2000 Volume 35 Issue 6 Pages 355-367
Industries and high population densities develop in costal areas, SLB (sea and land breezes), which act as a kind of local circulation. As a result, air pollution is a serious problem in coast areas. In order to understand overall flow fields of SLB accompanying sea breeze fronts, the flow in a water tank apparatus was visualized with styrofoam particles as tracers, and video images were taken. We calculated wind velocity vectors from the movement of tracer patterns between the visualized image, and were able to detect overall flow fields of SLB. We also carried out numerical calculations using a non-hydrostatic model, in which a two-dimensional laminar layer and Boussinesq approximation was assumed. The followipg phenomena were observed in the water tank experiment:(1) the vertical mixingflow was shown to be Benard type convection, and sea breezes crossed on to land from the coastline when the land temperature rose;(2) weak land breezes blew when land temperatures decreased. These characteristics observed in the water tank experiment were reproduced by numerical calculation. Using the universal function for SLB proposed by Ueda, a maximum wind speed of 1mm/s obtained in the water tank experiment was equivalent to 4.8m/s in field observations, and a mixing layer height of 10mm obtained in the water tank experiment was equivalent to 560m in field observations. From these results, we postulate that the water tank experiment can simulate actual SLB.
Next, we investigated the influence on mass diffusion of the flow fields using both non-hydrostatic and hydrostatic models. The non-hydrostatic model was able to reproduce Benard type convection, sea breeze front, and the strong perturbation of vertical flow that were observed in the water tank experiment. Temperature fall occurring at the time of the sea breeze front passage was also reproduced. But the hydrostatic model was not able to reproduce these characteristics of SLB. However there was no significant difference between the maximum mass concentration calculated by the mass diffusion simulation using either flow field. But the time to maximum concentration was delayed as the point was kept away from the point source, when the hydrostatic flow field model was used.
