Abstract
This study clarifies the vertical structure of diurnal variation in local wind systems, including sea breezes over the Kanto region, by focusing on temporal changes in the wind system structure caused by large-scale sea-breeze development. Because direct observation of the atmosphere's vertical motion is difficult, we developed an alternative index in which the quantity of divergence is calculated using observed surface wind data. However, observed surface wind data are inappropriate for calculating divergence fields because of differences in the observation heights of wind at different stations. Therefore, a roughness parameter corresponding to the wind direction at each observation point is estimated from land utilization data around the observation points based on the empirical formula proposed by Kuwagata and Kondo (1990). The adjustment of wind velocity is based on the logarithmic law of wind velocity. After adjustment, the wind velocity in the area near Tokyo Bay and absolute values of the quantity of divergence increased.
Typical sea-breeze days were selected, and temporal changes in the characteristics of the average wind and divergence fields were examined. Next, correlation coefficients of the quantity of divergence were calculated between marked divergence areas and each grid point in the study area. This analysis was performed to understand the relationship between the divergence and convergence areas and to identify areas where the diurnal variation in the divergence value is equal to that of marked divergence areas. At 09:00, a divergence area formed in Tokyo Bay (area TB), and sea-breeze circulation was observed between area TB and the convergence area in the neighborhood of Tokyo. At 11:00, with sea-breeze development, the influence of the Sagami Bay sea breeze in area TB weakened the correlation with the convergence area of Tokyo. Conversely, a valley wind developed in North Kanto, and valley wind circulation was observed between the divergence area around the prefectural border between Gunma and Saitama (area GS) and the convergence area of a neighboring mountainous district. It appears that typical valley wind circulation developed on a comparatively small scale; moreover, at that time, the valley wind circulation of North Kanto appeared to be independent of the seabreeze circulation of South Kanto. After midday, the original correlation level with area TB was observed in the area between Kanagawa and Sagami Bay, which suggested a switch from the typical sea-breeze circulation to a large-scale sea breeze. With this large-scale sea-breeze development, the valley wind circulation in North Kanto became indistinct. Simultaneously, area TB developed a negative correlation with area GS. When the large-scale sea-breeze development was marked, the divergence in area GS corresponding to valley wind circulation weakened.
