Subtropical Rainband along the Western Side of a Slow-moving Ridge

Abstract

The large-and meso-scale features of a subtropical rainband which caused thunderstorms over western Japan (*33N/130E) in 25*27 July 1981 are studied. This case aroused our interest because the convective activities persisted for about three days in the west side of a slow-moving ridge.Radar observations showed a persistent convective echo band of *800km length extending in NNW*SSE direction. This echo band was not accompanied with front-like strong thermal gradient. Satellite IR imageries showed that the rainband was a part of a long subtropical cloud zone formed along the northern periphery of the extremely moist tropical maritime airmass in the lower troposphere.The enhancement of cloud zone and the formation of the rainband was owing to the incessant feeding of moisture accompanied with the northward intrusion of the moist tropical maritime air along the western hem of the slowmoving ridge.Real data forecast experiments on the cloud zone and rainband were made using a coarse-and a fine-mesh primitive equation model. While only the large-scale zone of weak precipitation relevant to the observed cloud zone was produced in the coarse-mesh (380km-mesh) model, the narrow rainband with low-level convergence was simulated in the fine-mesh (77km-mesh) model. The "dry experiment" (a experiment without condensation) did not produce any low-level convergence zone. This indicated influence of condensation process on the formation of this subtropical rainband.