江西省2001年4～6月重要天气过程评述

1暴雨过程 每年的 4～ 6月正值我省汛期， 2001年 4～ 6月每天≥ 10县、市的暴雨天数有 8 d（表 1）。与历史同期相比，属偏少年份。但其中 4月 19日 20时～ 20日 20时暴雨和大暴雨总站数之多，与历史同期相比，属第 5位。其它过程均为一般过程。 表 1 2001年 4～ 6月暴雨天数一览表 序号日期(月、日)站数(站)暴雨中心暴雨落区暴雨大暴雨强度(mm)站点 14月7日20时～8日20时1994广昌全省 24月19日20时～20日20时2011199金溪赣南 34月28日20时～29日20时104119乐平全省 45月6日20时～7日20时1198黎川赣北、赣中 55月8日20时～9日20时1070赣…     

江西省2000年10～12月重要天气过程评述

1暴雨天气过程 (1)10月 19日 20时～ 20日 20时，受高空低槽东移影响，我省的吉安、抚州、赣州 3市有 22个县市出现了暴雨天气。暴雨中心位于兴国，强度为 72 mm。本次暴雨范围之大居历史同期第 2位，但强度一般。 (2) 10月 21日 20时～ 22日 20时，受低层切变影响，我省的鹰潭、抚州 2市和上饶市南部以及宜春市北部有 20个县市出现了暴雨天气。暴雨中心位于上饶，强度为 93 mm。本次暴雨范围之大居历史同期第 3位，但强度一般。 2冷空气过程 10～ 12月全省共出现了 3次强冷空气过程（表 1）。 3大雾天气过程 10～ 12月全省共出现雾日 2…     

商丘暴雨日及分布特征研究

对商丘国家观象台1954--2005年月报表中挑取的符合暴雨日条件的142个样本分析，结果表明：商丘暴雨日具有明显的季节性，频发于7、8月份；暴雨日年平均2．73个；日暴雨量最大（193．3mm）不超过200mm；最长连续暴雨日数不超过2日；连续暴雨日降水量累计（223．9mm）不超过250mm；1h最大降水量不超过70mm。暴雨目的年代际变化特征明显，20世纪80年代后暴雨日出现较晚，60--90年代的暴雨日数递减，90年代后有增加趋势；大暴雨日数自60年代起有逐步增加趋势。暴雨日对月、年降水量有显著贡献，4—9月暴雨日对月降水量的贡献很大，且从4月到7月暴雨目的贡献呈递增趋势。一年内暴雨日出现5次时，当年的年雨量为偏多年份。     

玉林市两次冬季暴雨过程分析

利用常规观测资料、自动站雨量资料以及6h一次的NECP FNL分析资料(水平分辨率为1°×1°)等,对2016年1月27日20时~28日20时和2013年12月13日20时~16日20时玉林市两次冬季暴雨过程进行诊断分析,结果表明:两次冬季暴雨过程均主要受南支槽东移和切变线的影响,西南气流为这两次过程提供了大量的水汽。低层辐合高层辐散促进上升运动发展,触发了大气层结的不稳定度,而K指数对暴雨的预报有一定的指示作用。     

副高偏弱偏南 局部暴雨成灾

1主要天气事件1997年4～6月主要天气事件有4月3～4日暴雨冰雹雷雨大风、5月8日清远～从化一花都特大暴雨洪水为月13一16日全省持续性暴雨～大暴雨、6月22～26日持续性大～暴雨。1．14月3～4日，粤西北、粤西南和粤中出现冰雹、暴雨到大暴雨、雷雨大风。清远、茂名两市有7个县降了冰雹，使清新县和高州市的16个镇受灾，2．6万多间房屋损坏，清新县有万余亩农作物受浸，高州市大片农作物被毁损失O．6亿元。1．25月8～9日，清远、从化、花都降特大暴雨。·8日02～20时清远市清城区源潭镇降雨934．3mm，为我省1951年以来24／J‘时雨量之最，导…     

