Unusual Weather due to Irregular Activity of Polar Airmass and Associated Heavy Rainfall.

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The polar airmass dominates the weather in Japan which is located in the east of the Asian Continent. Baiu and Akisame fronts which form between the polar airmass and the tropical airmass are related to an activity of the polar airmass. An irregular behavior of the polar airmass sometimes causes unusual weather in Japan. Objectives of this research are the unusual weather due to the irregular activity of the polar airmass and the associated heavy rainfall.The NMC grid point data were analyzed by means of a statistic method to investigate the activity of polar airmass. High correlations were found in 500hPa height field during summer season in the southern hemisphere and during winter season in the northern hemisphere. A height anomaly in the southern hemisphere was relatively uniform in the zonal direction in comparison with that in the northern hemisphere.Behavior of the polar airmass was simulated in a laboratory experiment. While a rotating apparatus was axisymmetric, the axisymmetric flow was not stable and a non-axisymmetric disturbance developed. The pattern of the non-axisymmetric flow was similar to that of a polar air outbreak.A heavy rainfall associated with the Baiu front was observed during an intensive observation period in the Kyushu District in July 1988. We studied a structure and mechanism of a rainband caused the heavy reinfall. A retrieval method was developed to reveal a thermodynamic field in the rainband. The retrieved field showed that the evaporation of raindrop and a vertical shear were important to maintain the intense rainband.^1 "ORI" is the abbreviation of "Ocean Research Institute".A numerical prediction experiment of the heavy rainfall was performed. The observed rainfall was simulated well and the environmental condition of the rainfall was studied. A numerical model with high horizontal resolution was used to find a detailed structure of the heavy rainfall. The rainband was simulated and its vertical structure was clarified.