Impacts of Urban Heat Island Circulation and Thermal on Fair-Weather Cumulus Clouds Forming over the Urban Area

  • Yuta KUNO, 2013: Impacts of Urban Heat Island Circulation and Thermal on Fair-Weather Cumulus Clouds Forming over the Urban Area .

Abstract

In this paper, effect of time variation of flow containing both urban heat island circulation (UHIC) and thermal that generate fair-weather cumulus clouds over urban area is investigated by numerical experiments. The numerical experiments are conducted using the WRF model in large eddy simulation mode (WRF-LES) with 50 m horizontal resolution.

  The results indicate that generation of fair weather cumulus clouds has a time variation. At first, fair-weather cumulus clouds occur sporadically over the urban area. Then, with time progresses, fair-weather cumulus clouds start to appear intensively at the central part of the urban area. Moreover, in the grass land which corresponds to downward flow region of UHIC, area with no fair-weather cumulus clouds expands with time progress. On the other hand, in the grass land away from the urban area which is not effected by the downward flow of UHIC, it fair-weather cumulus clouds occur sporadically due to thermal.

  Furthermore, to investigate the effect each from UHIC and thermal in generating fair-weather cumulus clouds, UHIC and thermal are distinguished according to the difference in their horizontal scales. The results are as follows: (1) During the time when fair-weather cumulus clouds appear sporadically over the urban area, these clouds over the central part of urban area mainly  occur  by  thermal.  (2)  During  the  time  when  fair-weather  cumulus  clouds  occur intensively at the central part of the urban area, UHIC is suggested to intensify generation of fair weather cumulus over the central part of urban area, by moving thermal to the central part of urban area. (3) UHIC weakens the updrafts of thermal at the grass land near the urban area. (4) UHIC lowers cloud formation level of the fair-weather cumulus over the urban area.

  Moreover, since the development of UHIC was influenced by the scale and width of urban areas, sensitivity experiment with different widths of the urban area are conducted. The results are as follows: (1) The narrower the urban width is, the more fair-weather cumulus clouds occur over the grass land near the urban area. (2) The narrower the urban width is, the more sequence-like fair-weather cumulus clouds generate early. (3) The wider the urban is, the longer the period when fair-weather cumulus clouds over the central part of urban area mainly occur by thermal.

  Key Words:fair-weather cumulus clouds, thermal, urban heat island circulation, WRF-LES