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

Abstract

Convective momentum transport (CMT) associated with the Madden?Julian oscillation (MJO), tropical waves, squall and nonsquall mesoscale convective systems (MCSs), and the diurnal cycle is studied by examining the momentum budget residual X = (X, Y) deduced from the objectively analyzed in situ observations during the Tropical Ocean Global Atmosphere Coupled Ocean?Atmosphere Response Experiment (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 |X| and ITBB (an index for deep convection), averaged over the intensive flux array (IFA), is analyzed. Signals of disturbances with periods ≥1 day in |X| generally evolve in phase with those in ITBB. During the convective phase of MJO, signals in both |X| and ITBB with shorter periods are also enhanced. Frequency distribution of IFA-mean E = ?v?·?X in the troposphere is examined. The mean E is positive, that is, kinetic energy (K) transfer is downscale, about 60%?65% of time in the lower troposphere below 500 hPa, and between 200 hPa and the tropopause. However, in the upper troposphere, between 350?200 hPa, upscale and downscale K transfers occur with nearly equal frequency. Different frequency distributions near the surface, the middle troposphere, and near the tropopause suggest the existence of different regimes of K transfer associated with various convective and boundary layer 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 2.5° ? 2.5° objective analysis. Couplets of vorticity and vorticity budget residual Z appear in the upper levels with nonsquall MCSs. Upscale K transfer is found in the line-normal direction of a squall line. During the westerly wind phase of the MJO, convection appears to play dual roles. First, as the westerlies are initiated in the lower troposphere, CMT is typically upgradient and may help maintain middle-level easterly shear. Thus 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.