Abstract
As a first attempt to establish a quantitative method of predicting the rainfall pattern of small scale, the effect of topography is investigated in detail based on the numerical prediction method. The calculation is carried out by dividing the atmosphere into two layers; one above 850 mb, the other below it. The vertical velocity for the upper layer is computed by the ω-equation under the assumption that the vertical distribution of ω is parabolic. For the lower layer, only the vertical motion due to the orographic effect is considered. By multiplying the water vapor condensation function to these vertical velocities and summing up the results, the amount of rainfall at the ground surface is calculated.
Grid distances used for the calculation are 300 km for the upper layer and 10 km for the lower layer respectively.
The method is tested for the case of typhoon Kitty which passedover Kanto District from Aug. 31 to Sept. 1, 1949. The result is satisfactory as far as the general feature of the distribution of rainfall is concerned. But the correlation coefficient between 24-hr. forecast and the observed amount of rainfall is 0.63 and indicating that the result is not satisfactory yet. To get better result, the effects of surface friction and of isallobaric wind should be considered.
