The third precipitation episode of China's great snowstorms of 2008 was analyzed using station observations and ECMWF six-hourly data. The variation of the shape of the upper-level subtropical jet played an important role in the rainfall over southern China. With the eastward movement of the trough, the jet shape changed from two straight jets to a tilting jet over China and then it moved southward. With these variations, the south–north movement of ascending flow and precipitation area over southern China occurred.
用NCEP 1°×1°再分析资料、MICAPS系统地面站资料、TRMM卫星降水资料以及自动站雨量资料分析了2011年梅雨后期发生在江苏省南京市的一次暴雨过程。结果表明:此次暴雨发生在中高层高空深槽前部、低空切变线附近、低层低涡和地面低压的东北部,是西风带低值系统及其引导的低层低涡与西进北上的"马鞍"台风共同作用产生的。925 h Pa上中国东南沿海位于低层低涡东北部的东南偏东气流影响下,将来自于西太平洋的水汽源源不断向暴雨区输送,为暴雨的发生提供了良好的水汽条件。不仅如此,此东南偏东气流还使得华南和东南沿海地区暖平流明显。低层暖平流输送伴随的小幅增温和水汽输送导致的湿度层增厚,导致南京附近大气层结不稳定是暴雨发生的热力环境。随着"马鞍"台风的西进北上,其北侧副高中的干空气逐渐被卷至中国东南沿海附近,与大陆上的暖湿气流形成明显对比,形成了一条明显的能量锋。与此同时,在长江中游地区,残余的梅雨锋也表现为一条能量锋,两条能量锋在南京附近相接,造成明显的上升运动,从而触发了暴雨的发生。
After first reviewing historical and current difficulties in solving streamfunction and velocity potential in a limited domain,and describing recent developments in obtaining accurate solutions in a limited domain with arbitrary shape,a newly proposed approach is introduced and its application to a torrential rain event is reported.The results show that the newly developed method has advantages in capturing mesoscale information,compared with horizontal winds.
This paper investigates the effects of surface drag on upper-level front with a three-dimensional nonhydrostatic mesoscale numerical model(MM5). To this end, a new and simple potential vorticity intrusion(PVI) index is proposed to quantitatively describe the extent and path that surface drag affects upper-level front. From a PV perspective, the formation of the upper-level front is illustrated as the tropopause folding happens from the stratosphere. The PVI index shows a good correlation with the minimum surface pressure, and tends to increase with the deepening of the surface cyclone and upper-level front. The surface drag acts to damp and delay the development of upper-level front, which could reduce the growth rate of the PVI index. However, the damping presents different effects in different development stages. It is the most significant during the rapid development stage of the surface cyclone. Compared with no surface drag cases,the tropopause is less inclined to intrude into the troposphere due to the surface drag. Positive feedback between the surface cyclone and upper-level front could accelerate the development of the frontal system.
The energetics of the third stage of a snowstorm over China was analyzed using ECWMF data. The analysis of the energy budget for the Middle East trough and the western Pacific trough that developed toward China on 25–28 January 2008 showed the advection of the geopotential by the ageostrophic wind to be both a crucial source and the primary sink of the eddy kinetic energy centers associated with the troughs. The magnitudes of the energy conversion terms, interaction kinetic energy conversion and baroclinic conversion, were too small to explain the development of the energy centers and the jet streaks. The energy centers gained energy at their entrance regions via the convergence of the ageostrophic geopotential fluxes, and then lost energy at their exit regions by the same fluxes. At the entrance regions, the fluxes converged, increasing the geopotential gradient, which generated a stronger geostrophic wind and higher kinetic energy, resulting in an ascending motion in this area. When the troughs moved to China, the ascending motion caused by the convergence of the fluxes at entrance region intensified the snowstorms over central and southern China.
The effects of sea surface temperature(SST),cloud radiative and microphysical processes,and diurnal variations on rainfall statistics are documented with grid data from the two-dimensional equilibrium cloud-resolving model simulations.For a rain rate of higher than 3 mm.h 1,water vapor convergence prevails.The rainfall amount decreases with the decrease of SST from 29℃ to 27℃,the inclusion of diurnal variation of SST,or the exclusion of microphysical effects of ice clouds and radiative effects of water clouds,which are primarily associated with the decreases in water vapor convergence.However,the amount of rainfall increases with the increase of SST from 29℃ to 31℃,the exclusion of diurnal variation of solar zenith angle,and the exclusion of the radiative effects of ice clouds,which are primarily related to increases in water vapor convergence.For a rain rate of less than 3 mm.h 1,water vapor divergence prevails.Unlike rainfall statistics for rain rates of higher than 3 mm.h 1,the decrease of SST from 29℃ to 27℃ and the exclusion of radiative effects of water clouds in the presence of radiative effects of ice clouds increase the rainfall amount,which corresponds to the suppression in water vapor divergence.The exclusion of microphysical effects of ice clouds decreases the amount of rainfall,which corresponds to the enhancement in water vapor divergence.The amount of rainfall is less sensitive to the increase of SST from 29℃ to 31℃ and to the radiative effects of water clouds in the absence of the radiative effects of ice clouds.
