干冷空气对江苏热带低压倒槽大暴雨影响的数值试验

Numerical Study on the Effect of Cold Dry Air on a Rainstorm Caused by Tropical Depression Inverted Trough in Jiangsu Province

采用WRF中尺度数值模式,通过对2011年7月11—13日江苏持续性热带低压倒槽大暴雨的数值试验,揭示干冷空气强度变化对暴雨分布和强度影响的动力、热力机制。结果表明:对流层高层干冷空气加强不利于降水增强,一定湿度的干冷空气对降水有利;中层干冷空气增强有助于暴雨增强,但湿度太低不利于强降水持续;低层干冷空气愈强愈有助于暴雨增强。暴雨强度不仅与低层辐合和高层辐散耦合动力配置的强度有关,还与其维持时间有关,中层和低层干冷空气增强均有利于动力配置的增强和维持,对应于暴雨的增强和维持。中层干冷空气增强,低层锋区增强,降水增强;低层干冷空气增强(减弱),锋区明显加强(减弱),对应降水增强(减弱),暴雨中心东南(西北)移。暴雨中心湿位涡分量MPV1(500 hPa)和MPV2(800 hPa)维持“上正下负”配置,有利于降水增强。高层和中层干冷空气加强时,MPV1和MPV2峰值先于降水最大值出现;低层干冷空气加强,MPV1峰值先于降水最大值出现,MPV2峰值与降水最大值同时出现,对降水增强有先导和增幅效果,MPV1和MPV2的峰值愈大,降水愈强。

Using the WRF mesoscale numerical model,the dynamic and thermodynamic mechanisms of cold dry air intensity change on distribution and intensity of rainstorm are revealed by conducting numerical experiments on a continuous rainstorm caused by tropical depression inverted trough in Jiangsu Province during 11-13 July 2011.The conclusions drawn from this study are as follows.The enhancement of cold air in the upper troposphere leads to precipitation reduction,and the dry cold air with a certain humidity is favorable for precipitation.Enhancement of dry cold air in the middle troposphere is good for the increase of heavy rain,but the low relative humidity is not conducive to the continuation of heavy precipitation.The stronger the dry cold air in the lower troposphere,the more intensification of heavy rain.Besides,the intensity of rainstorm is not only related to the intensity of coupling dynamic configuration of convergence in the lower troposphere and divergence in the upper troposphere,but also related to its maintenance time.The enhancement of dry cold air in the middle troposphere and lower troposphere both benefit the enhancement and maintenance of configuration,corresponding to the heavy rain.Moreover,the enhancement of dry cold air in the middle troposphere contributes to the strengthening of the lower front area and precipitation.As the dry cold air in the low troposphere strengthens(weakens),the front area strengthens(weakens),corresponding to the precipitation increase(reduction)and the southeast(northwest)movement of the rainstorm center.In addition,the coupling effects of positive MPV1 at 500 hPa and negative MPV2 at 800 hPa above the rainstorm center positively enhance the rainfall.When the dry cold air in upper and middle troposphere strengthens,the extreme values of MPV1 and MPV2 both appear before the maximum precipitation.When dry cold air in the lower troposphere strengthens,the maximum MPV1 appears before the maximum precipitation,and the maximum MPV2 appears at the same time as the maximum precipitation,which respectively has leading and increasing effects on precipitation enhancement.The greater the maximum values of MPV1 and MPV2,the heavier the precipitation.