1993年8月1日に南九州で発生したライン状豪雨の数値シミュレーション

Abstract

The stationary band-shaped torrential rain, observed over southern Kyushu on 1 August 1993, was successfully simulated by a three-dimensional anelastic non-hydrostatic model. A warm rain scheme that explicitly predicts cloud water and rainwater is employed in the model. The Japan Spectral Model (JSM) had previously failed to reproduce the stationary feature of the rainband and had predicted the precipitation area to move eastward.The failure of JSM was examined by using a 10km-resolution model. The failure was traced to the use of the moist convective adjustment scheme. Neither coarse-resolution nor absence of hydrostatic water loading led to the failure. By the moist convective adjustment scheme stabilizing unstable layers, the air was cooled and dried in the lower adjusted levels. This drying reduced the near-surface moisture and cut off the supply of highly humid air required for the maintenance of the rainband.The causes of the organization and maintenance of the simulated stationary rainband were then investigated using 2km- and 5km-resolution models. Highly humid air in the lower layer was advected by southwesterlies, condensing over the region with a large temperature contrast in the north-south direction corresponding to the Baiu frontal zone. At first, shallow convective rolls formed parallel to the low-level westerlies. As they developed, individual cells grouped into severe convective systems, resulting in a marked depression of pressure in the lower layer by more than 1.5hPa over the region of (100km)2 for two hours after the condensation. This low pressure induced the convergence of winds. An inflow of intensified cold northwesterlies increased meridional temperature contrast, forming a strong convergence line and an organized rainband. The maintenance system of this rainband had the characteristic features of the back-building type, with significant vertical wind shear in the repeated generation of convective cells upstream of the rainband. For this torrential rain event, evaporation was not very important in the enhancement and maintenance of the rainband, while the relatively large meridional temperature contrast over the Baiu frontal zone played a significant role.The maintenance and enhancement processes of the low-level jet (LLJ) in the torrential rain event were also investigated by calculating the mean horizontal momentum budget and examining the trajectories of air parcels. The pressure gradient force produced by the convection-induced low pressure accelerated the mean horizontal velocity in the LLJ core, while horizontal advection acted to decelerate the jet. It was found that the acceleration and deceleration of the jet canceled each other to maintain the jet over southern Kyushu. The inflow of southwesterly winds into the lower layer below 1km was also accelerated, by 6ms-1 over 150min after the start of integration, by the pressure gradient force. These accelerated horizontal winds were transported upward by convection, a portion of which intensified the LLJ to the north of the rainband.