PARAMETRIZATION OF MOMENTUM TRANSPORT BY CONVECTION .1. THEORY AND CLOUD MODELING RESULTS

The theory of the mass-flux approach to parametrization of convective momentum-transport is presented. A cloud-resolving model is used to si mulate momentum transport by ensembles of deep convective clouds in tw o very different regimes: a mid-latitude cold-air outbreak,, and typic al convection forced by convergence. Idealized, unidirectional, wind p rofiles are used to simplify interpretation of the results. Diagnostic s relevant to the parametrization problem are presented, and it is sho wn that, for both regimes, the approximations inherent in the parametr ization equations are reasonable. The pressure gradients inside clouds play an important role in determining horizontal velocities in the cl ouds. For the cold-air outbreak with linear shear, the results suggest that these pressure gradients are proportional to the shear and to th e up/downdraught mass-fluxes. Sensitivity studies suggest that the res ults are not very sensitive to resolution or parametrization of subgri d-scale processes, giving some confidence that the results are reasona bly accurate. For the tropical case with a low-level jet, the pressure gradients change sign with the wind shear. In part II of this paper, these results will be used to develop and validate a mass-flux paramet rization of convective momentum-transport which is tested in single co lumn and global versions of the Meteorological Office Unified Model.