An analysis of a back-propogating thunderstorm using the three-dimensional wind fields retrieved by the dual-Doppler radar data

A back-propogating thunderstorm accompanied by a local severe rain and gust wind on 13 Semptember 2013 is analyzed based on the convetional, radar and automatic station data. The environmental conditions and back-propogating mechanism are analyzed using the dual-Doppler radar data. Shanghai was on the northwest edge of the subtropical high for several days, which provided sufficient moisture and unstable condition, and the surface wind convergence triggered the initial convection in northern Shanghai. The outflow and surface wind convergence triggered new cells on its southwest side constantly, thereby causing a southwestly thunderstorm propagation vector in the opposite direcion of the steering flow. When the propagation speed was larger than the steering speed, the thunderstorm appeared to move backward whose travelling direction was also opposite to the steering flow. The study of the back-propogating mechanism shows that a new cell is triggered by the convergence of the gust front generated by the strong downdraft with the environmental southwest wind in the southwest of the old cell, and the stronger updraft is caused by the collision with the initial surface wind convergence line. After the development of the new cell, it produces severe rainfall and strong downdraft, which can strengthen and maintain the gust front. The gust front moves to the southwest and triggers a new cell again. The three-dimensional wind fields retrieved by the dual-Doppler radar data indicate that each convective cell dispalys a dissipation, maturation and intialization stage in turn from northeast to southwest. Therefore, a cycle of "a new convective cell regenerates and develops with moving slowly to the northeast after maturation and producing severe rain, strong downdraft and gust and then triggering a new cell on its southwest" is formed. Consequently, the main reason of the back-propogating is the dynamic uplift of the southwestly-moving gust front together with the surface wind convergence line, which form a local frontogenesis area situated on the southwest side of the storm that coincides with the convective cell regeneration zone.