Vertical fluxes of momentum due to cumulus convection are a significant but poorly understood contributor to atmospheric circulations, particularly in the tropics and subtropics. Reliable observations of cumulus momentum flux are scarce. It is very difficult to judge the accuracy of the widely varying parameterizations of cumulus momentum fluxes currently used in different global weather and climate models. Tropical convection also exhibits many types of mesoscale organization. An important issue for parameterization is whether the cumulus momentum fluxes can be predicted adequately without knowing the mesoscale organization of the convection.

This project will bolster observational knowledge of cumulus momentum fluxes using three extensive and complementary new data sets. Specifically, momentum fluxes will be computed (1) as budget residuals in global reanalyses for the TOGA-COARE region, other convective regimes, and as zonal averages, (ii) from an extensive set of dual-Doppler airborne radar analyses of tropical mesoscale convective systems from TOGA-COARE, and (iii) from output from cloud ensemble model simulations of tropical convection. The TOGA-COARE experiment is particularly promising to study because the persistent deep convection coupled to the large temporal variations in vertical wind shear can be expected to produce large and easily detectable changes in the cumulus momentum fluxes.

The goals of the project are to quantify the significance of cumulus momentum fluxes on large scale transient and mean circulations in the tropics, particularly the intraseasonal oscillation, and (ii) to determine the relative importance of the circulations in the convective versus stratiform region of mesoscale convective systems in producing the momentum fluxes, in a variety of wind shear regimes, and (iii) to compare the results to several different parameterizations that have been proposed.

These analyses should help to improve the performance of weather and climate models in the tropics.

National Science Foundation (NSF)
Division of Atmospheric and Geospace Sciences (AGS)
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Pamela L. Stephens
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University of Washington
United States
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