Doppler Radar Observation of Convergence Band Cloud Formed on the West Coast of Hokkaido Island. II: Cold Frontal Type

Abstract

A convergence band cloud moved southwestward through the Ishikari Plain on 20 January, 1986. In this study, we mainly used a single Doppler radar to examine the kinematic and radar echo structures of a convergence band cloud of the cold frontal type. The band cloud, approximately 40km in width, shifted southwestward and accompanied an advancing land-breeze front. The front was geometrically and dynamically similar to that of a gravity current found in laboratory experiments. Kelvin-Helmholtz instability waves with wavelength of 1-2km were observed along the interface of the land breeze and the northwesterly monsoon wind. Two small (-6km) vertical circulations similar to those seen in the warm frontal type were embedded in the mesoscale (-15km) vertical circulation. A low-level convergence and a resultant updraft which were stronger than that of the warm frontal type were located ahead of the front. A distinct subsidence, which is not found in the warm frontal type, followed the front. The updraft caused strong convective echoes ahead of the front, while, owing to the subsidence, the echoes behind the front rapidly weakened. The height of maximum updraft coincided with that of the potential instability layer. The updraft and the potential instability may be respon- sible for the formation of the convergence band cloud. Based on our observations, we constructed a conceptual model of the formation of the convergence band cloud.