中国南部沿海暴雨东风波特征及SST影响机制研究

The features of storm easterly waves along the Southern China Coast and the SST influence mechanism

分析1999—2013年影响我国南部沿海的东风波,可分为3类:偏南东风波、西行东风波以及近海东风波。太平洋副热带高压是影响3类东风波特征的关键系统,其西伸与北进直接引导东风波路径及活动位置。东风波的分类合成结构特征显示:强涡度中心指示东风波槽中心,强涡度中心通常位于850 h Pa及以下。东风波低层为强辐合场,槽后有整层的垂直上升区。偏南东风波波槽轴线随高度向西倾斜,西行东风波和近海东风波波槽轴线近乎垂直。合成诊断还显示,东风波的海上移动有向SST(Sea Surface Temperature,海表温度)大值趋暖的趋势。数值模拟证实,增强东风波槽前SST暖中心的强度,将引起槽区低层和槽后中层出现负变高中心,同时SST的增温将通过感热与潜热促使东风波槽强度加强,将进一步地增强东风波暴雨强度和雨带的北移。并增强中低层流场的气旋式气流成分,增强低层辐合场,维持深厚垂直上升运动层。典型西行东风波个例分析显示,螺旋度与东风波强度成正比,东风波纬向位温偏差显示东风波在热力场上具有"上暖下冷"的不稳定垂直结构。东风波涡度增强时,扰动动能向分层扰动位能转化。东风波强度减弱时,分层扰动位能向扰动动能转化。

By analyzing the easterly waves that influence Southern China Coast during 1999-2013,the easterly waves can be classified into three types：south path easterly waves,westward easterly waves and offshore easterly.The Pacific ocean high is a key system which controls the typical paths and positions of the three types of easterly waves especially through its motions towards west and north.The composite analysis based on different types of easterly waves shows that the strong vorticity center,which mostly locates under 850 hPa level,denotes the easterly wave center.As for the structures of the easterly waves,there are a strong convergence area in its lower part and a vertical rising area over the whole level on its back side.The trough axis tilts westwards with its height for the south path easterly wave,but it is almost vertical for the westward and offshore easterly waves.The composite analysis also shows that the easterly wave tends to move towards the warmer SST area.The numerical simulation results confirm that by warming up the SST warm center in front of the easterly wave trough,it will cause minus geo-potential height at low level of the easterly wave trough area and minus center of geo-potential height at mid level of its back side.It shows that the warming up of SST would increase the intensity of the easterly wave by sensitive and latent heats.It will also increase the intensity of the easterly wave precipitation,push the rainfall area northward and add more cyclonic component of wind fields in lower part of the easterly wave.The added component will support stronger convergence at low level and maintain the rising vertical velocity over the whole level.The analysis of a typical westward easterly wave shows that the helicity is positively correlated with the intensity of the easterly wave.Potential temperature zonal difference shows that the easterly wave has a structure of warm top and cool bottom.When the vorticity of the easterly wave increases,the perturbation kinetic energy transforms to the layer perturbation potential energy;while the easterly wave weakens,the latter transforms to the former.