A squall line on 14 June 2009 in the provinces of Jiangsu and Anhui was well simulated using the Advanced Regional Prediction System(ARPS) model. Based on high resolution spatial and temporal data, a detailed analysis of the structural features and propagation mechanisms of the squall line was conducted. The dynamic and thermodynamic structural characteristics and their causes were analyzed in detail. Unbalanced flows were found to play a key role in initiating gravity waves during the squall line's development. The spread and development of the gravity waves were sustained by convection in the wave-CISK process. The squall line's propagation and development mainly relied on the combined effect of gravity waves at the midlevel and cold outflow along the gust front. New cells were continuously forced by the cold pool outflow and were enhanced and lifted by the intense upward motion. At a particular phase, the new cells merged with the updraft of the gravity waves, leading to an intense updraft that strengthened the squall line.
基于陕西WARMS模式、FY2F卫星云图、多普勒天气雷达和地面加密观测等资料,分析总结了2015年8月3日西安致灾大暴雨过程(以下简称"8.3"大暴雨)的环境条件与中尺度特征。结果表明:该过程强度大、突发性强、降水落区集中,中低层快速东移南压的冷式切变线和地面冷锋是其主要影响系统,地面切变辐合偏弱、整层偏南水汽输送及其辐合不明显是大暴雨持续时间短、范围小的重要原因;地面冷锋后部偏北风遇秦岭北麓地形作用形成初始对流,高层北路冷空气侵入导致不稳定能量增大,二者共同作用触发对流与能量强烈释放,形成β中尺度对流系统,产生大暴雨;低层辐合、高层辐散和垂直上升运动中心偏强而无次级环流,造成暴雨范围小、持续时间短;暴雨区主要位于对流云团云顶亮温(TBB)梯度大值区,与3 h显著正变压中心梯度大值区和切变线交汇点南侧对应;雷达强回波区呈垂直塔状,质心低,属热带海洋型降水回波。在不稳定层结尤其是低层超绝热状态下,加强雷达资料分析研判,跟踪紧邻山地杂波的、孤立的、中心像素点反射率超过60 d Bz的小尺度对流单体发展,可提前发布秦岭北麓暴雨预警。
By employing the energy-Casimir method, a three-dimensional virtual pseudoenergy wave-activity relation for a moist atmosphere is derived from a complete system of nonhydrostatic equations in Cartesian coordinates. Since this system of equations includes the effects of water substance, mass forcing, diabatic heating, and dissipations, the derived wave-activity relation generalizes the previous result for a dry atmosphere. The Casimir function used in the derivation is a monotonous function of virtual potential vorticity and virtual potential temperature. A virtual energy equation is employed(in place of the previous zonal momentum equation) in the derivation, and the basic state is stationary but can be three-dimensional or, at least, not necessarily zonally symmetric. The derived wave-activity relation is further used for the diagnosis of the evolution and propagation of meso-scale weather systems leading to heavy rainfall. Our diagnosis of two real cases of heavy precipitation shows that positive anomalies of the virtual pseudoenergy wave-activity density correspond well with the strong precipitation and are capable of indicating the movement of the precipitation region. This is largely due to the cyclonic vorticity perturbation and the vertically increasing virtual potential temperature over the precipitation region.
