscale ABL evolution.     

秦岭地区气溶胶对地形云降水的抑制作用

以华山站为影响站, 周围的西安、渭南和华阴作为对比站, 通过影响站与对比站降水之比——地形强化因子(Ro)的变化趋势以及Ro与能见度关系的分析, 定量研究了秦岭地区气溶胶对地形云降水的抑制作用。Ro的演变分析表明: 有观测以来Ro逐年递减, 减幅为14%～20%, 即影响站与对比站相比降水量减少了14%～20%; Ro的减少趋势与能见度递减、气溶胶递增相吻合, 说明气溶胶的增加抑制了地形云降水。Ro的递减主要是减少了中小雨 (日雨量小于30 mm) 的天数, 这种影响对浅薄的生命期较短的地形云降水作用更明显, 对于华山站, 30 mm以下的降水都会受到入云气溶胶的抑制作用, 而西安站为5 mm以下, 入云气溶胶浓度越高, 就有越厚的降水云受气溶胶影响而抑制降水; 在以动力强迫抬升为主的春秋季, 气溶胶抑制华山地形云降水20%左右, 最大可达25%; 在热对流条件下, 气溶胶对地形云和对平原地区云的抑制作用基本相当。不同风速风向下Ro的变化趋势表明, Ro递减随风速增大而加快, 迎风向 (240°～30°) 大风 (≥5 m/s) 时减少降水超过30%。由Ro与能见度关系的定量分析发现, 当能见度在14 km时Ro为1.8左右, 随着能见度的降低Ro逐渐减小, 当能见度小于8 km时,R0约为1.2, 减小了30%左右; 华山对于华阴的Ro与能见度呈线性关系, 相关系数达0.81。最后, 根据研究结果归纳出气溶胶抑制秦岭地区地形云降水的初步物理模型。     

A unified theory of orographic influences upon cyclogenesis

Summary Cyclogenesis is known to take place frequently near the principal mountain complexes of the earth. However, a coherent and comprehensive theory of orographic cyclogenesis has never been offered in the literature. We propose here a unified theory of cyclogenesis in the presence of orography of various configurations, based on the generalization of a theoretical model concerning the interaction of baroclinic waves with local orography. This model has recently proved to be successful in accounting for some basic properties of Alpinecyclogenesis (Speranza et al. 1985). We consider, in particular, cyclogenesis in proximity of the Rocky Mountains (both to the west of the ridge, in the Gulf of Alaska, and to the east of it, over North America) and in proximity of the Himalaya-Tibetan Plateau.With regard to cyclogenesis over the Gulf of Alaska, results of numerical experiments, performed with the ECMWF model in order to isolate the orographic effects in realistic conditions, are also presented and compared with the theoretical results.

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Eine vereinheitlichte Theorie orographischer einflüsse auf die zyklogenese

Zusammenfassung Zyklogenese findet bekanntlich häufig in der Nähe der Hauptgebirgskomplexe der Erde statt. Dennoch wurde bis jetzt in der Literatur noch keine einheitliche und umfassende. Theorie orographischer Zyklogenese angeboten. Wir schlagen hier eine vereinheitlichte Theorie der Zyklogenese bei Vorhandensein einer Orographie mit unterschiedlichen Konfigurationen vor. Die Theorie basiert auf der Verallgemeinerung eines theoretischen Models über die Wechselwirkung von baroklinen Wellen mit der lokalen Orographie. Dieses Modell hat sich kürzlich bei der Erklärung einiger grundlegender Eigenschaften alpiner Zyklogenese als erfolgreich erwiesen (Speranza et al. 1985).Wir betrachten im besonderen die Zyklogenese in der Nähe der Rocky Mountains (sowohl westlich der Kette im Golf von Alaska als auch östlich davon über Nordamerika) und in der Nähe des Himalaya-Tibet-Plateaus.Hinsichtlich der Zyklogenese über dem Golf von Alaska werden auch Ergebnisse von numerischen Experimenten, die mit dem ECMWF-Modell durchgeführt wurden, um die orographischen Effekte unter realistischen Bedingungen zu isolieren, präsentiert und mit den theoretischen Ergebnissen verglichen.

