Brian E. Mapes
R/CDC1 325 S. Broadway, Boulder, Colorado, 80303 USA
brian.mapes@noaa.gov
NOAA-CIRES Climate Diagnostics Center (CDC)
National Center for Atmospheric Research (NCAR)
Domain-average momentum budgets are examined in several multi-day 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 2 and 3 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 which is overwhelmingly negative. The variation of this energy damping with shearflow kinetic energy and convection intensity (measured by rainrate) gives a `cumulus friction' coefficient around -40% to -80% per cm of rain in 3D runs. Large scatter reflects the effects of varying convective organization. Two-dimensional runs overestimate this friction coefficient for the v (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.