The effects of land-use alteration on the sea breeze and daytime heat island in the Tokyo metropolitan area

The changes of a sea breeze and a daytime heat island due to land-use alteration during an 85 year period (1900-1985) have been numerically simulated. The domain of interest is the Kanto Plain (15000 km(2)), including the Tokyo metropolitan area. This urban area is located in the southern part of the plain and consists of many cities in Tokyo and its suburbs. The horizontal scale of the area is about 40 km and has increased by a factor of four during the 85 year period. The simulations were conducted under a summer synoptic condition with weak gradient wind and almost clear sky. The model is based on the three-dimensional anelastic equations, taking into account the hydrostatic assumption. First, it was confirmed that the simulated wind field and temperature distribution with using the land-use data for 1985, agreed with observed data. The simulations were then conducted using the land-use data for 1950 and 1900. From comparison among the three simulations, the following two major conclusions were obtained: (1) Land-use alteration modified the wind system over the Kanto Plain. In particular, the simulated sea breeze front in 1985 was more clearly defined around the northern end of the Tokyo metropolitan area. The time required for the sea breezes to reach inland areas increased by two hours. (2) The warming due to land-use alteration is found over the Tokyo metropolitan area and the northwestern part of the Kanto Plain. In particular, the area of the most prominent warming is found in the northern end of the Tokyo metropolitan area. Intensity of daytime heat island in the area were estimated as 3-4 degrees C and 2-3 degrees C during the 85 year period, and latest 35 years respectively. The above warming is confirmed to result from the enhanced sensible heat flux and the change of interaction between the boundary layer heating and sea breeze front.