2011 Volume 120 Issue 2 Pages 341-358
The present study analyzed the influence of the urban heat island (UHI) phenomenon on the local wind system in Tokyo based on observational data acquired during two summers in 2003 and 2004. Data from Tokyo's high density observation system, the Metropolitan Environmental Temperature and Rainfall Observation System (METROS), was used. This study focused on the local wind system at nighttime, which is considered to greatly influence the nocturnal UHI. We first carried out some case studies on days that showed typical diurnal variations of the local wind system. A composite analysis was then conducted based on the full two-summer dataset.
The behavior of the local nocturnal wind system was found to be closely related to the temperature distribution in Tokyo. In the coastal area, the northerly wind was much weaker and lasted for a shorter time than in the northern and central areas of Tokyo. From the relationship between the temperature gradient and the north-south wind speed, a calm zone was predicted around the center of the Tokyo UHI where the nocturnal temperature gradient was small. The existence of this calm zone was confirmed by the case study, and its formation is thought to be caused by wind convergence due to the UHI.
The calm zone was also evident from nighttime to early morning from a composite analysis of eleven days on which typical diurnal wind variations were observed. It also appeared during a composite analysis of 47 weak-wind days (38% of days in July and August over two years) when wind speeds at nighttime were low, suggesting that it is a common summer phenomenon in Tokyo.
