Occurrence and migration of meso-β-scale precipitation area over the Kanto district in summer

Abstract

The purpose of the present study is to clarify the regional characteristics of the occurrence and migration of the meso-β-scale precipitation area over the Kanto district in summer and its relationship with the surface and upper-level wind system. The data sets used in this study are the hourly data of Radar-AMeDAS precipitation with spatial resolution of about 5 km × 5 km, surface wind data at 145 AMeDAS stations, and upper-level wind data at Tateno for July and August 1995-1998. As the first step, 131 fine weather days when no synoptic-scale disturbances or cloud systems appeared in and around the Kanto district were selected as target days using daily weather charts at 00 UTC and Radar-AMeDAS precipitation data. In this study, a precipitation area in which the area receiving ≥ 1 mm/hour developed into an area 20 km × 20 km or larger is defined as a meso-β-scale precipitation area, and its occurrence location and migration path are analyzed. To determine the migration path of the precipitation area objectively, the method of pattern matching based on the temporal preservation of the precipitation distribution pattern was applied to 261 selected cases. Moreover, wind data of AMeDAS stations are converted into zonal and meridional wind components at grid points at approximately 11 km intervals. Meso-β-scale precipitation areas with a life time of 3 hours or more (187 cases) are likely to occur over the mountainous areas around the Kanto Plain in the afternoon and over the Kanto Plain in the evening. Notable high-frequency areas of occurrence are located around Mt. Yamizo, Mt. Nantai, Mt. Haruna, and the Chichibu Mountains. Most of the precipitation areas occurring in the vicinity of Mt. Yamizo, Mt. Nantai, and the Chichibu Mountains tend to stagnate in their site of occurrence. Precipitation areas that occur near Mt. Haruna mostly tend to migrate east-southeastward through the vicinity of Maebashi/Kumagaya and reach the southern part of Ibaraki Prefecture. Precipitation areas of both the stagnant type and migratory type occur when the upper-level (500-700 hPa) wind is westerly, corresponding to the migration direction of the migratory type. Although the wind speed of the migratory type appears slightly greater than that of the stagnant type, the correlation between wind speed and the migration speed of the precipitation area is not significant. Therefore the migration of the meso-β-scale precipitation area cannot be explained solely by the upper-level wind. In the surface wind system, a few sea breeze systems are observed as southerly and southeasterly wind from Sagami Bay and Tokyo Bay and as easterly wind from Kashima-Nada. In the migratory-type precipitation area, a convergence zone is observed between these sea breeze systems with west-northwest to east-southeast orientation in the central part of the Kanto Plain, which coincides with the migration path of precipitation areas from Mt. Haruna. Moreover, the center of the convergence zone migrates east-southeastward as the sea breezes withdraw. However, in the stagnant type, the convergence area that appears near the site of occurrence stagnates and the convergence zone to the east of the precipitation area can barely be observed. It is concluded that the eastward migration of meso-β-scale precipitation areas is controlled mainly by the appearance and location of the convergence zone due to the sea breeze systems with different wind directions rather than by the upper-level wind.