Genesis and Evolution of Hierarchical Cloud Clusters in a Two-Dimensional Cumulus-Resolving Model

Abstract

A two-dimensional cloud ensemble model is integrated over a basin-scale domain with prescribed sea surface temperature (SST), to study the formation and evolution of cloud clusters over a large-scale warm pool. Neither a basic zonal flow is prescribed nor is a single perturbation initially given. The results show that deep convective clouds appear in hierarchical clustered patterns and are limited to the area of warm SST above 28°C. The most fundamental cloud cluster in the model has a horizontal scale of a few hundred kilometers, in which new cumulus clouds are generated at the leading edge of a propagating surface cold-air pool—the “gust front.” It may last for days and propagate for a long distance if the background flow is broad and persistent as is the case in the low-level convergence zone of the SST-induced background flow. The largest hierarchical propagating cloud systems in the model have horizontal scales up to 3000 km and consist of up to four cloud clusters that are generally of gust front type. The constituent cloud clusters are generated intermittently and have life spans of 12–36 h. The internal heating of the constituent clusters collectively induces an overall troposphere-deep gravity wave. The overall wave travels in the direction of the tropospheric deep shear at a speed determined by the thermodynamic asymmetry in the wave created by the transition from warm and moist incoming air in the front to drier and cooler air in the rear. The development of new cumulus clusters in the front region of the hierarchical system is due to the combined effect of the overall wave and the gravity waves excited by the constituent clusters on the lower-tropospheric stability. When there are no interruptions from outside the cloud system, new cloud clusters developed intermittently from shallow disturbances hundreds of kilometers ahead of the existing deep convection. The resulting hierarchical cloud pattern resembles the observed equatorial super cloud cluster (SCC) in the time–longitude diagram. However, the life spans of the constituent clusters of the system are shorter than that in the observed SCC.