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 ūX⊤ζū, v̄Y⊤v̄, and v̄ . X. the IOP-mean vertical distributions of the frictional force due to subgrid-scale eddies and the rate of kinetic energy transfer (E = -v̄ · 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.