Structures of Cold Air During the Development of a Broad Band Cloud and a Meso-β-scale Vortex : Simultaneous Two-Point Radiosonde Observations

Abstract

Simultaneous radiosonde observations were made at Funadomari on Rebun Island and at Soya Cape in order to study the kinematic and thermodynamic structure of a cold air mass that penetrates into the Sea of Japan from the Sea of Okhotsk through the Soya Straits when broad band cloud appears on the west coast of Hokkaido. The time interval between sonde observations was ∼ 2 hours. In the period from 26 to 27 Feb. in 1990, we were able to observe the developing stage of the broad band cloud and obtain the kinematic and thermodynamic structure of a meso-β-scale vortex which landed on Rebun Island just prior to the development of the cloud. At altitudes below 1 km, the static stability was higher at Soya Cape than at Rebun Island. It is confirmed that the cold air mass which blew over Rebun Island was closely related to the development and movement of the broad band cloud. The depth of the meso-β-scale vortex defined from the wind field was ∼ 4 km. The level of the cloud top of the vortex, defined as the region where the relative humidity was greater than 70 %, was ∼ 2.5 km at Rebun Island. There was a transient layer whose kinematic and thermodynamic structures were different from those inside the "cloud region" and the environmental air, at least on the leading side of the "cloud region". There appeared a meso-β-scale high temperature region with a strong wind (more than 50 m s^-1) just below the tropopause and right above the "cloud region" of the low-level vortex. This upper-level meso-scale disturbance moved with the low-level vortex, and its structure changed in accordance with the developing stage of the low-level vortex.