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ABSTRACT
Intense summer heat in Tokyo, Japan, and its suburban areas between 1990 and 2010 was statistically analyzed. Sample days were selected from among days with a sea breeze and sufficient sunshine duration, because sea breeze is the dominant summertime meteorological system in the region. Excess in the daily maximum temperature of an inland urban site in Tokyo over a site facing the outer sea where the sea breeze originates was selected as a key index. A comparison of a group of days for which the temperature excess was large with an opposing group of days for which it was small revealed a distinct difference in diurnal wind variations: persistent southwesterly winds overwhelmed the sea breeze in areas facing the outer sea in the former case, whereas in the latter case a diurnal sea-breeze cycle was typically observed in those areas as well as in the inland areas. This difference could furthermore be connected to differences in the pressure gradient in the region, that is, differences in the synoptic-scale pressure field. As a result, slight shifts in the pressure field affect urban heat, suggesting that, in addition to general warming, changes in the pressure field resulting from future climate change can be of great importance.
1. Introduction
Increased air temperature in urban areas compared with surrounding rural areas is a well-known phenomenon called the urban heat island (UHI). This phenomenon is most clearly observed in large cities. In the case of Tokyo, the annual average temperature has increased at a rate of 3.28C (100 yr)21 for the period between 1931 and 2011 (JMA 2012). For the same period, 17 observatories in remote small towns recorded an average temperature increase of 1.58C (100 yr)21. This increase is considered to be mainly a response to global climate change in the Japan region, and the difference in Tokyo's temperature is attributed to the UHI effect.
Special attention is paid to daily maximum temperatures Tmax in summer because the peak electric power demand for air conditioning is directly dependent on it. Also, there is a marked increase in the number of cases of heat stroke in hot summers. Global warming will possibly aggravate these circumstances, even if the UHI effects remain at the present levels. The response of...