Quasi-Equilibrium States in the Tropics Simulated by a Cloud-Resolving Model. Part I: Specific Features and Budget Analysis

Abstract

A series of long-term integrations using the two-dimensional Goddard Cumulus Ensemble (GCE) model were performed with various imposed environmental components. Vertical wind shear, minimum surface wind speed (only used for computing surface fluxes), and radiation are found to be the three major components that determine the quasi-equilibrium temperature and water vapor fields simulated in this study. The genesis of a warm/wet quasi-equilibrium state is mainly due to either strong vertical wind shear along with strong surface winds or large surface fluxes, while a colder/drier quasi-equilibrium state is due to weak (mixed wind) shear along with weak surface winds. Latent heat flux and net large-scale temperature forcing dominate the beginning stages of the simulated convective systems, then considerably weaken in the final stages leading to quasi-equilibrium states. Radiation is necessary in establishing the quasi-equilibrium states but is not crucial to the considerable variation between them. A warmer/wetter thermodynamic state is found to produce more rainfall, as convective clouds are the leading source of rainfall over stratiform clouds even though they occupy much less area. Convective clouds are more likely to occur in the presence of strong surface winds (latent heat flux), while stratiform clouds (especially the well-organized type) are favored in conditions with strong wind shear (net large-scale forcing). The convective systems, which consist of distinct cloud types due to the variation in horizontal winds, are also found to propagate differently. Convective systems with mixed-wind shear generally propagate in the direction of shear, while systems with strong, multidirectional wind shear propagate in a more complex way. Cloud-scale eddies are found to transfer the heat and moisture vertically and assist in balancing the heat (Q1) and moisture (Q2) budgets and in reaching a quasi-equilibrium state. Atmospheric stability, CAPE, and mass fluxes are also investigated and compared between the various quasi-equilibrium states.