On Convective Clouds at Owase in the Cases of Heavy Rain

Abstract

The problem of convective clouds at Owase in the cases of heavy rain is studied by a quasi steady-state one-dimensional jet model of cumulus convection which includes dynamic entrainment and simple cloud physical processes with no glaciation process.In this study the author assumes hypothetically that one of the characteristic mechanisms at Owase in the cases of heavy rain lies in the horizontal convergence at subcloud layer and updraft velocity at cloud base reflects the convergence.The height of cloud and the profiles of many variables obtained from our cloud model depend upon the updraft velocity at cloud base. This property is different from that of modified parcel method or bubble model of cumulus convection.First, to test whether this cloud model correctly predicts the heights reached by clouds the author compares observational results with those obtained from this numerical model on the case in Aug. 30th 1971. By this comparison it is indicated that the coincidence is pretty well. Then, as examples of heavy rain, 51 cases for these ten years are adopted and the numerical computations are repeated. The main characteristics obtained are as follows.(1) The height of most convective clouds is less than 5 km above the ground surface and the ratio which it exceeds 6 km is only 5 to 18 percent of all the convective clouds whose heights exceed 1 km under the assumption with less than 2 m/s of updraft velocity and no excess temperature at cloud base. (2) The frequency distribution of the height of cloud top is rather concentrated in the ranges of 1 to 2 km and 3 to 4 km except very shallow clouds.From these results it is inferred that small scale or shallow convective clouds whose heights are less than 5 km are generated with considerable frequencies around the Owase area in the cases of heavy rain.