Convection over the Pacific warm pool in relation to the atmospheric Kelvin-Rossby wave

Deep convection over the western tropical Pacific warm pool is analyzed in terms of its relation to the atmosphere Kelvin-Rossby wave, which dominates the large-scale Row during the austral summer. The study uses Doppler rada r data collected by aircraft and ship radars during different rime periods in the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response E xperiment to characterize the mesoscale circulations si organized convectiv e cloud systems occurring throughout the season. The study focuses on conve ction in two contrasting phases of the wave: the "westerly onset region" Ju st west of the point within the wave where low-level easterlies change to w esterlies, and the "strong westerly region" (or "westerly wind burst") lyin g between the large-scale counterrotating eyres of the Kelvin-Rossby wave. In the westerly onset region the zonal wind component had midlevel easterli es overlying low-level westerlies. In the strong westerly region a deep lay er of westerlies extended from the surface up to the upper troposphere. wit h a maximum of westerly component at about the 850-mb level. The different vertical shear of the zonal wind In these two regions of the wave led to di fferent momentum transport by the mesoscale circulations that develop into very large "super convective systems" (cloud tops colder than -65 degrees C over regions of similar to 300 km or more in lateral dimension). The super convective systems developed strong midlevel inflow jets. The direction of the jet was determined by the environmental shear. which in turn was deter mined by the dynamics of the large-scale wave. In the westerly onset region , the large-scale shear determined that the jet had an easterly component. In the strong westerly region, the jet bad a westerly component. In both ca ses, the inflow intensified within the cloud system as the convective cells of the super convective system filled a broad region with a deep stratifor m Ice cloud, from which ice particles fell. Evidently, as the particles sub limated and melted, they cooled the air at midlevels in the cloud system. T he cooling evidently modified the mesoscale pressure field in the system so as to accelerate the Row of ambient air into the system and to encourage t he inflow to subside. In this way, the mesoscale inflow to super convective systems transported easterly momentum downward in the westerly onset regio n and westerly momentum downward in the strong westerly region, so that the mesoscale momentum feedback of the mesoscale inflow jets were negative in the westerly onset region and positive in the strong westerly region (accel erating the westerly wind burst). These momentum transports by the broad me soscale midlevel inflow of super convective systems affected broad horizont al regions and were sometimes different in sign from the momentum transport s of individual convective-scale cells in the same system.