Convective Momentum Transport over the Tropical Pacific: Budget Estimates

Abstract

ECMWF and NCEP?NCAR reanalyses are used to calculate convective momentum transport (CMT) as a momentum budget residual over several tropical oceanic convective regions, including the TOGA COARE Intensive Flux Array (IFA). Sources of uncertainty are quantified through intercomparison, and methods for minimizing uncertainty are discussed. The authors examine how the environmental shear of the time-mean, and time-varying, flow in these regions is modified by CMT, and compare the budget results with some parameterizations. The zonal residual is a significant term in the time-mean reanalysis budgets in all regions below 850 mb. Since the subcloud turbulent layer typically does not extend above 940 mb, this suggests a possibly important role for CMT by shallow convection. In the rest of the troposphere, the residuals are small, but a slight tendency for downgradient transport of zonal momentum, and smoothing of the environmental wind profile, is suggested. The main budget uncertainties associated with pressure gradient forces and horizontal advection are substantially reduced by considering a larger domain, and by averaging over long periods. The Gregory et al. CMT parameterization scheme shows qualitative agreement with the time-mean budget residuals in all regions above 850 mb. The Schneider and Lindzen scheme shows poorer agreement. Neither parameterization captures the residual accelerations seen below 850 mb.