Role of the Heated Landmass on the Evolution and Duration of a Heavy Rain Episode over a Meiyu-Baiu Frontal Zone

Abstract

A series of numerical simulations on a heavy rain episode over a Meiyu-Baiu frontal zone was conducted, to demonstrate the role of the heated landmass over mainland China on the evolution and duration of the rainfall. The case studied here was a long-lived convective rainband, which formed over the Yangtze River basin, and brought heavy rainfall reaching 380 mm in 20 hours. The synoptic-scale situation was characterized by the indistinct meander of the upper-level flow, and the significant low-level southerly inflow from a subtropical high. Simulations using a cloud-resolving non-hydrostatic model were performed within a domain covering central and southern China, to reproduce land-surface heating to the south of the front. Sensitivity experiments without shortwave radiation, without terrain, and with modified land-surface conditions, were conducted to examine the factors effective in the evolution and duration of the rainband. The control simulation reproduced the rainband, and its reproducibility was good when compared to the observational evidence. The evolution resulted from the latent instability, due to the inflow of the warm and moist air from the fine-weather area to the south of the front. The rainband was not reproduced when the surface heating effect was excluded. The duration became short as the heated area was reduced, and was represented as a function of the meridional width of the heated area, and meridional velocity component in the lower troposphere. These results indicate that land-surface heating, over fine-weather areas to the south of the front, is a crucial factor for the rainband evolution and its duration.