NOTES AND CORRESPONDENCE Convective Eddy Momentum Tendencies in Long Cloud-Resolving Model Simulations

Abstract

Domain-average momentum budgets are examined in several multiday cloud-resolving model simulations of deep tropical convection in realistic shears. The convective eddy momentum tendency F, neglected in many global circulation models, looks broadly similar in two- and three-dimensional simulations. It has a large component in quadrature with the mean wind profile, tending to cause momentum profile features to descend. This component opposes, and exceeds in magnitude, the corresponding large-scale vertical advective tendency, which would tend to make features ascend in convecting regions. The portion of F in phase with the mean wind is isolated by vertically integrating F?·?u, yielding a kinetic energy tendency that is overwhelmingly negative. The variation of this energy damping with shear flow kinetic energy and convection intensity (measured by rain rate) gives a ?cumulus friction? coefficient around ?40% to ?80% per centimeter of rain in 3D runs. Large scatter reflects the effects of varying convective organization. Two-dimensional runs overestimate this friction coefficient for the ? (out of plane) wind component and underestimate it for the u (in plane) component. Another 2D artifact is that 460-hPa-wavelength shear is essentially undamped, consistent with the descending jets reported by Held et al. in a free-running 2D cloud model.