莱芜“7.12”暴雨初步分析

1984年7月12日,菜芜、沂源出现了一次大暴雨。4小时降水120毫米,中心雨量达260毫米,是近10年来所未有的。这次暴雨过程主要集中在11日20时到12日8时。暴雨开始前受低空切变和地面静止锋影响。从11日20时起,随着低涡和地面气旋波北上东移,在暖区内出现了暴雨,本文利用现有的资料,对这次暴雨的成因进行了分析。     

江西省2000年4～6月重要天气过程评述

4～ 6月是我省的汛期, 2000年汛期共有 6次较明显的暴雨过程： 　　（ 1) 4月 19日 20时～ 20日 20时受地面弱冷空气影响,赣北和赣中北部有 14个县市出现暴雨。与历史同期相比,该过程属一般过程。 　　（ 2） 受中层暖切变影响, 4月 24日 20时～ 25日 20时赣北和赣中北部有 28个县市出现暴雨, 6个县市出现大暴雨。这是 2000年入汛以来的第 1场大范围暴雨过程,范围之大居历史同期 第 1位,强度之大居历史同期第 5位。 　　(3)受低层切变和高空低槽共同影响, 5月 29日 20时～ 31日 20时我省北部有 14个站次出现暴雨, 10个站次出现大暴…     

一次江淮暴雨过程中干空气侵入的诊断分析

利用中尺度数值模式MM5(V3)，对2003年7月4日20时至5日20时发生在江淮地区的一次梅雨锋暴雨过程进行模拟，并采用模式输出的具有较高分辨率和动力协调性的网格资料，就干空气侵入及其对暴雨过程的作用进行了诊断研冤。结果表明，此次暴雨过程中有明显的干空气侵入，且干空气侵入对暴雨过程产生重要作用：干空气侵入将对流层高层高位涡带入低层，促进了对流层低层气旋及对流运动的发展，继而引起降水的增强。     

一次梅雨暴过程中潜热的计算分析

本文针对1991年7月5日20时-6日20时一次江淮梅雨过程，计算了潜热的释放，并揭示出潜热具有以下特点：（1）在整个梅雨锋暴雨过程中有大量凝结潜热释放，且在梅雨锋暴雨的强盛时期潜热释放最多。（2）在此次梅雨锋暴雨过程中对流凝结潜热的释放明显大于大尺度稳定凝结潜热的释放，且主要集中在650hPa-550hPa，它的强弱变化与实际降水强度变化成正比。     

“94.8.6”深圳特大暴雨分析和预报

受9413号热带气旋南侧低槽影响，深圳市8月5日20时至6日20时普降暴雨到特大暴雨，其中罗湖区田雨量达312．1mm，是本地区有资料以来的最高纪录。龙岗区的布吉镇、葵冲和宝安区的公明镇为大暴雨（图1）。这场特大暴雨的特点是：范围小、雨势强，过程最大一小时降雨量69．7mm，且伴有雷暴。罗湖区受灾最严重，市区几条道路水深1．5米，最深3米，交通一度瘫痪。许多民房进水，黄贝岭附近山体滑坡，全市受灾人数达21万人，死亡4人，直接经济损失5千万元人民币。图15日20时～6日20时深圳市雨量分布图1环流形势与影响系统8月3～4日，500hPa副高脊…     

Summer dryness in a warmer climate: a process study with a regional climate model

