安定した総観条件下における関東地方の地上風系場と収束・発散分布の日変化 夏季海陸風日と冬季季節風日について

Abstract

In the present study, climatological features of wind systems over Kanto district are documented in terms of diurnal variation in wind distribution and divergence field under stable synoptic conditions. At first, the days when no synoptic scale disturbances existed in the area of 30-40°N and 134-144°E have been selected by using daily weather charts at 09 JST, 21JST and 09 JST of next day in the period of 1993-1995. Two typical synoptic conditions are obtained in terms of direction and velocity of surface gradient wind, and potential sunshine duration; that is, summerr days with small pressure gradient that correspond to the summertime land-sea breezes days (hereafter, refer to these cases as SLED: 30 cases), and the winter monsoon days (WMD: 41 cases) prevailing strong north-northwest wind. In the next step, the hourly grid point data of wind components at about 11km interval is prepared by the objective analysis based on the wind data of 170 observation points (AMeDAS). The structures of average wind distribution and divergence field are examined for SLSD and WMD by using the grid point data.As for SLSD, a convergence zone as the invasion limit of the sea breeze (sea breeze front) appears along the south coast of Kanto district, and the valley breeze circulation begins to form in the northern part of Kanto plain around 09 JST. Along the west coast of the Tokyo Bay, a small scale convergence zone due to the beginning of sea breeze from the Tokyo Bay appears in the late morning to early afternoon. An extended sea breeze (ESB) with south-southeast wind covers consecutively from the southern coast of Kanto district to western part of Gunma Pref. is established around 15 JST. The area where the wind direction of ESB changes from south to southeast in the southern part of Saitama Pref. corresponds to a marked convergence zone with east-west oriented axis. This convergence zone maintains as far as the ESB-like wind system is continued. In the western edge of this convergence zone, a small scale vortex with cyclonic circulation is formed in the early evening when the mountain breeze begins to blow from the mountainous district of northern Kanto. This vortex is maintained in the confluence point of the ESB and mountain breezes from the Kanto Mountains and the mountainous district of northern Kanto. The western margin of the ESB comes to correspond to the convergence zone with these mountain breezes including the wind system that spouts from the Izu Peninsula, and retreats eastward due to the expansion of these mountain breezes. The vortex moves eastsoutheastward on the convergence zone formed in the ESB (or its remaining wind system). It reaches to the north of the Tokyo Bay in the early morning. Though the south wind prevails over Boso Peninsula and southern part of Ibaragi Pref, all night, in the eastern part of Tokyo and Saitama Pref, the west wind from the Kanto Mountains blows as a mountain breeze after retreating the ESB around midnight, then it changes into the north wind from the mountainous region of the northern Kanto in the early morning.On the other hand in WMD, though the wind velocity increases considerably in daytime, the diurnal variation in the characteristic feature of wind system is rather small. When the north wind component is strong as taken up in this analysis, the north wind in the western part of Kanto plain forms a shear line in the Sagami Bay between the west wind which blows south coast of Tokai-Kanto district. This shear line is detected as a clear stationary convergence zone. Moreover, another stationary convergence area, where the north wind is weakened toward the windward due to the existence of the Boso Hills, appears on the Tokyo Bay side of the Boso Peninsula. The deceleration of the north wind in the southern part of the Boso Peninsula facilitates the invasion of west wind blowing along the south coast of Tokai-Kanto district to the Boso Peninsula. Therefore, the topography of the