Abstract
This study uses statistical evaluation by correlation analysis to examine the effects of thermal environment on the frequency of convective precipitation in the Greater Tokyo Area between 12:00 and 18:00 on summer days from 1997 to 2006. To extract the frequency of convective precipitation we used Automated Meteorological Data Acquisition System radar data to obtain detailed rainfall distribution maps, and to extract the urban thermal environment we used surface temperature data from a National Oceanic and Atmospheric Administration weather satellite. Results were a coefficient of determination of 0.01, indicating no clear relation between surface temperature and convective rain frequency in the study area. Examining the convective rain frequency distribution map in conjunction with an elevation map of the area indicates that higher elevation is a better predictor of increased frequency of convective rainfall than is surface temperature. Because this indicates that orographic precipitation has a large influence in the study area, we used an elevation map to exclude hilly and mountainous regions, regions bordering flat areas (under the assumption that wind could easily move orographic precipitation to such areas), and regions containing marine areas. Doing so resulted in a coefficient of determination of 0.38, a clear signal that differences in the thermal environment in the Greater Tokyo Area have an effect on the frequency of convective precipitation. We next focused on metropolitan Tokyo, the most developed part of the region and the part experiencing the most frequent occurrences of convective precipitation, and we performed correlation analysis considering parameters related to buildings. Results indicate that orographic precipitation has a strong influence in metropolitan Tokyo as well, so we excluded those areas that were excluded from the Greater Tokyo Area analysis and again performed correlation analysis. However, we found no clear relation of convective precipitation frequency with surface temperature or building parameters.
Highlights
Cities are spaces dedicated to human activity, and their unchecked growth has resulted in numerous problems
Comparison with a topographic map created with Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM) shows that these areas have relatively high altitude (Figure 3), suggesting that rainfall in these areas is not convective precipitation due to an urban thermal environment, but rather orographic precipitation due to geographically induced updrafts
To investigate whether urban heat islands (UHIs) affect mesoscale circulation in urban areas and contribute to convective precipitation, we performed statistical evaluation by correlation analysis to examine the effect of thermal environment on the frequency of convective precipitation in the Greater Tokyo Area
Summary
Cities are spaces dedicated to human activity, and their unchecked growth has resulted in numerous problems One such problem is the development of the urban heat islands (UHIs). There have been reports of increased urban rainfall in Japan [16,17], and many reports indicating UHIs as a contributing factor [18,19,20,21,22] Japanese megacities such as Tokyo already demonstrate the UHI effect, yet continue to expand to the point of assimilating smaller nearby towns. The present study examines the urban and suburban areas of Tokyo (Figure 1), using statistical evaluation by correlation analysis to examine the effects of thermal environment on convective precipitation frequency. The present study examines summer (July and August) surface temperatures in the daytime (12:00 to 18:00) from 1997 to 2006
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