Abstract
A convective shower-cloud is numerically simulated, entrainment and the drag force of raindrops being taken into account, and the initiation of a downdraft in the cloud is studied. Rainfall is considered in the model in accordance with a relation of itss intensity with the water content of raindrops. The relation is in satisfactory agreement with observations and has some evidences on the theoretical basis.
It is concluded that the downdraft is initiated in a cloud by the drag force of raindrops, when the air-temperature in the cloud is still higher than that of the surrounding atmosphere. Further, it is found that in a cloud with larger horizontal size, the updraft and the downdraft are more vigorous, and the downdraft is more rapidly initiated by the accumulation of larger amount of liquid water.
