Numerical experiments on the formation of urban climate under various geographical conditions

Abstract:

Geographical conditions make a specific local circulation for each condition. The circulation around urban area affects a diurnal change of urban atmosphere, and then urban warming intensifies or weakens depending on the geographical condition. Comparing the effects on the diurnal change, those on the climatological change as an urban climate are insufficiency investigated. Understanding the geographical effects can be considered as one of the necessary factors to generalize urban climate. Therefore, in this study, formations of urban climate were represented for an inland city, a coastal city and a basin city using ideal experiments and were compared among them to access the effects of geographical conditions on urban climate.

For the formation of urban climate in an inland city, the mixing layer grows up as the urbanization makes progress in the night, and thus, the temperature change seems to be become small. The tendency of temperature rises during the nighttime decelerates with developing of urbanization. By contrast, the tendency during the daytime accelerates because it takes time for a heat island circulation to reach inside of the urban area when the city expands. For the formation in a coastal city, the temperature rise amount is smaller than that of the inland city. Sea and land breeze play an important role in the trend of urban climate in the coastal city. Thickness of sea breeze decides the height of mixing layer over the city during the day, especially in summer. The rise of daily maximum temperature accelerates. For the formation in a basin city, strong stable layer exists over the urban area throughout a day and the development of mixing layer by heat from the urban constricts in the layer. The tendency of temperature rise accelerates. The geographical effects appear most significantly on the daily minimum temperature. The temperature difference between the coastal city and the inland city is larger than that between the basin city and the inland city for all stage of urban development. The trends of daily mean temperature rise show a deceleration for the inland city and the coastal city and acceleration for the basin city.