地形追随垂直运动方程在南疆极端暴雨中的诊断分析

Diagnostic Analysis of Terrain Following Vertical Motion Equation in Southern Xinjiang Extreme Rainstorms

针对2021年6月15~17日发生在昆仑山脉北坡的南疆极端暴雨过程,本文综合考虑地形对暴雨发生、发展的作用后,利用地形追随坐标控制方程并采用Boussinesq近似推导建立了地形追随坐标的非静力平衡广义垂直运动方程。诊断结果表明,经向气压梯度力耦合经向散度项(项一)、垂直气压梯度力耦合纬向散度项(项二)和非绝热加热经向梯度项(项三)是激发暴雨垂直运动发展演变的三个主要强迫项。项一体现了偏北风逐渐增强,在昆仑山脉的阻挡下导致经向辐合增强,触发了垂直上升运动。经向气流辐合始终是对流活动最主要的强迫过程,其次为纬向气流辐合。在地形追随坐标形式下,经向和垂直气压梯度能够增强项一和项二。对流发展阶段,水汽辐合与非绝热加热过程增强了非绝热加热经向梯度,促进了垂直上升运动发展。在地形的影响下,对流层中高层西风过山气流波动特征明显。重力波活动导致的高层辐散进一步促进了山脉迎风坡对流活动。经向和纬向气流辐合、非绝热加热过程以及重力波活动等多个因素共同造成了此次南疆极端暴雨。

Recently,during 15–17 June 2021,an extreme rainstorm occurred on the northern slope of Kunlun mountain in southern Xinjiang,China.After considering the effects of geography on the rainstorm,a terrain-following,nonhydrostatic generalized vertical motion equation was built under Boussinesq approximation and terrain-following governing equations.Diagnostic results indicated that the three main forcing terms stimulating vertical motion development during the rainstorm were meridional pressure gradient force coupled with meridional divergence(first term),vertical pressure gradient force coupled with zonal divergence(second term),and meridional gradient of diabatic heating(third term).The first term reflected that the gradually increased northern flow led to the growth of meridional convergence under the blockage of Kunlun mountain,which activated the ascending motion.The lead forcing process for the convection was the meridional convergence,followed by zonal convergence.Meridional and vertical pressure gradients had an amplification effect on the first two terms in a terrain-following coordinate frame.The meridional gradient of diabatic heating was intensified by the water vapor convergence and diabatic heating processes during the convection development stage,thereby promoting the ascending motion.Westerly winds over the Kunlun mountain exhibited evident wave characteristics in the middle and upper troposphere under the influence of geography.The convection in the Kunlun mountain windward slope was strengthened by the upper divergence induced by a gravity wave.Thus,the rainstorm in southern Xinjiang was caused by the combined effects of meridional and zonal convergences,diabatic heating,and gravity wave activities.