The Australian monsoon and its mesoscale convective systems

Abstract

The 1987 Australian monsoon was observed with satellites, rawinsondes, radar and aircraft. These data are presented, with theory filling the gaps, in illustration of its dynamics.The engine of the monsoon is its embedded mesoscale convective systems (MCSs). Ten MCSs were explored with airborne Doppler radar. They all exhibited multicellular convection, in lines or arcs along the edges of cold pools, aging and evolving into areas of stratiform precipitation. This temporal evolution can be divided into three stages: "convective," "intermediary," and "stratiform." Doppler radar divergence profiles for each stage show remarkable consistency from one MCS to the next.Convective areas had low-level convergence, with its peak elevated off the surface, and divergence above $\sim$8 km altitude. Intermediary areas had very little divergence through the lower troposphere, but strong convergence near 10 km altitude, associated with upper-tropospheric ascent. Stratiform areas had midlevel convergence between divergent layers. These divergence profiles indicate thermal forcing of the monsoon by the convection, in a form more useful than heating profiles.The response of the atmosphere to thermal forcing is considered in chapter 2. Thermal disturbances travel through a stratified fluid at a speed proportional to their vertical depth. A heat source with complex vertical structure excites disturbances ("buoyancy bores"), of many depths, that separate themselves out with distance from the heat source. Hence the deeper components of a heat source can be found at greater distances from the heat source, at any given moment and also in the limit of long time in a rotating or dissipative fluid.Low-level dynamical processes initiate deep convection within the active cyclonic areas of the monsoon trough, despite the warm core aloft and the consequent (small) decrease in CAPE. In 1987, four tropical cyclones were generated in the monsoon by this runaway positive feedback loop.Two forcing terms appear in the vorticity equation, representing the two dynamical effects of MCSs. Vertical momentum transport causes small-scale vortex pairs in the upper troposphere, while net MCS divergence "stretches" vorticity at many scales. Divergence and vorticity profiles averaged over all north Australia show only the "gravest mode" vertical structure, while profiles in the smaller Gulf of Carpentaria, near the convective heat source, indicate finer vertical structure.