A Study of Mechanism of "Yamase" Events and Their Prediction

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Observational and theoretical studies have been made to understand mechanism of "Yamase" and the results obtained are summarized as follows.1. A composite analysis of the five-day mean 500 hPa weather charts for summer seasons from 1958 to 1989 has been performed and the characteristic atmospheric circulation pattern is found to be associated with Yamase-type weather in the northern Japan.A factorial analysis is applied to the daily mean temperature anomalies obtained at the 16 weather stations in the east side of the northern Japan during June and July for the three years from 1990 to 1992. Anomalously low temperatures on the Pacific Ocean side are observed about 2 days later than those in the Okhotsk Sea side.2. A mechanism to cause the cool and humid weather in Yamase events is investigated with use of a two-dimensional numerical model which includes processes of cloud formation and long-wave radiation. A prediction experiment has also been made for the remarkable Yamase period from 17th to 19th July 1993 with use of a revised version of the Japan Spectral Model, JMA.The following three problems are shown to be crucial to clarify mechanism of intense Yamase events with fog and/or stratus : (1) thermodynamical conditions of the lower atmosphere in the high pressure system over Okhotsk Sea, (2) role of surface buoyancy flux during the southward movement of cooled airmass from Okhotsk Sea, and (3) interaction among a) amount of water vapor phase change b) cooling rate of cloud-top radiation and c) rate of entrainment into a mixed layr on the evolution of the moist convective mixed layr.