Actual Condition of Heavy Rainfall in Summer in the Tokyo Metropolitan Area and Influence of Urbanization on Precipitation

  • Takuro HANYU, 2007: Actual Condition of Heavy Rainfall in Summer in the Tokyo Metropolitan Area and Influence of Urbanization on Precipitation .

Abstract

 This dissertation describes essential features of the local whether when heavy rainfalls occur in the Tokyo metropolitan area and effects of the urbanization on the precipitation.
 First, spatial distribution of the precipitation amount was examined using the Radar AMeDAS analysis precipitation data in the past 17 years (1988-2005) that differs from the statistical period of the earlier studies. As a result, we found that there is more precipitation in the metropolitan area than its surroundings.
 Second, case study was carried out in detail, focusing on three events that heavy rainfalls occurred in the metropolitan area. Here, we concern about water vapor and cloud top height as well as surface wind system and atmospheric stability. The following characteristics are found. (a) The cold air existed in the middle troposphere, and the atmospheric stability was low while Japan Island was covered with Pacific high. (b) Surface winds converged in the central part of the urban area during the several hours before rainfall occurs in the metropolitan area. (c) The water vapor tends to increase before the rainfall occurs over the surroundings of the urban area, whereas it has slightly increased in the center part of the urban area. (d) The cloud top height increases again when the precipitation system reach the city. Additionally, we investigated surface wind system and atmospheric stability about the other events that Japan Island is covered by the Pacific high. As a result, surface winds tend to converge in the urban area and atmospheric stability is low in general on the rainfall days.
 Third, numerical experiments were carried out for the two cases; one is for the control case with the urban areas and another for the experiment case without the urban areas. Simulated period is one month (July, 2000). Comparison between the two cases shows that the control case has more precipitation amount than that of the experimental case over the metropolitan area. However, a careful judgment should be needed to conclude the urban effects on the precipitation because there were differences in precipitation between the control case and the experimental case for the other areas as well.
 Last, the similar experiment was carried out for the precipitation event observed on a day (July 4, 2000). In this case, ensemble experiment that adapts the different initial condition and physics scheme was performed. Some members show positive impacts, i.e., the city enhanced precipitation. However, the others did not. The impacts of the initial condition and physics scheme were larger than that of the urbanization.
 These imply that the urbanization increases total precipitation amount, however its impact is very small. Improving the accuracy of numerical models and more ensemble experiments should be significant.

The paper was based on a portion of his Guraduation thesis was published in Geographical Review of Japan(Japanese)(2010).
Geographical Review of Japan(Japanese)
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The paper was based on a portion of his Guraduation thesis was published in Journal of Heat Island Institute International(Japanese)(2010).
PDF(Full Text)Download(Japanese)