The Impact of Thermally Induced Local Circulation on Summertime Precipitation in Kinki District

  • Fuki KUDO, 2013: The Impact of Thermally Induced Local Circulation on Summertime Precipitation in Kinki District .

Abstract

The present study utilizes statistical analysis and numerical experiment to examine how thermally induced local circulation affects summertime precipitation in Kinki district, where mountains and sea are very close to urban area. From statistical analysis, the distribution of radar echo frequency shows clear temporal variation in summertime weak wind days; high frequency areas are found over the mountains during 12:00-18:00, and lower elevation area of southern Kyoto prefecture during 15:00-18:00. Moreover, in summertime westerly wind days, radar echo frequency around southern Kyoto prefecture tends to be relatively higher than other areas. In order to investigate the influence of thermally induced circulation to the temporal variation of precipitation, three-dimensional idealized numerical experiments are conducted under generally calm summer day condition. The simulated distribution of precipitation has clear temporal variation, which corresponds to high radar echo frequency area. From sensitive experiments with different terrain and land-use, it is indicated that precipitations over mountain during 12:00-15:00 occurs without ocean. This is because valley wind develops in early afternoon and precipitable water vapor (PWV) increases around mountain area in the afternoon. On the other hand, sea breeze tends to develop later than valley wind, transporting moisture to inland of Kinki district. It is considered that precipitation during 15:00-18:00 is mainly influenced by transporting moisture of sea breeze. Moreover, urban heat island in Osaka tends to shift convergence zone of sea breeze toward Osaka city. It also increases moisture above urban area, and causes an increase of evening precipitations near Osaka city. Furthermore, as a result of sensitive experiments with different initial wind condition, westerly wind tends to strengthen northerly surface wind from Japan Sea. It converges with westerly wind from Osaka bay, and increase precipitations around southern Kyoto prefecture.

Key Words: precipitation, thermally induced circulation, sea breeze, valley wind, urban heat island, radar, numerical experiment