ANALYSES OF VARIANCE AND FLUX BUDGETS IN CUMULUS-TOPPED BOUNDARY-LAYERS

A large-eddy simulation(LES) of a cumulus-topped boundary layer (CTBL) has been made, based on observations gathered on 10 September 1974 du ring the GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment (GATE). On this day there were light winds, making buoyancy production the only important mechanism for generating turbulent kine tic energy. Cumulus clouds with a depth of a few hundred meters covere d about 10% of the sky. A comparison between LES results and observed variances and fluxes will be made. In order to understand the dynamics of the cumulus, we will investigate the physical mechanisms by which variances and fluxes are produced in CTBL's. This will be done by anal yzing the budget equations which describe the time evolution of the va riances and fluxes. The production, transport and destruction terms, t hat appear in these equations, will be calculated explicitly with the LES model. Budgets are calculated from the GATE simulation and two sli ghtly modified GATE simulations, in which only the initial water vapor specific humidity is increased. As a result in these two modified GAT E simulations the vertical extent of the clouds and the cloud cover is larger. In this way, the influence of the cumulus clouds on the budge ts can be investigated more thoroughly. Finally, the budgets are compa red with the results obtained from an earlier simulation based on the Puerto Rico Field Experiment and other results found in the literature .