Effects of Mountains and Urban Areas on Daytime Local-Circulations in the Osaka and Kyoto Regions

Abstract

In the current study, the effects of topography and urbanization on the daytime local-circulations in the area of the Osaka—Kyoto plain were investigated using a mesoscale atmospheric model. The main results obtained from the numerical and observational data analyses are as follows: 1) The stagnation of the inland penetration of the sea-breeze front that occurs in the peripheral portion of the Tokyo urban area does not clearly occur in the peripheral portion of the Osaka urban area. The stagnation is attributed to the heat-island circulation that develops over the peripheral portion of the urban area. Because the northern peripheral portion of the Osaka urban area faces mountains, the valley circulation weakens the heat-island circulation; the about 600-m mountains surrounding the Osaka and Kyoto urban areas are the most suitable to cause the weakening of the heat-island circulation. The valley circulation also induces the surface-air-temperature increase over the plain, through an adiabatic process due to its downward flow; the temperature increase strengthens the horizontal pressure gradient between the sea-breeze circulation and inland areas. 2) The surface air over the suburban region between the Osaka and Kyoto urban areas remains drier than that over the Osaka and Kyoto urban areas, until the arrival of the sea-breeze front, even though urban areas were thought to be drier than suburban areas. This dryness results from the fact that the valley circulation transports drier air downward from the upper levels over the suburban area, and transports the surface-air moisture supplied from the ground surface into mountainous areas. The valley circulation also appears to play an important role in the transport of pollutants from the suburban area into mountainous areas. 3) A chain flow, which flows downward from the upper levels over the Osaka urban area to the lower levels over the Kyoto urban area, forms ahead of the sea-breeze front. This flow can result in the transport of pollutants emitted from the Osaka urban area into the inland Kyoto area prior to the transport of pollutants by the sea-breeze circulation. Additionally, the chain flow intensifies the downward flow of the valley circulation over the suburban region between the Osaka and Kyoto urban areas.