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

Brian E. MapesNOAA–CIRES Climate Diagnostics Center, Boulder, Colorado

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Xiaoqing WuNational Center for Atmospheric Research, Boulder, Colorado

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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 υ (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.

Corresponding author address: Dr. Brian E. Mapes, NOAA–CIRES Climate Diagnostics Center, R/CDC, 325 S. Broadway, Boulder, CO 80303.

Email: bem@cdc.noaa.gov

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 υ (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.

Corresponding author address: Dr. Brian E. Mapes, NOAA–CIRES Climate Diagnostics Center, R/CDC, 325 S. Broadway, Boulder, CO 80303.

Email: bem@cdc.noaa.gov

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