Earlier GCM studies have expressed the concern that an enhancement of greenhouse warming might increase the occurrence of summer droughts in mid-latitudes, especially in southern Europe and central North America. This could represent a severe threat for agriculture in the regions concerned, where summer is the main growing season. These predictions must however be considered as uncertain, since most studies featuring enhanced summer dryness in mid-latitudes use very simple representations of the land-surface processes ("bucket" models), despite their key importance for the issue considered. The current study uses a regional climate model including a land-surface scheme of intermediate complexity to investigate the sensitivity of the summer climate to enhanced greenhouse warming over the American Midwest. A surrogate climate change scenario is used for the simulation of a warmer climate. The control runs are driven at the lateral boundaries and the sea surface by reanalysis data and observations, respectively. The warmer climate experiments are forced by a modified set of initial and lateral boundary conditions. The modifications consist of a uniform 3 K temperature increase and an attendant increase of specific humidity (unchanged relative humidity). This strategy maintains a similar dynamical forcing in the warmer climate experiments, thus allowing to investigate thermodynamical impacts of climate change in comparative isolation. The atmospheric CO ₂ concentration of the sensitivity experiments is set to four times its pre-industrial value. The simulations are conducted from March 15 to October 1st, for 4 years corresponding to drought (1988), normal (1986, 1990) and flood (1993) conditions. The numerical experiments do not present any great enhancement of summer drying under warmer climatic conditions. First, the overall changes in the hydrological cycle (especially evapotranspiration) are of small magnitude despite the strong forcing applied. Second, precipitation increases in spring lead to higher soil water recharge during this season, compensating for the enhanced soil moisture depletion occurring later in the year. Additional simulations replacing the plant control on transpiration with a bucket-type formulation presented increased soil drying in 1988, the drought year. This suggests that vegetation control on transpiration might play an important part in counteracting an enhancement of summer drying when soil water gets limited. Though further aspects of this issue would need investigating, our results underline the importance of land-surface processes in climate integrations and suggest that the risk of enhanced summer dryness in the region studied might be less acute than previously assumed, provided the North American general circulation does not change markedly with global warming.     

Continental vegetation as a dynamic component of a global climate model: a preliminary assessment

A simplified vegetation distribution prediction scheme is used in combination with the Biosphere-Atmosphere Transfer Scheme (BATS) and coupled to a version of the NCAR Community Climate Model (CCM1) which includes a mixed-layer ocean. Employed in an off-line mode as a diagnostic tool, the scheme predicts a slightly darker and slightly rougher continental surface than when BATS' prescribed vegetation classes are used. The impact of tropical deforestation on regional climates, and hence on diagnosed vegetation, differs between South America and S.E. Asia. In the Amazon, the climatic effects of removing all the tropical forest are so marked that in only one of the 18 deforested grid elements could the new climate sustain tropical forest vegetation whereas in S.E. Asia in seven of the 9 deforested elements the climate could continue to support tropical forest. Following these off-line tests, the simple vegetation scheme has been coupled to the GCM as an interactive (or two-way) submodel for a test integration lasting 5.6 yr. It is found to be a stable component of the global climate system, producing only ~ 3% (absolute) interannual changes in the predicted percentages of continental vegetation, together with globally-averaged continental temperature increases of up to + 1.5 °C and evaporation increases of 0 to 5 W m^–2 and no discernible trends over the 67 months of integration. On the other hand, this interactive land biosphere causes regional-scale temperature differences of ± 10 °C and commensurate disturbances in other climatic parameters. Tuning, similar to the q-flux schemes used for ocean models, could improve the simulation of the present-day surface climate but, in the longer term, it will be important to focus on predicting the characteristics of the continental surface rather than simple vegetation classes. The coupling scheme will also have to allow for vegetation responses occurring over longer timescales so that the coupled system is buffered from sudden shocks.     

