Convective eddy momentum tendencies in long cloud-resolving model simulations

Domain-average momentum budgets are examined in several multiday cloud-reso lving model simulations of deep tropical convection in realistic shears. Th e convective eddy momentum tendency F, neglected in many global circulation models, looks broadly similar in two- and three-dimensional simulations. I t has a large component in quadrature with the mean wind profile, tending t o cause momentum profile features to descend. This component opposes, and e xceeds in magnitude, the corresponding large-scale vertical advective tende ncy, which would tend to make features ascend in convecting regions. The po rtion of F in phase with the mean wind is isolated by vertically integratin g F . u, yielding a kinetic energy tendency that is overwhelmingly negative . The variation of this energy damping with shear flow kinetic energy and c onvection intensity (measured by rain rate) gives a "cumulus friction'' coe fficient around -40% to -80% per centimeter of rain in 3D runs. Large scatt er reflects the effects of varying convective organization. Two-dimensional runs overestimate this friction coefficient for the upsilon (out of plane) wind component and underestimate it for the u (in plane) component. Anothe r 2D artifact is that 460-hPa-wavelength shear is essentially undamped, con sistent with the descending jets reported by Held et al. in a free-running 2D cloud model.