Mesoscale analysis of a typical heavy rain event caused by returning flow in the warm sector in southern China

Data from conventional surface observations, FY-2E satellite, Doppler weather radar and NCEP/NCAR reanalysis with 1°×1° resolution were used to analyze mesoscale features of a heavy rain event in the warm sector which occurred in Southern China on 8 May 2013. Results are as follows. (1) This event occurred at the back of high pressure ridge transformed over the sea. It is triggered by the ascending movement resulted from the surface convergence line caused by topographic effect where a strong extreme low-level southeast jet stream met bell mouthed terrain. The event hence belongs to a heavy rain by returning flow in the warm sector in southern China because it was not affected by cold air. (2) Successively originating, moving eastwards and strengthening of four β-mesoscale convective systems (MβCS), in which the first MβCS lasted for 6 hour, are the direct cause that led to this event. A new MβCS originates in the southwest side of an earlier MβCS when it dissipates, which result in several going through the same area and hence forming a "train effect". (3) Being different from a heavy rain case in the warm sector in southern China earlier whose water vapor concentrates at 850 hPa or 925 hPa, the water vapor in this event comes mainly from the extreme low-altitude (950 hPa) southeast jet stream, which maintains energy in the low-level. Intrusion of the weak dry and cold air in the middle level has provided a favorable condition for the development and maintenance of MCS. The large vertical helicity, which shows the coupling between deep vorticity column and strong ascending movement, has provided dynamic mechanism for occurrence of heavy rain. (4) Analysis of wind field data at the regional AWSs showed that the mesoscale convergence line formed by strengthening southeasterly located in Yangjiang to Enping area in the nighttime and ground northeasterly (or breeze) makes small scale convective cloud clusters strengthen to MCS. The new mesoscale convergence line is formed by outflow at the back side of MCS and southeast airflow from the sea, and triggers the new MCS again, which is characterized by backward propagation. Key words: heavy rain in warm sector；mesoscale convergence line；MCS；mesoscale