中国大范围雷暴大风事件(Derechos)研究: 时空分布、环境背景和对流系统形态特征

2017年7月9日，天津市西南地区出现一次伴随短时强降水、冰雹、下击暴流的强雷暴大风天气过程，其中天津市静海区独流镇先后发生两次下击暴流大风。利用多普勒天气雷达数据、三维变分风场反演资料、地面加密自动站观测资料等，对此次强雷暴大风天气过程中下击暴流的成因进行了初步探讨。结果表明：(1) 本次过程发生在500 hPa冷涡后部和高空前倾槽形势下，上干冷下暖湿形成静力不稳定层结，对流发生前对流层中下层具有明显的条件不稳定、较大的对流有效位能和下沉对流有效位能，西南风与东南风形成的中尺度辐合线在低层高温背景下触发对流，较弱的垂直风切变环境使得对流风暴初期以脉冲风暴形式出现。(2) 下击暴流大风发生前伴随雷达回波图上反射率因子核心下降、中层径向辐合和低层中气旋等特征。(3) 降水粒子的拖曳作用对独流镇两次下击暴流大风的发生均起到重要作用；此外，第一次下击暴流大风的发生还与负浮力和干空气夹卷有关，第二次下击暴流大风的发生则与中气旋后侧扰动气压垂直梯度有关。

The distinction between large-scale and mesoscale contribution to severe convection: A case study example

A severe thunderstorm gale event with several short-term rainfall,hail and downburst occurred in the southwest of Tianjin on 9 July 2017, during which two downbursts attacked successively Duliu Town,Jinghai District,Tianjin. Based on the Doppler weather radar data,the three-dimensional variational wind retrieval data,the intensive observations from the automatic weather stations and other data,we have conducted a preliminary study on the causes of downburst during this severe thunderstorm gale event. The results indicate that (1) this downburst event is due to the cold vortex at 500 hPa and the forward-tilted trough,as well as a statically unstable stratification formed by the dry and cold air in the high and the warm and humid air in the low. There are obvious conditional instability and large CAPE and DCAPE in the lower and middle troposphere before the convection,which is triggered by the mesoscale convergence line formed by the southwesterly and southeasterly winds with high temperature in the low atmosphere. The weak vertical wind shear environment makes the convective storm appear in the form of pulse storm in the early stage. (2) Before the downburst occurs,it corresponds to the decline of reflectivity factor core and the MARC and the low-level mesocyclone in the radar echo map. (3) The dragging effects of precipitation particles play an important role in the occurrence of two downbursts in Duliu Town. In addition,the occurrence of the first downburst gale is also related to the negative buoyancy and the entrainment of dry air,and that of the second downburst gale is related to the vertical gradient of disturbed air pressure behind the mesocyclone.