Abstract
The distribution of mass transfer coefficient (k) around two dimensional models was discussed in the preceding paper. This time, the effects of air flow around cuboid model and regularly arranged city-block models on the ground evaporation are investigated in the same manner (using the salinity change technique). Many model experiments on air flow in built-up area have been made previously, but none of the reports clarified its effect on surface flux, especially latent heat flux at all. The distribution of k around cuboid model shows complex pattern in accord with air flow, and cannot be explained well by only mean and turbulent velocities near the surface (Fig. 3 and 6). When cuboid models are arranged continuously in the direction of stream, the conspicuous variation of k is restricted within first three array and seems to be constant after that (Fig. 8). Regarding city-block models, the local distribution of k on the street is more complex, but some systematic tendencies are revealed (Fig. 10). As a genenal, k is large in peripheral part of built-up area, especially wind-ward, and decreases gradually to the center of it. However, in detail, k is larger at cross-street than other part of street (Fig. 10 and 11). Averaging over the lot, k is proportional to mean and turbulent velocities near the surface in spite of the differences of model height, model size, and arrangement pattern (Fig. 21). In the case of equal height, the building coverage dose not affect on averaged k value seriously, within limits of this experiment (Fig. 19). On the contrary, the change of k is remarkable when city-block consists of the models unequal height (Fig. 23). Therefor, the vertical variation of surface component have a large effect on the turbulent diffusivity in urban area compared to its horizontal density.
