Convective Momentum Transport Observed during the TOGA COARE IOP. Part I: General Features

Abstract

The momentum budget residual X = (X, Y) is estimated with objectively analyzed soundings taken during the Tropical Ocean and Global Atmosphere Coupled Ocean?Atmosphere Response Experiment (TOGA COARE) intense observing period (IOP; November 1992?February 1993) to study the effects of convective momentum transport (CMT) over the western Pacific warm pool. The time series of X and Y exhibit multiscale temporal behavior, showing modulations by the Madden?Julian oscillation (MJO) and other disturbances. The power spectra of X, Y, and ITBB (an index of convective activity) are remarkably similar, showing peaks near 10, 4?5, and 2 days, and at the diurnal period, suggesting a link between deep cumulus convection and the acceleration?deceleration of the large-scale horizontal motion, via CMT, which is being modulated by various atmospheric disturbances. The temporal behavior of X and Y can be described as fractals from 1/4 to ?20 and from 1/4 to ?16 days, respectively. Their fractal characteristics are reflected in the very large standard deviations around the small IOP means. From the analyses of the quantities uX/|u|, ?Y/|?|, and v?·?X, the IOP-mean vertical distributions of the frictional force due to subgrid-scale eddies and the rate of kinetic energy transfer (E = ?v?·?X) are determined. The frictional deceleration and downscale energy transfer take place in a deep tropospheric layer from the surface to 300 hPa. In addition, a concentration of large friction and energy transfer exists in a layer just below the tropopause, suggesting the contribution of momentum detrainment from the top of deep cumuli. The IOP-mean frictional deceleration and downscale energy transfer in the lower troposphere are ?0.5?1.0 m s?1 day?1 and ?1.0 ? 10?4 m2 s?3, respectively. The product of eddy momentum flux with the large-scale vertical wind shear shows that the momentum transport is, on the average, downgradient; that is, kinetic energy is converted from the large-scale motion to convection and turbulence.