A Dual-Doppler Radar Study of Longitudinal-Mode Snowbands. Part I: A Three-Dimensional Kinematic Structure of Meso-γ-Scale Convective Cloud Systems within a Longitudinal-Mode Snowband

Abstract

Observations of snowstorms were carried out around Ishikari Bay, Hokkaido, Japan, from December 1991 to February 1992. On 15 January 1992, a longitudinal-mode snowband was observed by a dual-Doppler radar system. The snowband was composed of meso-?-scale (?20 km) convective cloud systems. The three-dimensional kinematic structures and organization processes of the meso-?-scale systems were studied in detail. Strong band-parallel winds appeared in the upper-north rear of the meso-?-scale systems. Since their speed was faster than that of the cellular radar echoes, the winds formed rear-to-front currents. Increasing in volume and speed, the rear-to-front currents developed into meso-? scale and penetrated toward the lower front and lower south of the systems. Consequently, the rear-to-front currents caused strong meso-?-scale convergence and the enhancement of updrafts at their leading edges. The transport of the band-parallel momentum greatly contributed to the successive development of new convective cells within the systems and the organization of the meso-?-scale systems in the snowband. Ice/snow particles were transported by the updrafts toward the rear and north of the systems in the overlying stable layer. They then evaporated outside the clouds. The downdraft caused by evaporative cooling played an important role in the transport of band-parallel momentum from the upper to the lower levels and from the outside to the inside of the snowband.