Long-term Change and Spatial Anomaly of Warm Season Afternoon Precipitation in Tokyo

The long-term trend of precipitation at the central part of Tokyo was analyzed using digitized hourly data for 118 years (1890. 2007). It was found that "no preceding precipitation" (NPP) cases, defined as not preceded by >= 1 mm precipitation for the last six hours, showed an increasing trend of precipitation as a rate of 30%/century or more from afternoon to early evening of the warm season. Analysis for spatial precipitation patterns for NPP cases, using hourly data on the AMeDAS network for the recent 30 years (1978-2007), also showed a positive anomaly in Tokyo in the afternoon of the warm season. These facts suggest the reality of the urban heat-island effect on the increase of warm season short-term precipitation in Tokyo.