夏季の東京都心部における対流性降水の降水強度と気温場および地上風系場

The purpose of this study was to clarify the relationship between the intensity of connective rainfall in central Tokyo and the distribution of surface temperature and wind systems over the Kanto Plain in summer. Data sets used in this study include the hourly data on Radar-AMeDAS rainfall with a spatial resolution of about 5km×5km, surface temperature, and wind data at 92 AMeDAS stations for July and August 1995-1999.<br>First, 192 days were selected, during which neither synoptic-scale disturbances nor rainfall systems appeared in and around the Kanto district, using daily weather charts at 00 UTC and Radar-AMeDAS rainfall data. The rainfall events that occurred during these days were derived from the connective system, and 25 cases of convective rainfall events were recorded over the central Tokyo area (15km×15km). The rainfall intensity in central Tokyo is defined as the maximum hourly rainfall within the area during each rainfall period. Moreover, wind data at AMeDAS stations were converted into zonal and meridional wind components, and interpolated to approximately 11km interval grid points to calculate wind divergence.<br>Intense rainfall occurred in central Tokyo when the surface wind system with southerly wind from Sagami Bay and easterly wind from Kashima-Nada was observed (type E-S: 5 cases). In contrast, when northeasterly wind (type NE: 12 cases) or southerly wind (type S: 6 cases) blew over the Kanto Plain, less intense rainfall occurred. In the case of intense rainfall (type E-S), a strong convergence area was observed over central Tokyo because of the southerly and easterly winds from Sagami Bay and Kashima-Nada. A weak convergence area appeared in cases of less intense rainfall (types NE and S) in central Tokyo, corresponding to the decrease in the speed of northeasterly (type NE) or southerly (type S) wind blowing over the Kanto Plain.<br>Regardless of the rainfall intensity, the temperature over the inland area including central Tokyo was rather uniform, and the low-temperature area was distributed only in the coastal area of the Kanto Plain. Moreover, the correlation coefficients between the rainfall intensity in central Tokyo and the temperature difference between Tokyo and the respective AMeDAS stations were significant only in the coastal area. A decrease in temperature in the coastal area was observed several hours before the occurrence of convective rainfall in central Tokyo, corresponding to the appearance of cold air advection in the sea. Next, the temperature difference between Tokyo and the surrounding area was enhanced, and a relative high-temperature area appeared over the inland area including central Tokyo. That is, the high temperature area over the inland area in these cases was a result of the wind system blowing from the sea. Therefore, it is difficult to explain the occurrence of the convergence area and intensity of convective rainfall in central Tokyo based on the temperature distribution alone.<br>Before the temperature difference between Tokyo and surrounding area was enhanced, a change in the wind system occurred corresponding to the development of a thermal local low over the Chubu Mountains, which is considered to be more influential than the high temperature area over the inland Kanto Plain for the wind system as far as rainfall intensity is concerned.