An Integrated View of the 1987 Australian Monsoon and Its Mesoscale Convective Systems. I: Horizontal Structure

AbstractThis observational study is organized around three issues: the forcing of the monsoon flow by embedded deep convection, the thermodynamic conditions that support the convection, and the structures of the mesoscale convective systems (MCSs) that constitute much of the deep convection.The monsoon convective cloudiness occurred predominantly offshore, over the earth's warmest waters. Week‐long periods of widespread deep convection caused cycles of monsoon spin‐up, culminating in tropical‐cyclone formation. Momentum transports by the convection also created smaller‐scale vortex pairs in the upper troposphere.Sounding data suggest that the convection was modulated by low‐level processes, not by large‐scale deep forced ascent. Triggering by mesoscale boundary‐layer cold‐pool boundaries and coastlines determined specifically where convection occurred, while several positive feedbacks acted to keep regional thermodynamic conditions favourable. Hence monsoon convection, once initiated, persisted as self‐exciting ‘superclusters’, composed at any instant of many distinct MCSs.The observed MCSs all contained areas of deep convection and stratiform precipitation areas. Stratiform precipitation usually evolved in place from convective cells, although precipitation also occasionally fell from overhanging anvils created by upper‐level shear. the MCSs within similar synoptic wind environments tended to have similar mesoscale structures. This fact reflects the effects, illustrated herein, of environmental winds upon the relative motions of cold pools (which trigger new convective cells) and stratiform precipitation areas (which evolve from old convective cells).