Numerical analysis of a thermotopographically-induced mesoscale circulation in a mountain basin using a non-hydrostatic model

Summary The boundary-layer wind field during weak synoptic conditions is largely controlled by the nature of the landscape. Mesoscale (sub-synoptic) circulations result from horizontal gradients of sensible heat flux due to variation in local topography, variation in surface-cover, and discontinuities such as land-sea contrasts. Such flows are usually referred to as thermally-driven circulations, and are diurnal in nature and often predictable. In this paper we use a state-of-the-art non-hydrostatic computer model to shed light on the physical mechanisms that drive a persistent easterly wind that develops in the afternoon in the Mackenzie Basin, New Zealand. The easterly – Canterbury Plains Breeze (CPB) – is observed early in the afternoon and is often intense, with mean wind speeds reaching up to 12 m s⁻¹. Although computer modelling in mountainous terrain is extremely challenging, the model is able to simulate this circulation satisfactorily. To further investigate the mechanisms that generate the Canterbury Plains Breeze, two additional idealized model experiments are performed. With each experiment, the effects of the synoptic scale wind and the ocean around the South Island, New Zealand were successively removed. The results show that contrary to previous suggestions, the Canterbury Plains Breeze is not an intrusion of the coastal sea breeze or the Canterbury north-easterly, but can be generated by heating of the basin alone. This conclusion highlights the importance of mountain basins and saddles in controlling near-surface wind regimes in complex terrain.     

A cold air lake formation in a basin — a simulation with a mesoscale numerical model

Summary A formation of a cold air lake in a basin is studied with a mesometeorological model.A dynamic Boussinesq hydrostatic mesoscale numerical model is developed in a staggered orthogonal grid with a horizontal resolution of 1 km and with a varying vertical grid. The topography is presented in a block shape so that computation levels are horizontal.The mesometeorological model is tested in three idealized topography cases (a valley, a single mountain, a basin) and test results are discussed.In an alpine basin surrounded by mountains and plateaus the air is supposed to be stagnant at the beginning of the night. Due to differences in radiation cooling an inversion layer is formed in the basin and local wind circulation is studied by model simulations.With 14 Figures     

Effects of a Fence on Pollutant Dispersion in a Boundary Layer Exposed to a Rural-to-Urban Transition

Simultaneous particle-image velocimetry and laser-induced fluorescence combined with large-eddy simulations are used to investigate the flow and pollutant dispersion behaviour in a rural-to-urban roughness transition. The urban roughness is characterized by an array of cubical obstacles in an aligned arrangement. A plane fence is added one obstacle height h upstream of the urban roughness elements, with three different fence heights considered. A smooth-wall turbulent boundary layer with a depth of 10h is used as the approaching flow, and a passive tracer is released from a uniform line source 1h upstream of the fence. A shear layer is formed at the top of the fence, which increases in strength for the higher fence cases, resulting in a deeper internal boundary layer (IBL). It is found that the mean flow for the rural-to-urban transition can be described by means of a mixing-length model provided that the transitional effects are accounted for. The mixing-length formulation for sparse urban canopies, as found in the literature, is extended to take into account the blockage effect in dense canopies. Additionally, the average mean concentration field is found to scale with the IBL depth and the bulk velocity in the IBL.     

Evaporation from a blanket bog in a foggy coastal environment

Frequent fog severely restricts evaporation from blanket bogs in Newfoundland because it more than halves the radiant energy input, and it eliminates the vapor pressure deficit, resulting in evaporation at the equilibrium rate (average = 0.99 during fog). During these periods, there is no surface resistance to evaporation because the bog has been wetted by fog drip, and although the latent heat flux dominates over sensible heat (average = 0.8), both are small. In contrast, the surface dries during clear periods, increasing the surface resistance to evaporation so that sensible heat becomes more important ( = 1.05). When the mosses are dry, evaporation is below the equilibrium rate ( = 0.87), although the higher available energy ensures that actual evaporation is higher. During clear periods, daily evaporation averaged 2.5 mm, compared to 1.1 and 0.7 mm for fog and rain, respectively. The suppressed evaporation at this site is important in maintaining appropriate hydrological conditions for blanket bog development.     

