Tokyo—one of the largest cities in the world—exhibits intensive urban land use, urban sprawl, and high anthropogenic heat exhaustion, which produce characteristic urban temperature distributions. Typical features of the nocturnal temperature distribution from November to February under conditions of clear skies and weak winds were analyzed for the Tokyo metropolitan-ward area using data from spatially high-density observations from the Automated Meteorological Data Acquisition System (AMeDAS) of the Japan Meteorological Agency (JMA), municipal air pollution monitoring systems, and Extended-METROS from 2006/2007 to 2009/2010. To analyze the structure of temperature distributions in the Tokyo-ward area, hourly temperature data at 124 stations were interpolated into grid points with 1-km intervals.
When high temperatures in central Tokyo were expressed as a difference in AMeDAS temperature between central Tokyo (Otemachi) and an average of four points outside the Tokyo-ward area, the weather conditions most strongly affecting this temperature difference at 06:00 Japan Standard Time (JST), prior to sunrise, were cloud amount followed by wind speed. The temperature difference at night under clear-sky and weak-wind conditions (average cloud amount of 2/10 or less and average wind speed of 3m/s or less) generally showed an abrupt increase during the several hours before and after sunset followed by a gradual increase, reaching maximum levels before sunrise (06:00 JST). When the temperature difference before sunrise was large, the difference would evidently increase even after midnight; however, if this difference was small, it would tend to decrease after midnight.
The high temperatures in the metropolitan area were concentrated throughout the night near the southern area of Chuo ward. In particular, it is noted that several zones of steep horizontal temperature gradients were detected in the metropolitan-ward area. These zones (e.g. starting from the southern part of Saitama Prefecture, it appeared through the border between Nerima and Itabashi wards, Toshima, Shinjuku, and the area stretching from western Setagaya to the southwest) appeared during the period several hours before and after sunset, and gradually became more noticeable near sunrise. The distribution of temperature-decrease rate around sunset also showed steep horizontal gradient zones, which corresponded to regions exhibiting steep horizontal temperature gradient zones between the inner city and outer area.
