CLOUD-RESOLVING MODELING OF TROPICAL CLOUD SYSTEMS DURING PHASE-III OF GATE .1. 2-DIMENSIONAL EXPERIMENTS

A formal framework is established for the way in which cloud-resolving numerical models are used to investigate the role of precipitating cl oud systems in climate and weather forecasting models. Emphasis is on models with periodic lateral boundary conditions that eliminate unreal istic numerically generated circulations caused by open boundary condi tions in long-term simulations. Defined in this formalism is the conce pt of large-scale forcing and the cloud-environment interactions that are consistent with the periodic boundary conditions. Two-dimensional numerical simulations of the evolution of cloud systems during 1-7 Sep tember 1974 in phase III of the Global Atmospheric Research Program At lantic Tropical Experiment (GATE) are conducted. Based on the above fo rmalism, a simple technique is used to force an anelastic cloud-resolv ing model with evolving large-scale horizontal wind field and large-sc ale forcing for the temperature and moisture obtained from the GATE da ta. The 7-day period selected is characterized by transitions of the c loud systems through several regimes, in response to evolving large-sc ale forcing and vertical wind shear as an easterly wave passes over th e region. The observed nonsquall cloud clusters, squall lines (squall clusters), and scattered convection are all simulated. Model-produced budgets of heat and moisture compare well with GATE observations. It i s argued that differences between simulations and observations (most a pparent in the relative humidity) result from the treatment of condens ed water. In particular, the lack of observed fields to prescribe forc ing for the upper-tropospheric ice, together with the periodic lateral boundary conditions, results in a middle and upper troposphere that i s too moist by 10%-20%. A key conclusion is that tropical convection, forced in a simple way by large-scale analysis, is sorted into specifi c regimes as a result of dynamic control by the wind shear. The quanti fication of this large-scale control is fundamental to the concept of convective parameterization. Furthermore, the cloud-resolving model re sults by design satisfy the large-scale budgets and, therefore, can be applied directly to the strategic problem of convective parameterizat ion in weather forecasting and climate models.