Joint Centre for Mesoscale Meteorology (JCMM)
JCMM INTERNAL REPORT NO.54
Parametrization of momentum transport by convection. I: Theory and cloud modelling results
by R Kershaw and D Gregory
The theory of the mass flux approach to parametrization of convective momentum transport is presented. A cloud-resolving model is used to simulate momentum transport by ensembles of deep convective clouds in two very different regimes: a mid-latitude cold-air outbreak and tropical convection forced by convergence. Idealised, uni-directional, wind profiles are used to simplify interpretation of the results. diagnostics relevant to the parametrization problem are presented, and it is shown that, for both regimes, the approximations inherent in the parametrization equations are reasonable. The in-cloud pressure gradients play an important role in determining the in-cloud horizontal velocities. For the cold-air outbreak case with linear shear, the results suggest that these pressure gradients are proportional to the shear and to the up/downdraught mass fluxes. Sensitivity studies suggest that the results are not very sensitive to resolution or parametrization of subgridscale processes, giving some confidence that the results are reasonably accurate. For the tropical case with the low-level jet, the pressure gradients change sign with the wind shear, and the strength depends on the organization of the convection. Both down-gradient and up-gradient momentum transports are simulated. In part II of this paper, these results are used to develop and validate a mass-flux [parametrization of convective momentum transport which is tested in single column and global versions of the UK Met Office Unified Model.