A Study of the Response of Deep Tropical Clouds to Mesoscale Processes: Three-Dimensional Numerical Experiments

Abstract

A three-dimensional numerical cloud ensemble model has been developed to investigate the collective feedback effects of cloud systems on the large-scale environment. An observed large-scale lifting is imposed continuously in the model. Small amplitude random perturbations in the form of temperature fluctuations are also continuously fed into the model at low levels to simulate random thermals over the tropical ocean. The model allows several clouds of various sizes to develop simultaneously inside the domain. An integration of the model is made for six hours of simulated time in order to allow large numbers of convective clouds to develop. Following each simulation, the collective feedbacks of cloud systems on the large-scale temperature, moisture and horizontal momentum fields are computed. Horizontal and time averages of various relevant variables are also computed to elucidate the statistical properties of the clouds. The model was applied to a case of a well-defined ITCZ rainband over the eastern tropical Atlantic ocean. Nine simulations are made under the same large-scale conditions. The location, number and configuration of the clouds that form in the model are usually different in each of the nine simulations, but after an hour or two all simulated clouds assume a band structure instead of being randomly distributed. The orientations of the bands resemble the observations, and the bands are aligned along the direction of the lower tropospheric wind shear. The differences among these simulations on the cloud heating and drying effects are small. The model results for the total heating and moistening effects are also in fairly good agreement with those estimated from observations. The vertical transports of v-momentum (parallel to the simulated rainband) are essentially the same in all of the simulations, but the vertical transports of u-momentum (normal to the rainband) are quite different in some of these simulations. The physical process involved is the generation of horizontal momentum by the pressure gradient force in the momentum equation. This generated horizontal momentum can be selectively transported vertically by clouds. In order to examine whether different large-scale forcing, environmental wind shear or microphysical processes result in different cloud ensembles or different cloud heating and moistening profiles, three additional experiments are studied. The simulated clouds developed randomly instead of assuming a preferred elongation in a case of no vertical wind shear. No collective vertical transports of horizontal momentum by clouds occur in this case. It is also found that the collective cloud feedback effects on temperature and moisture are sensitive to both magnitude of lifting and cloud microphysical processes. A comparison of the three-dimensional model simulation with a two-dimensional simulation is also made.