水汽对梅雨期切变线大暴雨影响的数值试验

A numerical study on the effect of water vapor on a rainstrom related to shear line during Mei-yu period

利用WRF(Weather Research Forecasting)中尺度数值模式对2010年7月12—13日梅雨期影响江苏的江淮切变线大暴雨过程进行数值试验,重点研究切变线南侧水汽强度、垂直厚度和输送位置变化对降水发生、发展的影响,并揭示湿位涡对江淮切变线降水的指示性。结果表明:对流层高层水汽对降水强度和雨带分布影响较小;中层水汽对整体雨带形态的维持起了重要作用;低层水汽强度的变化主要对大暴雨区域及大暴雨中心降水强度存在影响;而水汽输送位置离切变线越近越有利于暴雨的发生发展。同时,切变线南侧水汽变化对江淮切变线和西南风低空急流发生发展,以及相关高低空散度和上升运动也存在影响,切变线南侧水汽供应越充足、水汽强度越强、水汽柱愈深厚、输送位置离切变线越近,则高低空散度发展耦合愈充分,垂直上升运动愈旺盛,切变形势及切变线上低涡越活跃,相应的降水强度越强、雨带分布越宽阔连续。分析发现湿位涡(Moist Potential Vorticity,MPV)对江淮切变线降水有较好的指示性,且以正压项MPV1的影响和指示为主。MPV1负峰值的出现指示降水峰值出现,当MPV1<-1.5 PVU时,切变线附近有小时降水量大于20 mm的短时强降雨发生。在MPV1<0条件下,若︱MPV2︱>0.05 PVU且尤其当MPV2>0时,降水强度明显增强,而MPV1为负、MPV2为正维持时间越久、︱MPV1︱和︱MPV2︱峰值越大,则江淮切变线降水持续时间越久、强度越强。

A numerical study was carried out by using the Weather Research Forecasting(WRF)model on a rainstrom related to shear line over the Yangtze-Huaihe River basin from July 12 to 13,2010,during Mei-yuperiod.This study focuses on the impact of intensity,vertical thickness,and transportation position change of water vapor on the south side of the shear line on the occurrence and development of the rainstorm,and reveals the indications of the wet potential vorticity on the Jianghuai shear line precipitation.Results show that:the upper tropospheric water vapor has relatively little effect on precipitation intensity and rainband distribution,whereas the middle tropospheric water vapor plays an important role in maintaining the overall rainband shape.The change of the low-level water vapor intensity mainly affects the shape of heavy rain area and the precipitation intensity in the heavy rain center.The closer the water vapor transports to the shear line,the more favorable it is for the occurrence and development of heavy rain.Moreover,the change of water vapor on the south side of the shear line has an impact on the occurrence and development of the shear line and southwesterly low-level jet,as well as the high and low level divergence and upward movement.The more water vapor supply is on the south side of the shear line,the stronger the water vapor intensity,the deeper the water vapor column,and the closer the transportation position are to the shear line,the more fully the coupling between the high and low altitude divergence development,the more vigorous vertical upward movement,and the greater the shear deformation potential.Consequently,more active low vortex are on the shear line,and stronger corresponding precipitation intensity and wider rainband distribution are obtained.It is also found that the Moist Potential Vorticity(MPV)is a good indication of precipitation related to shear line over the Yangtze-Huaihe River basin,and the influence and indication of MPV1 are relatively more important.The occurrence of a negative peak of MPV1 indicates the peak of precipitation will occur.When MPV1<-1.5 PVU,there will be a short-term heavy rainfall with hourly rainfall greater than 20 mm near the shear line.For MPV1<0,if︱MPV2︱>0.05 PVU and especially when MPV2>0,the precipitation will significantly enhance.If the negative MPV 1 and positive MPV 2 maintain longer,and the︱MPV 1︱and︱MPV 2︱become larger,the rain will be heavier and last longer.