PRIMARY ANALYSIS OF THE LONGEST-LASTING SNOWFALL IN BEIJING

Five-day snowfall, which occurred in Beijing on 19—23 December 2002 and became the longest-lasting snowfall since 1841, is compositely analyzed. Five-day composite circumfluence showed that the snowfall took place in the constant zonal circulation. During the period, there was a strong Mongolia High on the surface. And the ridge of high pressure protruded southward our country’s east. On 850 hPa, Hetao area has a northward trough, and Beijing lied in the positive vortex of the trough. On 500 hPa, a front crossing Asia from west to east located 40~45°N. On 300 hPa, Beijing situated positive vortex of the axis of the jet, which was straightly zonal. The vertical section indicated that Beijing was convergent on the low-lever whereas divergent on the high-level. And there were two fronts. In a conclusion, Beijing was controlled by the large-scale circumfluence that was in favor of snowfall.The analysis of Huabei area during the continuous snowfall showed that the deformation field locating the north of trough had evident vapor convergence. Moisture flux indicated that vapor came from the return flow of the Yellow Sea, but not the return flow of Bo Sea. The warm advection in the east of Hetao trough was the cause of ascending.