Investigation of rainstorm warning and its difference under different synoptic conditions in Shanghai

Based on the hourly observation data of 79 automatic weather stations in Shanghai from 2010 to 2020 and the rainstorm warning signal data issued by the Shanghai Meteorological Center and District Meteorological Centers from 2016 to 2020, the spatial and temporal distribution characteristics of the rainstorm in Shanghai are analyzed using statistical metrics such as trend analysis and cumulative frequency. On the basis of statistical analysis of the frequency and timeliness of rainstorm warning signals at different levels, the difference of the timeliness of rainstorm warning signals under different synoptic conditions was studied. The results show that the rainstorm meeting the 6-h rainfall standard presents a significantly increasing trend, particularly for the rainstorm with the "Blue " warning. The rainstorm meeting the 6-h rainfall standard is the minimum in July, while the rainstorm meeting the 1-h rainfall standard is mroe in July than in June, which indicates that the frequency of short-time rainstorm in Shanghai is higher in July due to the control of subtropical high. The rainstorm in Shanghai usually lasts relatively short in the afternoon, while the longer rainstorm usually occurs in the morning. The frequency of heavy rainstorm is the highest in the urban area and Pudong near the Huangpu River, which may be related to the urban heat island effect. The release time of early warning signal in Shanghai presents a feature of bimodal distribution. The first peak appears at 12:00, and the release of early warning signals from 12:00 to 17:00 is relatively concentrated. The second peak appears at 6:00 in the morning.The average duration of early warning signals of all levels is 5.9 hours. The average hit rate of rainstorm early warning in Shanghai is 82.4%, and the average rainstorm warning time is 1.3 hours. The severe convective rainstorm at the edge of the subtropical high has the shortest warning timeliness, and the rainstorm near the typhoon body or the peripheral spiral rain belt is easy to be missed, which requires special attention in future.