Mesoscale and Microscale Structures of Snow Clouds over the Sea of Japan : Part III: Two Types of Circulations in Snow Bands Associated with a Wind-Speed-Increase Zone Preceding Cold-Air Outbreaks

Airflow structures in two snow bands, one in 1990 and the other in 1992, over the Sea of Japan have been studied by dual-Doppler radar and supplementary observations. These snow bands appeared in a wind-speed-increase zone, which moved into the experiment area several hours before the outbreak of the winter monsoon. In the 1990 case, dropsonde observations showed that the stratification of low-level air behind the trailing edge of the band was convectively unstable, and that a cold pool was situated beneath the band. A cold air presence was confirmed, also for the 1992 case, from surface temperature analyses. The mass transport budget analyses revealed the relative importance of a band-normal wind component to a mean airflow structure in the both snow bands. Their airflow structures in the band-transverse vertical cross-sections were, however, completely different from each other. The circulation in the 1990 case was characterized by the ascending inflow from the trailing edge and the descending outflow from the leading edge. For the 1992 case, the circulation was the reverse of the 1990 case, the ascending inflow being from the leading edge and the downdraft occurring in the very narrow region around the trailing edge. The different circulations in the two snow bands were explained by vertical wind shear in the band-transverse components of the horizontal winds.