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With 11 Figures     

A meso-scale three-dimensional dynamical model of orographic rainfall

Summary A mesoscale three-dimensional (3-D) dynamical model is presented to diagnose orographic rainfall, with particular reference to the Western Ghats (WG) in India. This model has two parts, namely, a dynamical part and a thermodynamical part. In the dynamical part the vertical velocity induced by a mesoscale elliptical orographic barrier has been computed using the perturbation technique. In the thermodynamical part rainfall intensity (RFI) has been computed using the computed vertical velocities, with the help of continuity of moisture and mass. The computed RFI has been compared with observed RFI as well as with that computed by 2-D model. The present study shows that during the southwest monsoon season (SWMS), orographic rainfall enhancement in the WG area appears to be solely due to the vertical shear of the basic flow and its variation with height. Stability appears to have very little influence on it. The spatial distribution of RFI across the barrier shows that there are four regions of maximum rainfall, one primary on the windward side behind the peak of the barrier and three secondary on the leeward side. The symmetry in the locations of these secondary rainfall maxima appears to be critically dependent on the component of basic flow parallel to the major ridge axis of the barrier.     

On frictional and orographic effects on precipitation in coastal areas

Methods have been designed to calculate orographic and frictional influences on precipitation in coastal regions. A model computes the vertical fluxes of water vapor at the top of the planetary boundary layer due to these two effects. The horizontal downwind displacement of falling precipitation is also allowed for. The procedures are applied over southwestern British Columbia to monthly amounts and maximum 24-h rainfalls for return periods of 2–50 yr. Correlation coefficients between observed and computed precipitation vary from 0.65 to 0.97, the average being 0.84.     

正压地形扰动的数值试验

本文用正压模式对一种水平差分格式在有复杂地形下的计算稳定性及动力性能作了多种检验,模拟了在不同结构的基流上扰动的传播,得到了较为理想的结果.     

Numerical simulations of orographic effects on NE Colorado snowstorms

Summary In this paper we show using a three-dimensional mesoscale model that terrain forcing has a major effect on the observed snowfall distribution under upslope conditions in northeast Colorado. Two upslope snowstorms (in 1982 and in 1985) were simulated. The effects are strongly dependent on the orientation of the prevailing wind with respect to terrain orientation.It is suggested that an approach such as discussed in this paper could be used in a forecasting mode, combined with traditional tools such as the NWS Limited Fine Mesh Model (LFM) or Nested Grid Model (NGM). In both of these cases the LFM correctly predicted the development of these storms. However, due to its much coarser resolution it was unable to resolve the terrain-forced variation evident in the simulations presented here.With 15 Figures     

Prediction of orographic precipitation using cartesian coordinates and a single prognostic equation for the water substance

The three-dimensional compressible non-hydrostatic mesoscale model ADREA, developed at NCSR Demokritos for wind field and dispersion predictions in complex terrain, is used to perform two-dimensional simulations of orographically induced precipitation. The model makes use of a simple cartesian grid by allowing blocked areas in the grid volumes and grid faces which are crossed by the irregular topography (the so-called porous-medium concept). Additionally, since we are seeking both simplicity and low computational requirements, only one prognostic equation for the whole of the water substance is implemented. The model equations are based on the theory developed for multicom-ponent/multiphase mixtures and give full account of the effects arising from the relative motion between water particles and moist air. The evaluation of the feasibility of this approach is obtained by comparing model predictions with measurements and analytical results concerning orographic precipitation episodes in the mountain barrier of Sierra Nevada.     

夏季青藏高原动力和热力的强迫定常扰动

本文应用一个球面和σ坐标原始方程线性定常模式,模拟夏季青藏高原的动力和热力强迫作用下的定常扰动.计算得到的定常扰动,在对流层上层槽脊分布和实况基本相符,但南亚高压的位置稍偏东,中太平洋高空槽较弱.在对流层中层计算的扰动振幅较实况强,特别是西太平洋副高.对偏差的可能原因进行了讨论.文中比较了不同基本气流、热源的垂直分布和热力耗散的影响,结果表明不同的热源垂直分布可以影响定常扰动的位相分布;还指出线性模式中,随高度增强的热力耗散机制对解释定常扰动的位相和结构可能是重要的.