A Two-Dimensional Numerical Study of the Impact of a City on Atmospheric Circulation and Pollutant Dispersion in a Coastal Environment

The urban impact on the sea breeze is studied by means of a mesoscale model with a detailed urban parameterisation. Four simulations are carried out on an idealised two-dimensional flat domain. In the base case, half of the domain is characterised by seaand the other half by rural land. In the urban case, an urban area 10 km wide is added near the shoreline. Simulations are performed for a moist rural soil (weak sea breeze) and for a dry rural soil (strong sea breeze). Results are analysed in order to evaluate the impact of the city on the wind, temperature and turbulent kinetic energy fields. The dispersion of a passive tracer emitted near the coastline is, also, used in the comparison. Results show that the city accelerates the sea-breeze formation in the morning (combinations of urban circulation and sea breeze), but it slows thesea-breeze front penetration. Moreover, the presence of the city enhances the recirculation processes and strongly modifies the pollutant dispersion. These effects are enhanced for a moist rural soil.     

江淮气旋发生发展中尺度系统特征数值模拟研究

本文应用中尺度模式MM5对发生在江淮流域的气旋进行了数值模拟，并用模式输出的高时空分辨率、动力协调资料、HUBEX试验期间的加密观测资料进行江淮气旋的中尺度诊断研究。结果表明：从大尺度角度看，江淮气旋的生成可分为静止锋上的波动和倒槽锋生两种类型，但从中尺度角度分析，它们具有如下共同特点：（1）它们的生成源地均在大别山西侧，大别山地形均使得气旋发展增强，移速减慢。（2）在发展阶段，700hPa层以下的温压场的斜压结构是气旋发展的重要因素，当冷平流与暖平流呈现西北－东南的偶极子型时，气旋发展；当冷暖平流偶极子呈东西型时，气旋发展进入成熟期。（3）气旋中的中尺度雨带与Wave-CISK湿条件对称不稳定区有密切关系，辐合上升区出现在气旋暖锋前部和冷锋后部50－100km范围内。     

Sea-Breeze Modification Of The Growth Of A Marine Internal Boundary Layer

A numerical mesoscale model is used to make a high-resolutionsimulation of the marine boundary layer in the Persian Gulf, duringconditions of offshore flow from Saudi Arabia. A marine internal boundary layer(MIBL) and a sea-breeze circulation (SBC) are found to co-exist. The sea breeze develops in the mid-afternoon, at which time its frontis displaced several tens of kilometres offshore. Between the coastand the sea-breeze system, the MIBL that occurs is consistent with a picture described in the existing literature. However, the MIBL isperturbed by the SBC, the boundary layer deepening significantly seaward of the sea-breeze front. Our analysis suggests that thisstrong, localized deepening is not a direct consequence offrontal uplift, but rather that the immediate cause is the retardation of theprevailing, low-level offshore windby the SBC. The simulated boundary-layer development can be accounted for by using a simple 1D Lagrangian model of growth driven by the surface heatflux. This model is obtained as a straightforward modification ofan established MIBL analytic growth model.     

华北平原中尺度对流复合体发生的环境和条件

中尺度对流复合体（MCC）会给华北平原带来暴雨灾害。在对MCC普查的基础上，选取发生在华北平原上的3个典型个例合成出代表MCC发生前的环境场，并通过诊断分析考察了华北平原发生MCC的条件。结果表明，华北平原发生MCC的环境既不同于我国南方的MCC，也不同于北美。对于具有明显夜发性的MCC，所得到的MCC发生环境和条件，对预报当天傍晚华北平原是否有MCC发生有一定的参考价值。     

由大尺度观测提取中尺度信息的可能性研究

利用浅水方程模式和变分四维同化方法对由大尺度观测提取中尺度信息的可能性进行了模拟试验研究。试验的初始场是在平直气流上叠加一个半径为 5倍格距的圆形涡旋。分别设定观测点的间距是 5 ,7和 9倍格距 ,观测的时间间隔分别是 1,2和 3h ,同化时间为 6h或 12h ,考察同化产生的初始场。试验结果是 :观测点的间距是 5倍格距时 ,由观测给出的分析场不能反映涡旋的基本特征 ,而在同化产生的初始场中 ,涡旋的基本结构清楚 ,但强度有所削弱。增加观测次数或者延长同化时间 ,可以改善同化的结果。对于移动较快的系统 ,同化的结果较好。当观测点的间距是 9倍格距时 ,同化的结果与分析场相比已无明显改进。     

