Actual condition and influence of urban on local rainfall in the Tokyo metropolitan area.

  • Sanae OTAKA, 2012: Actual condition and influence of urban on local rainfall in the Tokyo metropolitan area. .

Abstract

In this study, a case study regarding local rainfalls observed in the Tokyo metropolitan area was nducted using ground observation data, sonde data,radar data. First, three cases of localized rainfall in the Tokyo metropolitan area in July and August 2010 were selected and the essential features were investigated in terms of wind,temperature, and stability index. As a result, there are various factors on each case (for example, wind convergence near the urban area, wind type E-S,temperature rise in the afternoon). In all cases, the atmosphere was unstable.

In order to investigate the influence of the city for the precipitation, the WRF model was used under idealized atmospheric conditions. In control experiment, the simulated results were verified against the observations to discuss the impact of urbanization.After that, "no urban experiment" and "strong urban experiment" are conducted. The initial condition was the average of convective precipitation days in summer. As a result, it was confirmed that the simulated rainfall and winds were similar to the observed features. In urban sensitivity experiments, the existence of city had a major impact on precipitation. In "no urban experiment", rainfall did not occur in the metropolitan area. Contrary to this, in the "urban experiment" and "strong urban experiment", rainfall occurs in the evening in and around the metropolitan area. Occurrence time of rainfall in "strong urban experiment" was earlier than the occurrence time of rainfall in "urban experiment". The reason for this results are (i) increase of sensible heat flux in the city, (ii) convergence of cold outflow and sea breeze
(iii) increase of precipitable water vapor due to sea breezes.

Furthermore, sensitivity of the precipitation to the environmental atmospheric stability and wind is examined.Only when the atmosphere is a little unstable, localized rainfall occurs. Wind direction and wind speed greatly affects the position of the rainfall occurs,but the localized rainfall has occurred in the Tokyo metropolitan area for all cases.

From these results, the urban can cause the localized rainfall in the Tokyo metropolitan area under a little unstable atmosphere.

Keyword:Local rainfall,Tokyo metropolitan area , WRF , Idealized Numerical Experiment ,Urban climate