Numerical experiments on the weak-wind region formed ahead of the sea-breeze front

A weak-wind region, whose horizontal scale is comparable to that of an urban area, is occasionally formed ahead of the sea-breeze front, over a large coastal urban area (Yoshikado and Kondo 1989; Yoshikado 1990; Ohashi and Kida 2001). In this study, the characteristics and the formation mechanism of the weak-wind region were investigated, using a 2D mesoscale atmospheric model. The following results were obtained: The weak-wind region is created by the development of the heat-island circulation over the urban area; the heat-island circulation (i.e., the urbanward pressure-gradient-force) weakens the inlandward ambient wind during the morning hours. At that time, the sea-breeze circulation is inessential in creating this weak-wind region. The weak-wind region cannot be created under the influence of the seaward ambient wind. Subsequently, the weak-wind region is persistently formed ahead of the sea-breeze front, and gradually moves inlandward. A long-term balance between the urbanward pressure-gradient-force, and the turbulent mixing, causes such a phenomenon. Thus, interactions among the sea-breeze circulation, heat-island circulation, and inlandward ambient wind play an important role in the above processes. The spatial scale of the weak-wind region strongly depends on the urban size (width), i.e., the spatial scale of the heat-island circulation developing over the urban area, during the daytime hours.