Daytime Boundary Layer Evolution in a Deep Valley. Part II: Numerical Simulation of the Cross-Valley Circulation

Abstract

The thermally induced circulation in a deep valley during fair weather and weak synoptic wind conditions is simulated by a two-dimensional numerical model, in order to investigate the daytime planetary boundary layer evolution observed in the Ina Valley, a deep, two-dimensional valley in Japan. The numerical model can simulate the observed structure of the PBL fairly well, along with the daytime variations of the observed valley surface air temperature and surface pressure. The numerical simulations suggest that the thermally induced cross-valley circulation creates a two-layer PBL structure. That is, a turbulent mixed layer develops due to sensible heating from the surface, reaching to heights of about 500?1000 m above the valley floor, while a quasi?mixed layer is formed above the turbulent mixed layer by the heat transport of the cross-valley circulation. The quasi?mixed layer is a new feature of the PBL. The upper limit of the quasi?mixed layer corresponds to the top of the cross-valley circulation, being somewhat higher than both sides of the mountains. The quasi?mixed layer can be clearly distinguished during the daytime in a deep valley having a depth of greater than about 1500 m. Since the quasi?mixed layer has a slightly stable stratification, the magnitude of the coefficient of vertical turbulence in this layer is much less than that in the turbulent mixed layer. The results of the simulations reveal that the thermally induced cross-valley circulation transports heat from the mountainous regions to the central part of the valley, while water vapor is transported in the opposite manner. The potential temperature becomes horizontally uniform during the afternoon, except in the shallow layer of the upslope flow along the side slopes. On the other hand, the daytime distribution of specific humidity in the valley is rather complex, being affected not only by the cross-valley circulation, but also by the ambient wind along the direction of the cross valley. Water vapor tends to be accumulated over the mountainous regions during the daytime, resulting in the formation of cumulus clouds. Visible images observed by the NOAA satellite confirm the development of cumulus clouds over the mountainous regions in the Ina Valley during the afternoon.