Abstract

An attempt is made to assess the effects of the compensating downward motion associated with the updraft upon the development of a cumulus cloud. The model consists of two circular concentric air columns: the inside column (core) corresponds to the updraft region (cloud area) and the outside conc... Show moreAn attempt is made to assess the effects of the compensating downward motion associated with the updraft upon the development of a cumulus cloud. The model consists of two circular concentric air columns: the inside column (core) corresponds to the updraft region (cloud area) and the outside concentric annular column corresponds to the downward motion region (cloudless area). The combined cell is surrounded by the atmosphere at rest. The governing equations of both the updraft and the compensating downward motion are derived from the conservation equations of momentum, heat, moisture and mass. The set of differential equations is solved numerically to show how the vertical velocity, temperature, specific humidity and liquid water content in and out of the cloud area will change with height and time, and to examine the effect of the downward motion upon the development of the cloud. The main results are the following: When the environmental atmosphere is relatively dry and without a steady source of moisture, tall clouds never develop. The compensating downward motion seems to have little influence on the evolution of the cloud in this case. When the environmental atmosphere is relatively moist, the structure of the single column updraft tends to a steady state, if we neglect the effect of the compensating downward motion. However, with the compensating downward motion, no tall cloud is maintained unless there is a steady source of moisture at the cloud base. It is apparent that the compensating downward motion acts as a "break" which prevents the maintenance of a tall cumulus cloud. Show less