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

WEN-WEN TUNG AND MICHIO YANAI*
Department of Atmospheric Sciences, University of California, Los Angeles,
Los Angeles, California

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ABSTRACT

The momentum budget residual, , is estimated with objectively analyzed soundings taken during the TOGA-COARE Intensive Observing Period (November 1992-February 1993) to study the effects of convective momentum transport (CMT) over the western Pacific warm pool. The time series of and exhibit multi-scale temporal behavior, showing modulations by the Madden-Julian oscillation (MJO) and other disturbances. The power spectra of , , and (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 and 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 , , and , the IOP-mean vertical distributions of the frictional force due to subgrid-scale eddies and the rate of kinetic energy transfer ( ) 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 ~ 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, i. e., kinetic energy is converted from the large-scale motion to convection and turbulence.

Convective Momentum Transport Observed during the TOGA COARE IOP.
Part II: Case Studies

WEN-WEN TUNG AND MICHIO YANAI*
Department of Atmospheric Sciences, University of California, Los Angeles,
Los Angeles, California

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ABSTRACT

Convective momentum transport (CMT) associated with the Madden-Julian oscillation (MJO), tropical waves, linear and nonlinear mesoscale convective systems (MCSs), and the diurnal cycle is studied by examining the momentum budget residual, , deduced from the objectively analyzed in-situ observations during the TOGA COARE Intensive Observing Period (IOP, November 1992--February 1993). Using wavelet transform, time evolution of signals of these disturbances in the time series of and (an index for deep convection), averaged over the intensive flux array (IFA), are analyzed. Signals of disturbances with day in generally evolve in phase with those in . During the convective phase of MJO, signals in both and with shorter periods are also enhanced. Frequency distribution of IFA-mean in the troposphere is computed. E is positive, i.e., kinetic energy (K) transfer is downscale, in about 60-65% of time in the lower troposphere below 500 hPa. However, in the upper troposphere between 350-200 hPa, upscale and downscale K transfers occur with nearly equal frequency. Distinctly different frequency distributions near the surface, middle troposphere, and near the tropopause suggest the existence of different regimes of K cascades associated with different convective processes. Furthermore, the dependence of the direction of CMT on mesoscale convective organizations documented in many previous observations is found to be detectable at the grid-size of GCMs. Couplets of vorticity and vorticity budget residual, Z, appear in the upper levels with nonlinear MCSs. Upscale K transfer is found in the line-normal direction of a linear MCS. During the westerly wind phase of the MJO, convection plays dual roles. First, as the westerlies are initiated in the lower troposphere, CMT is typically upgradient and helps maintain middle-level easterly shear. The upscale K transfer may help trigger the westerly wind burst (WWB). Second, at the later stage with strong lower-to-middle-level westerlies, CMT is mostly downgradient and reduces the middle-level zonal wind shear. The first role seems to be played by shallower convection and the second by very deep convection.

*Corresponding author address: Dr. Michio Yanai, Department of Atmospheric Sciences,
University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1565
Email: yanai@atmos.ucla.edu