Characteristics of a back-building mesoscale convective system causing short-duration heavy rainfall in Zhejiang province

Back-building MCSs (Mesoscale Convective Systems) are highly conductive to 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 spatial and temporal distribution of back-building MCSs that triggered short-duration heavy rainfall during the warm seasons from 2015 to 2021 in Zhejiang province. Different organizational forms and environmental thermodynamic factors of different types are also explored. The results show that back-building MCSs in Zhejiang province exhibit significant monthly and diurnal variation patterns, i.e., MCSs mainly occur in June and July with peak hourly rainfall intensities of 30 and 50 mm in these two months, respectively. The MCS primarily form between 11:00 and 14:00 BT, with the highest frequency of formation occurring between 12:00 and 13:00 BT. The majority of MCSs have a duration of 12 h or less, with 10 h duration being the most common. The start time of backward propagation shows a quasi-bimodal pattern, which is 2—3 h later than the main formation time of the MCS. For 90% of the cases, the time of maximum hourly rainfall intensity occur within 0—2.5 h after the onset of the backward propagation. Based on the evolution characteristics of convective system organization, the back-building MCSs with short-duration heavy rainfall in Zhejiang province can be categorized into four types: Advective, quasi-stationary, turning, and propagating MCS, with about 42% occuring under the forcing of weak synoptic-scale system. The MCS usually occurs in an environment with medium convective available potential energy (CAPE), high humidity and appropriate vertical wind shear, but with different environmental factors for different types. The quasi-stationary MCSs account for the largest proportion (44.7%) and are characterized by significant environmental dynamic features, including strong atmospheric instability, large steering flow, and mid-to-lower-level vertical wind shear. They result in relatively weak maximum hourly rainfall intensity (the median is 50 mm/h). Propagating MCSs (accounting for about 17%) exhibit more distinct environmental thermodynamic characteristics with large CAPE and precipitable water (PW), and lead to the strongest maximum hourly rainfall intensity (the median is 70 mm/h).