Characteristics and Ambient Conditions of Mesoscale Convective Systems Moving Offshore in the Yangtze-Huai River Basin, East China

Abstract

This work for the first time studies the characteristics and environment conditions of mesoscale convective systems (MCSs) that move offshore (i.e., land to sea, LS) in the Yangtze-Huai River Basin (YHRB), East China. In the warm season (April–September), LS MCSs account for 25.5% of the total MCSs, exhibiting a decreasing trend from spring (April–May) to later summer (August–September). LS MCSs most frequently occur from late morning to early evening. Compared to land-only (LO) MCSs, LS MCSs form closer to the coastline and have significantly longer lifetimes, greater intensities and larger areas. Both two types of MCSs develop under favorable synoptic conditions, including upper-level jets, mid-level troughs, low-level southwesterly jets and surface cyclones. The environment conditions that distinguish LS MCSs from LO ones change with seasons. In spring, the offshore propagation of LS MCSs is driven dynamically by the cold, dry flow behind the mid-tropospheric short-wave trough in the midlatitudes, featured by stronger low-level vertical wind shear (VWS) but lower most unstable convective available potential energy (MUCAPE) than LO ones. Conversely in late summer, LS MCSs are mainly driven thermodynamically, with higher MUCAPE but weaker low-level VWS than LO ones. It is the low-level warm, moist southwesterly flow along the northwestern periphery of the Western Pacific Subtropical High that steers the LS MCSs toward the ocean. Revealing this seasonal reversal of dominant forcings can provide useful insights into the prediction of offshore propagation of MCSs and associated several convective weather in the coastal region of the YHRB.