On the Unstable Stratification Associated with Cloud Top Entrainment Instability over the East China Sea during Cold-air Outbreak Situations

Abstract

Evolution of a cloud-topped boundary layer in the East China Sea area in a situation of cold-air outbreak was examined for a case during 12-13 January 1991 based on the operational rawinsonde observation data and the NOAA AVHRR data received by Nagoya University. The analysis was made by comparing with that in the downstream region in the central area of the Japan Sea on 2 February 1991. (1) According to the brief analysis during the period from 11 January to 3 February 1991, the stratification which satisfies the condition for Cloud Top Entrainment Instability (CTEI) was observed at Naze (about 1000 km downstream from the continent over the East China Sea) in most cases of cold-air outbreak situations. (2) According to a case study for 12-13 January 1991, it was suggested that the release of CTEI seems to occur and this might affect the change in characteristics of low-level clouds in the downstream of Naze, even in situations of cold-air outbreak over the East China Sea. (3) On the other hand, the unstable stratification satisfying the CTEI condition was formed on passing from Saishuh-toh (about 500 km distant from the continent) to Naze in that case. Besides, the horizontal scales of cloud areas became larger and the cloud tops became somewhat flatter, so that the cloud layer became favorable for release of CTEI in downstream of Naze. (4) The large positive value of the total apparent source just below the top of the mixed layer and its rapid decrease with height above that layer contributed primarily to generating the stratification which satisfies the CTEI condition in the upstream of Naze. The much higher "degree of unsaturation" (large value of q^*-q) above the mixed layer in the East China Sea area than in the Japan Sea area, as well as the huge supply of heat and moisture from the underlying sea, seems to be one of the favorable factors for generating the CTEI condition there, even in situations of cold-air outbreak.