Vertical motion, diabatic heating, and rainfall characteristics in north Australia convective systems

Abstract

Very-high-frequency wind-profiler data are used to study the vertical draught structure within 13 tropical Mesoscale Convective Systems (MCSs) near Darwin, Australia during the 1989-90 and 1990-91 wet seasons. These studies are supported by single-Doppler radar, soundings, and surface rainfall data to correlate radar reflectivity, thermal buoyancy, and surface rainfall patterns with vertical air-motion structures. Because of Darwin's unique location at the southern tip of the Maritime Continent, vertical draughts in both the monsoon (maritime) and monsoon break (continental) convective regimes can be observed. The break-regime MCSs (six in total) were all squall lines, characterized by a leading line of convection with heavy precipitation and trailing stratiform rainfall containing a characteristic radar bright band. These MCSs exhibited a pronounced life-cycle pattern and were all sampled by the profiler in the mature to dissipating stages. In contrast, the monsoon systems (seven in total) were composed of regions of stratiform cloud with embedded convective bands which moved on-shore in the monsoonal flow. Results from the Darwin rain-gauge network indicated that the majority of the total rainfall in each MCS (break and monsoon) was associated with the convective portion of the system. The break-regime MCSs were all characterized by a low-level (4 km) updraught peak associated with convective cells on the leading edge of each squall line, trailed by deeper convective updraughts in the middle and upper troposphere. For the monsoon cases, the lower-troposphere convective updraughts were typically less than those in the squall lines, yet were stronger in the upper troposphere. The low-level differences in the convective updraughts were consistent with the smaller virtual-temperature excess in the monsoon soundings, as well as the larger vertical radar-reflectivity gradients observed in monsoon convection. Consistent with the differences in vertical air-motion patterns, diabatic heating and moistening profiles for the monsoon MCSs were characterized by a higher-level heating and drying peak compared with the break MCSs. The results have important implications for cumulus parametrizations in numerical models since the large-scale circulation is sensitive to the vertical distribution of diabatic heating in tropical MCSs.