Numerical Experiments Concerning the Origin of Cloud Clusters in the Tropical Atmosphere

Abstract

A large-domain, two-dimensional cloud convection model was used for the purpose of examining the natural properties of cloud convections under idealized conditions: the atmosphere received heat and moisture from an underlying uniformly warm water surface, and, at the same time, the atmosphere was cooled at a constant rate. Five experiments were carried out with different sets of microphysical processes.
Quasi-steady states naturally attained in the experiments showed different spatial and temporal structures of convection. In the case where rain was not generated, a cellular structure similar to the Benard convection appeared. In the case where rain was generated but did not evaporate, there appeared only a single deep narrow cloud whose lifetime was unrealistically long. In the case with the full set of the standard microphysics, there appeared a 'double-scale' structure. That is, a number of deep convective clouds having a horizontal scale of O(1km) and a lifetime of O(1hour) were generated, and these clouds were spontaneously organized to form several cloud systems having a lifetime longer than 10 hours. Each of the cloud systems induced a rainfall over a region having a width of 30-100km during its life cycle.
The principal mechanism for the generation of the double-scale structure in the last case is the formation of a cold air pool at the foot of each deep cloud by rain water evaporation. The formation of cold air limits the lifetime of individual cloud and thus determines the characteristic time scale of the shorter-lived, smaller-scale structure, i.e., individual cloud. At the same time, the cold air pool spreads out in the form of a density current and triggers new clouds at the edges of the pool. In this manner it produces and maintains the longer-lived larger-scale structure, i.e., cloud system.
The double-scale structure which was naturally obtained in the case with the full set of microphysics resembles the double-scale structure of the convection over the earth's tropical ocean; i.e., short-lived, small-scale cumulonimbi are organized to form longer-lived, larger-scale clusters of clouds. Furthermore, the role of the cold air pools in the maintenance of the cloud systems is consistent with that observed in the cloud clusters in the atmosphere. These strongly suggest that the double-scale structure in the earth's tropical atmosphere is the natural form of precipitating cloud convection driven by vertical differential heating. The results of the experiments also show that the origin of the double-scale structure of the tropical convection is the existence of cloud microphysical processes.