An integrated view of the 1987 Australin monsoon and its mesoscale convective systems. II: Vertical structure

AbstractThe vertical structure of monsoon thermal forcing by precipitating convection is diagnosed in terms of horizontal divergence. Airborne Doppler‐radar divergence profiles from nine diverse mesoscale convective systems (MCSs) are presented. The MCSs consisted of multicellular convective elements which in time gave rise to areas of stratiform precipitation. Each of the three basic building blocks of the MCSs—convective, intermediary, and stratiform precipitation areas—has a consistent, characteristics divergence profile. Convective areas have low‐level convergence, with its peak at 2‐4 km altitude, and divergence above 6 km. Intermediatry areas have convergence aloft, peaked near 10 km, feeding into mean ascent high in the upper troposphere. Stratiform areas have mid‐level convergence, indicating a measoscale downdraught below the melting level, and a mososcale updraught aloft.Rawinsonde composite divergence profiles agree with the Doppler data in t least one important respect: the lower‐tropospheric convergence into the MCSs peaks 2‐4 km above the surface. Rawinsonde vorticity profiles show that monsoonal tropical cyclones spin‐up at these elevated levels first, then later descend to the surface. Rawinsonde observations on a larger, continental scale demonstrate that a large horizontal scales only the ‘gravest vertical mode’ of MCS heating is felt, while the effects of shallower components of the heating (or divergence) profiles are trapped near the heating, as predicted by geostrophic adjustment theory.