Abstract
As a continuation of a previous study (Yamasaki, 1975), several numerical experiments are performed to investigate the properties of a CISK mode which is not essentially affected by surface friction. A set of equations used for time integrations includes equations for cloud water and rainwater with parameterized cloud physical processes such as autoconversion, collection and so on. The horizontal grid size is taken to be 200m in the convective area so that individual cumulus clouds may be properly described. For the save of computational time we adopt an initial condition such that formations of cumulus clouds are limited in a small area with a scale of only several tens of kilometers. The depth of the conditionally unstable layer is taken to be shallow so that the vorticity of the disturbance, which develops through its interaction with cumulus clouds, does not become large enough to cause appreciable frictional convergence.
Numerical experiments indicate that the advection of momentum as well as the pressure gradient force are important terms in the equation of horizontal motion. The convective transport of momentum acts to suppress rapid intensification of the meridional circulation which is caused by the temperature gradient due to collective effects of convective heating. The Coriolis force has only modifying effects on the growth and structure of the disturbance. Rainfalls in the convective area do not occur randomly but systematically to some extent. In particular, outward propagations of rainfall area are notably found. It is unlikely that cumulus clouds are successively formed when the autoconversion from cloud water to rainwater is not taken into account. It seems that the evaporation and drag force of rainwater play an essential role in the CISK mode under consideration.
The statistical effects of cumulus clouds on the large-scale heat and water vapor budgets as well as the behaviors of individual cumulus clouds are also discussed.
