Evaluation of urban effect based on the spatial and temporal characteristics of rainfall in Seoul, Korea

The effect of urbanization based on the spatial and temporal characteristics of precipitation in Seoul is investigated. Data used in this study are surface meteorological station data observed in Seoul and 4 cities nearby Seoul for the period of 1961 to 2009 (over 49 years) and 31 Automatic Weather System (AWS) stations data observed in Seoul in August 2004. The yearly precipitation amount during 40 years is increased with time but rainy days are decreased. This explains the daily precipitation intensity has increased. Precipitation amount and rainy days over 20 mm/hr referring as heavy rainfall during 50 years is increased with time. Once the rain is started, the chances for heavy rainfall are increased. The frequency of heavy rainfall has apparently increased since 1990. It is explained that the urbanization affected the intensity of convective precipitation. Heavy rainfall event often occurred in the afternoon and early morning of the day during summer season when the atmosphere was instable due to the approach or the passage of the front. The observed time of heavy rainfall event is advanced from the nighttime (19-24 LST) to late daytime (13-18 LST). From the case study for the period 3 to 6 August 2004 to understand the characteristics of spatial and temporal distribution of precipitation in Seoul, the rain storm was formed by urban heat island in the region of north-east of Seoul (downwind side) between 22-24 LST on 4th August. The localized strengthening of the convection due to the strong urban heat island has caused the precipitation. The analysis result in this study indicate that the substantial changes observed in precipitation in Seoul seem to be linked with the accelerated increase in the urban sprawl in recent decades which in turn has induced an intensification of the urban heat island effect. Therefore, the change of precipitation properties is induced by the urbanization and urban heat island can change local weather and climate.