Effects of Sea Breezes on Mitigating Urban Heat Island Phenomenon: Vertical Observation Results in the Urban Center of Sendai

Abstract

 The mitigation of the urban heat island phenomenon by local circulations of land and sea breezes has recently become a subject of major interest. This study aims to clarify the effects of sea breezes on the urban heat island phenomenon in Sendai, Northeast Japan, paying attention to the vertical mixing effects of tall buildings.
 In Sendai city, where the urban heat island phenomenon has developed along with sea breeze circulation, spatially dense observations of temperature were carried out with instrument screens at twenty-five elementary schools in and around the urban area from 2000 to 2004.
 When a sea breeze begins to blow, the air temperature in the coastal region peaks and does not rise during daytime. By comparing the warming quantity during the day when sea breezes do not blow, the cooling effects of the sea breeze are evaluated quantitatively. It was found that cooling effects are remarkable in May and June, and disappear in September. Cooling effects in the urban center do not differ from those in the suburban area, in spite of dense buildings and large number of roughness parameters. Because of the mixing function, the large number of roughness parameters is considered to be useful to pull the cool air mass of sea breezes down to the ground.
 Vertical observations of wind and air temperature at the Miyagi Prefectural headquarters, which is located in the central business district (CBD), were carried out from July 2007 to July 2008. When sea breezes begins to blow, downward air currents were observed at the windward walls of buildings, and the cooling effects of sea breezes were identified gradually from the tops of tall buildings to the ground. The horizontal distributions of air temperature during the day with sea breezes produce relatively cool areas near the coast and in the urban center. The cooling effects of sea breezes appear to be more remarkably in the urban center than in the residential area where there are no tall buildings.