Mechanisms for Morning Heavy Rainfall Corridors along the Meiyu Front over East Asia

Moist convection repeatedly develops in the midnight-to-morning hours of 11–15 June 1998 and yields heavy rainfalls (≥150 mm) in a narrow corridor ~1300 km long and ~160 km wide. Such multiday rainfall episodes with a regular diurnal phase have high impacts on regional hydrologic cycle and causes severe floods over East Asia, which is analogous to that over North America. Using the JMA nonhydrostatic model, we carry out a series of numerical experiments to examine the diurnal cycle and regional features of nocturnal convection in the corridor. The primary objective is to clarify the initiation and evolution of mesoscale convective systems in response to the environment, which itself varies both diurnally and regionally.

The model reproduces a train of mesoscale convective systems that simultaneously develop along the synoptic-scale Meiyu front and coincide with an arrival of monsoon air at night, consistent with observations. This process is primarily regulated by a nocturnal low-level jet (NLLJ) in the active southwesterly monsoon that forms southern China and extends to central China. The NLLJ mainly penetrates south–north-oriented valleys and strengthens moisture transport by more than 50%. It leads to three preferred regions of elevated conditionally unstable air at its northern terminus, where an enhanced low-level ascent overcomes small convective inhibition triggering new convection. A budget analysis on convective instability further shows that the intrusion of moist monsoon air can generate a large convective available potential energy prior to convection initiation, while free-atmospheric forcings are much weaker. The instability precondition is mostly induced by the NLLJ-related horizontal advection (100–190 J/kg per hour). Although the advection effect declines in convective phase, the instability production by vertical lifting increases to 100–160 J/kg per hour, suggesting that updrafts may exploit near-surface moisture to offset energy depletion for sustaining convection. The convection dissipates in morning with decaying LLJ and moisture. Thus, the multi-scale interactions among the diurnal forcings, mesoscale disturbances, synoptic-scale front, and regional monsoon are essential for the prediction of the morning-peak heavy rainfall corridor.