Convective momentum transport in 3D cloud resolving model simulations of TOGA COARE

Global climate model studies suggest that the cumulus momentum transport (CMT) in tropical convective cloud systems plays a significant role in the tropical mean circulation and transient variability. CMT is difficult to measure directly and it can depend on the detailed structure and organization of the convection. In global circulation models, CMT must be parameterized as part of the cumulus parameterization. We analyze CMT in a four month 3D CRM simulation of TOGA COARE with 1 km horizontal resolution, 64x64 horizontal gridpoints and 96 vertical levels. By comparing the 64x64 simulation with an otherwise identical 10 day 256x256 simulation, we establish that the CMT is insensitive to the domain size. We partition the CMT into contributions from unsaturated and saturated regions. Underlying most CMT parameterizations is the assumption that the CMT is dominated by large horizontal velocity anomalies in the cumuli. However, we find that the unsaturated regions also contribute significantly to the CMT. The unsaturated CMT comes from a combination of convective downdrafts and vertically propagating gravity waves. It has a quite different vertical structure than the saturated CMT, and should be explicitly predicted in future refinements of CMT parameterizations in global models.