A mechanism for the reversal of long-term average vertical velocities around East Asia during the cold season

The formation mechanism of a systematic vertical-velocity reversal over Eas t Asia was studied during the cold season by using MU radar data and global analysis data. Although active gravity waves may modify the vertical wind pattern in the stratosphere, clear reversal patterns are also observed in t he periods when gravity waves are quiet. The frontogenesis function is posi tive in all the reversal periods. These facts suggest that the reversal is a part of the frontogenetically forced secondary circulations associated wi th an upper tropospheric jet streak. Calculation of the geostrophically for ced secondary circulation following frontogenesis theory supports this conc lusion.