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

为了解“Yamase”的形成机制，我们进行了观测和理论研究，主要结果如下：1.对Yamase的形成机制进行了研究。本文对1958-1989年夏季5天平均500百帕天气图进行了合成分析，发现其特征大气环流型与日本北部Yamase型天气有关。对日本北部东部16个气象站1990-1992年3年6-7月的逐日平均气温距平进行了因子分析。太平洋一侧的异常低温比鄂霍次克海一侧的温度晚2天左右。利用一个包含云形成和长波辐射过程的二维数值模式，研究了Yamase事件中凉湿天气的形成机制。本文还用日本谱模式的修订版JMA对1993年7月17日至19日这一显著的Yamase期进行了预报试验。结果表明，以下三个问题对阐明伴随雾和(或)层云的强烈Yamase事件的机制至关重要：(1)鄂霍次克海高压系统中低层大气的热力学条件；(2)从鄂霍次克海来的冷却气团南移过程中地面浮力通量的作用。(3)水汽相变量、云顶辐射冷却速率和混合层夹带速率对湿对流混合层演变的影响。

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.