Parameterization of a momentum source in a tropical cumulus ensemble

Summary An eddy effect of tropical deep convection on the large-scale momentum, resp vorticity budget is investigated. The process is specified by a simple parameterization approach which is based on a concept of rotating clouds exerting a momentum on the large-scale flow. The cloud rotation is associated with the thermal properties of a cloud ensemble by the principle of conservation of potential vorticity. A decomposition of cloud classes is applied in consistency with the thermodynamical parameterization scheme of Arakawa and Schubert (1974).The parameterization is tested with observations of GATE74, Phase III. The data are processed on a B/C-scale grid (55km) in a region within 9N and 16N, and between 21W and 27W, and with a vertical resolution of 41 levels. The parameterization results correspond to the observed patterns, especially in situations with strong large-scale wind shear. The findings suggest that certain large-scalle flow regimes provoke convective scale momentum generation rather than redistributing large-scale momentum by convective circulations.With 10 Figures     

Spatial structure of a jet flow at a river mouth

The present work concentrates on the latest data measured in the Jordan river flow in lake Kinneret. Spectral characteristics of fluctuating velocity components have been obtained and processed. The three-dimensional structure of turbulence along the flow has been described. The main features of the jet flow turbulence in the river mouth are: a) The supply of turbulent energy changes due to different mechanisms along the flow. b) The structure of turbulence formed in the river decays rapidly along the flow, and c) In the sand area and beyond it, a significant generation of turbulent energy occurs. Quantitative estimations of the above effects were carried out.     

Atmospheric dispersion of a heavier-than-air gas near a two-dimensional obstacle

Flow over a two-dimensional obstacle and dispersion of a heavier-than-air gas near the obstacle were studied. Two species, one representing air and the other representing the heavier-than-air gas were treated. Equations for mass and momentum were cast in mass-averaged form, with turbulent Reynolds stresses and mass fluxes modeled using eddy-viscosity and diffusivity hypotheses. A two-equation k- turbulence model was used to determine the effective turbulent viscosity. Streamline curvature and buoyancy corrections were added to the basic turbulence formulation. The model equations were solved using finite difference techniques. An alternating-direction-implicit (ADI) technique was used to solve the parabolic transport equations and a direct matrix solver was used to solve the elliptic pressure equation.Mesh sensitivities were investigated to determine the optimum mesh requirements for the final calculations. It was concluded that at least 10 grid spaces were required across the obstacle width and 15 across the obstacle height to obtain valid solutions. A non-uniform mesh was used to concentrate the grid points at the top of the obstacle.Experimental measurements were made with air flow over a 7.6 by 7.6 cm obstacle in a boundary-layer wind tunnel. Smoke visualization revealed a low-frequency oscillation of the bubble downstream of the obstacle. Hot-wire anemometer data are presented for the mean velocity and turbulent kinetic energy at the mid-plane of the obstacle and the mid-plane of the downstream recirculation bubble. A single hot-wire probe was found to be suitable for determining mean streamwise velocities with an accuracy of 11 %. The downstream recirculation bubble was unsteady and had a length range from 3 to 8 obstacle lengths.The experimental results for flow over the obstacle were compared with numerical calculations to validate the numerical solution procedure. A sensitivity study on the effect of curvature correction and variation of turbulence model constants on the numerical solution was conducted. Calculations that included the curvature correction model gave a downstream recirculation bubble length of 5.9 obstacle lengths while excluding the correction reduced this length to 4.4.In the second part of the study, numerical calculations were performed for the dispersion of a heavier-than-air gas in the vicinity of the two-dimensional obstacle. Characteristics of an adiabatic boundary layer were used in these calculations. The densities of the contaminant gases were 0, 25 and 50% greater than the air density. Calculations were performed with the contaminant injection source upstream and downstream of the obstacle.Use of the pressure gradient model reduced the size of the dense gas cloud by as much as 12%. The curvature correction model also affected the cloud expanse by reducing the effective turbulent viscosity in the downstream recirculation bubble. The location of the injection source had the largest impact on the cloud size. The area of the cloud within the 5 % contour was three times larger for downstream injection than for upstream injection.