Local Circulations Developed in the Vicinity of Both Coastal and Inland Urban Areas. Part II: Effects of Urban and Mountain Areas on Moisture Transport

Abstract

The moisture distribution near the ground surface in and around the Japanese cities of Osaka and Kyoto was investigated. From the analysis of observed data, the atmosphere over the suburban areas between coastal Osaka and inland Kyoto was drier than that over Osaka and Kyoto during the daytime hours. This feature differs from results in previous studies and from expectations based on urban and suburban surface heat budgets. To understand the drying mechanism, numerical experiments were performed, using a simplified geometrical model consisting of a straight coastline, a square urban area on the coast, a square inland urban area, and a plateau mountain surrounding the urban areas. The following main results were obtained. First, suburban drying during the daytime was mainly caused by a valley circulation that developed over the surrounding mountain area. In addition, the two heat island circulations that developed over the two urban areas also caused suburban drying. As a consequence, the coexistence of mountain and urban areas caused more notable suburban drying. Second, the amount of suburban drying was greatest when the urban distance was 40‐50 km, which is roughly equal to the actual distance between the Osaka and Kyoto urban areas. Last, temporal changes in moisture and those of suspended particulate matter, SO 2, and NOx concentrations decreased before the arrival of the sea-breeze front. Thus, it is argued that moisture and pollutants were transported by the two heat island circulations that developed over Osaka and Kyoto and by the valley circulations and then were modified by the sea-breeze circulation.