Initiations of Mesoscale Convective Systems in the Middle Reaches of the Yangtze River Basin Based on FY-4A Satellite Data: Statistical Characteristics and Environmental Conditions

Based on the brightness temperature observed by the Fengyun-4A satellite, eight hundred mesoscale convective systems (MCSs) are identified in the middle reaches of the Yangtze River Basin during the warm seasons (April-September) of 2018-2021, and these MCSs are categorized into the quasistationary (QS) type and the outward-moving (OM; i.e., moving beyond the source region) type. Afterward, the initiations of the MCSs are backward tracked using a hybrid method of areal overlapping and optical flow. Then, the main features of QS-MCSs and OM-MCSs and their respective synoptic circulations and environmental parameters are analyzed. The QS-MCSs primarily occur in July and August and are mainly initiated in the afternoon. The OM-MCSs mostly occur in June and July with two initiation peaks at noon and late night, respectively. The QS-MCSs are mainly initiated in mountainous areas. In contrast, the OM-MCSs are mainly initiated in plain areas. Compared to the OM-MCSs, the QS-MCSs show notable diurnal variation in intensity and develop more rapidly. Circulations of a total of 285 days (without direct influences from tropical cyclones) are objectively classified into three patterns by using the k-means algorithm. Pattern-I, which is closely related to low-level jets, shows the most similar features to those of typical Mei-yu fronts, and it acts as the most favorable circulation type for MCS initiations. Pattern-II is dominated by northwesterlies, with a relatively stable layer in the low-level troposphere. Pattern-III features a dry-adiabatic or even a superadiabatic layer that contributes to decreasing the layer stability.