Dynamics and Prediction of a Mesoscale Extreme Rain Event in the Baiu Front over Kyushu, Japan

Abstract

A high-resolution version of the limited-area primitive equations model of the Bureau of Meteorology Research Centre is used to investigate the dynamics and prediction of the mesoscale, extreme rain event of 1 August 1993 over Kyushu. The model is multiply nested, uses high-order numerics, includes sophisticated parameterizations of physical processes, and has a capability to use cloud imagery from the Japanese Geostationary Meteorological Satellite (GMS) to initialize the vertical motion field. The short-term forecasts of the timing and distribution of rainfall are very encouraging. However, they are sensitive to many components of the forecast system, particularly the diabatic initialization and boundary layer parameterization. Large-scale flow changes during the event were 1) the maintenance of the subtropical ridge to the south of Japan and the development of a trough over Korea, which combined to maintain a warm and moist southwesterly flow over Kyushu, 2) the movement of a cold low from west to east of Japan which rearranged the westerly low-level flow and was associated with enhanced low-level convergence over southern Japan, and 3) the passage of an upper-level short-wave trough, consistent with sustained conditional instability and enhanced upper outflow over Kyushu. To highlight the importance of these flow changes in establishing a favorable, local convective environment, the authors show that many of the features, including the vertical divergence structures (but not the intensities), are reproduced from a ?no latent heating? simulation. Based on diagnostic calculations, the authors demonstrate the following: 1) Torrential rainfall developed in flows characterized by weak inertial stability and weak but sustained convective instability. 2) Increases in cyclonic vorticity by vortex stretching in the ascent (heavy rain) area was mostly counterbalanced by anticyclonic vorticity advection by the synoptic-scale flow. This balance was consistent with the low environmental cyclonic vorticity (weak inertial stability). 3) In this environment, large-scale ageostrophic circulations developed to enhance the low-level jet. The authors hypothesize that in these flows, with weak but sustained inertial stability and conditional instability, large-scale in?up?out divergent circulations developed unimpeded, with horizontal moisture convergence rapidly converted to moist ascent and heavy rain; and that a feedback between the convection and the strengthening low-level winds was established, such that the convection could enhance the low-level jet via the ageostrophic circulations, and the low-level jet could enhance the convection via the moisture supply and maintenance of conditional instability. The end result was a highly efficient rain system.