Modeling Case Study of the Japan-Sea Convergent Cloud Band in a Varying Large-Scale Environment

摘要

A prediction experiment is performed for a typical case of cold-air outbreak with large temporal variation over the Japan Sea in the Asian winter monsoon situation with a very-fine-mesh, primitive-equation model. The purposes are to elucidate the typical evolution of the Japan-Sea convergent cloud band (CCB) that responds to the large-scale environment varying according to the passage of a short-wave trough accompanied by a marked cold vortex aloft, and to diagnose the physical processes responsible for the formation of a paired middle-level jet and weak-wind zone analyzed along the CCB.The model simulates well the overall orientation and intensity of the CCB. The CCB reaches its peak intensity under the short-wave trough, where the layer of low stability in the troposphere is deepest and the largest air-mass transformation occurs. The CCB weakens as the stability of the lower troposphere increases. Net air-mass transformation over the sea decreases behind the trough and eventually it dies away, even though air-mass transformation in cold advection persists. Although the orientation of the CCB tends to be parallel with the large-scale flow, this is not always the case. This is because each part of it migrates, following its own local flow field which has both temporal and spatial variations according to the phase of travelling short waves. The variation in thermal structure may be primarily attributable to the location and orientation of the CCB which govern the relative influences of each lower-boundary forcing.A paired middle-level jet and weak-wind zone develop along the CCB when the CCB is in the mature stage. A sensitivity experiment suggests that the pair of wind anomalies are closely linked with the CCB. Analysis of the ageostrophic wind reveals that the jet is rather geostrophic and nearly balanced with the mesoscale temperature field featuring the CCB, while the weak-wind zone is highly sub-geostrophic. The former develops in a mass-momentum adjustment process due to concentrated diabatic heating along the CCB, while the latter is produced directly by rapid upward motion with diabatic heating in a baroclinic environment. The diabatic vertical displacement in cold advection explains the wind-speed minimum in the vertical observed along the ascending zone.