跨图幅任意区域DEM裁剪功能的实现

随着计算机技术和图形图像技术的发展，3维地形模型的研究越来越受到重视，为了实现GIS中3维地形的无级缩放和无缝漫游，任意区域的DEM裁剪是一个必须解决的问题，文中通过举例详细论述了任意区域DEM裁剪功能实现的思想和过程。     

中纬度中尺度对流系统研究的若干进展

中尺度对流系统（MCSs)与许多严重的气象灾害，特别是我国的暴雨洪涝密切相关。本文主要对中纬度中尺度对流系统研究的若干进展作了简要综述，其中包括：MCSs的时空分布；MCCs的平均生成环境；MCSs的组织模型和逆尺度演变；发展中的MCSs与其环境的相互作用；MCSs的动力学和不稳定度；MCSs的数值模拟；MβCS和MγCS的观测和模拟；MCSs的概念模型；MCSs的结构和机制研究的观测计划。我们希望通过本文有助于进一步了解有关中纬度MCSs的发生、发展、结构、机制以及概念模型等研究的若干进展。     

Mapping mesoscale currents by optimal interpolation of satellite radiometer and altimeter data

 Mapping the mesoscale surface velocity stream function by combining estimates of surface height from satellite altimetry and surface currents from sequential infrared (sea-surface temperature) imagery using optimal interpolation is described. Surface currents are computed from infrared images by the method of maximum cross-correlations (MCC) and are combined with altimeter sea-level anomaly data from the TOPEX/Poseidon and ERS satellites. The analysis method was applied to 6 years of data from the East Australian Current region. The covariance of velocity and sea-level data is consistent with the statistical assumptions of homogeneous, isotropic turbulence, with typical length scales of order 220 km and time scales of 10 days in this region. Augmenting the analysis of altimeter data with MCC velocity observations improves the resolution of the surface currents, especially near the Australian coast, and demonstrates that the two data sources provide consistent and complementary observations of the surface mesoscale circulation. The volume of MCC data is comparable to that from a satellite altimeter, but with a more variable distribution of spatial and temporal resolution. In concert with altimetry, satellite radiometer velocimetry represents a technique useful for retrospective analysis of currents from high-resolution satellite radiometer data-sets. Received: 3 July 2001 / Accepted: 16 November 2001     

Simulation of Meteorological Fields Within and Above Urban and Rural Canopies with a Mesoscale Model

Accurate simulation of air quality at neighbourhood scales (on order of 1-km horizontal grid spacing) requires detailed meteorological fields inside the roughness sub-layer (RSL). Since the assumptions of the roughness approach, used by most of the mesoscale models, are unsatisfactory at this scale, a detailed urban and rural canopy parameterisation, called DA-SM2-U, is developed inside the Penn State/NCAR Mesoscale Model (MM5) to simulate the meteorological fields within and above the urban and rural canopies. DA-SM2-U uses the drag-force approach to represent the dynamic and turbulent effects of the buildings and vegetation, and a modified version of the soil model SM2-U, called SM2-U(3D), to represent the thermodynamic effects of the canopy elements. The turbulence length scale is also modified inside the canopies. SM2-U(3D) assesses the sensible and latent heat fluxes from rural and urban surfaces in each of the computational layers inside the canopies by considering the shadowing effect, the radiative trapping by the street canyons, and the storage heat flux by the artificial surfaces. DA-SM2-U is tested during one simulated day above the city of Philadelphia, U.S.A. It is shown that DA-SM2-U is capable of simulating the important features observed in the urban and rural RSL, as seen in the vertical profiles of the shear stress, turbulent kinetic energy budget components, eddy diffusivity, potential air temperature, and specific humidity. Within the canopies, DA-SM2-U simulates the decrease of the wind speed inside the dense canopies, the skirting of the flow around the canopy blocks, warmer air inside the vegetation canopy than above open areas during the night and conversely during the day, and constantly warmer air inside the urban canopy. The comparison with measurements shows that the surface air temperature above rural and urban areas is better simulated by DA-SM2-U than by the `standard version' of MM5.