Study on the characteristics of back building Mesoscale Convective System of short-duration heavy rainfall in Zhejiang Province

Abstract：The back building MCS (Mesoscale Convective Systems) is highly likely to cause sudden, localized short-duration heavy rainfall. In order to reveal the characteristics of this type of MCS and its association with heavy rainfall, this study systematically studies the spatial and temporal distribution of back buliding MCS that triggered short-duration heavy rainfall during the warm season from 2015 to 2021 in Zhejiang Province, as well as the different organizational forms and environmental thermodynamic factors for different types. The results show that the back building MCS in Zhejiang Province exhibit significant monthly and diurnal variation patterns, mainly occurring in June and July, with peak hourly rainfall intensities of 30 and 50 mm, respectively. MCS primarily form between 11:00 and 14:00, with the highest frequency of formation occurring between 12:00 and 13:00. The majority of MCS have a duration of 12 hours or less, with 10-hour duration being the most common. The start time of back building shows a quasi-bimodal pattern, which is 2-3 hours later than the main formation time of the MCS. For 90% of the cases, the time of maximum hourly rainfall intensity occurred within 0-2.5 hours after the onset of the back building. Based on the evolution characteristics of convective system organization, the back building MCS with short-duration heavy rainfall in Zhejiang Province can be categorized into four types: advective, quasi-stationary, turning, and propagating MCS, with about 42% ocurred under the forcing of weak synoptic-scale system. The MCS usually occur in an environment with medium CAPE, high humidity and appropriate vertical wind shear, but with different environmental factors for different types. The quasi-stationary MCS account for the largest proportion (44.7%) and are characterized by significant environmental dynamic features, including strong atmospheric instability, larger steering flow, and mid-to-lower-level vertical wind shear, resulting in relatively weaker maximum hourly rainfall intensity (the median is 50mm/h). While propagating MCSs (accounting for about 17%) exhibit more distinct environmental thermodynamic characteristics, with larger CAPE and PW, leading to the strongest maximum hourly rainfall intensity (the median is 70mm/h).