A Numerical Study of Rainfall Associated with a Baiu Front: A Case of Niigata-Fukushima Heavy Rainfall in 2004

Abstract

Numerical experiments are performed to simulate and understand Niigata-Fukushima heavy rainfall in July 2004. The initial condition is adopted from global analysis (GANAL) data of JMA, and another numerical experiment is made with modified initial moisture field to better simulate the rainfall. A mesoscale-convection-resolving model is used, and the horizontal grid size is taken to be 1/18 degrees (about 5 km).It is emphasized that latent instability (positive buoyancy of rising air or CAPE) is important to explain convective activity and the rainfall, in contrast to a previous study which suggested the importance of convective instability. Latent instability is weak or absent in the GANAL data, but the observed heavy rainfalls and satellite images suggest that latent instability should be present at the initial time. A slight modification of the moisture field from the GANAL data gives rise to rainfall patterns which are more similar to those indicated by Radar-AMeDAS data in the early 10 hours. The rainwater distribution is discussed, compared with latent instability distribution. It is also suggested that neither convective instability nor equivalent potential temperature in the boundary layer are a good measure to explain the observed rainfall distributions.