Meso-β to -γ-Scale Wind Circulations Associated with Precipitating Clouds near Baiu Front Observed by the MU and Meteorological Radars

Abstract

During the Baiu season (17 June-8 July 1991) we carried out simultaneous tropospheric observation by using the MU (Middle and Upper atmosphere) radar (VHF band, Kyoto University) and meteorological radars (C band of Osaka Meteorological Observatory, X band of Hokkaido University and C/Ku band of Kyoto University). Vertical distributions of three components of wind field and precipitation particles were observed by the MU and C/Ku-band radars, respectively. The C- and X-band radars were used to investigate horizontal distributions of precipitating clouds in the meso-α and -β scales, respectively. Several meso-β and -γ-scale cloud systems were observed around a meso-α-scale cyclone center during 4-5 July when rainfall was the strongest in the whole observational period. They were divided into two groups of convective clouds i) near a surface warm front and ii) near a surface cold front, and iii) one group of stratiform clouds on the north-western side of the surface cold front. In i), a remarkable updraft inside a precipitating cloud extending up to an altitude of 14km was produced by a convergence (inflows coming from the front and rear of the precipitating cloud) at an altitude of 4-5km and by strong southerly wind in the middle troposphere. In ii), a narrow rainband with gust front was seen at the leading edge of the surface cold front.Two meso-γ-scale rotor circulations were found in front of and inside the rainband, respectively. In iii), south-easterly (north-westerly) ascent (descent) flows were observed above (inside/under) the cold frontal surface extending up to an altitude of about 9km. Below the cold frontal surface, there was a dry region without precipitation, and a part of the descending westerly flow returned to the back of the precipitating cloud. In this study, vertical structures of meso-β and -γ-scale cloud systems with characteristic wind flows as mentioned above were revealed by the detailed three components of wind field in both clear and precipitating atmosphere. They were presented as smaller cloud systems in the hierarchical structure of cloud clusters near the meso-α-scale cyclone in the central region of the Japan Islands.