Relationships among Structures, Development Processes, and Heating Profiles for Two Mesoscale Convective Systems in Inactive Phase of a Large-Scale Disturbance over Northern Australia during the Southern Summer in 1998-1999

Abstract

The relationships among structures, development processes, and diabatic heating profiles of a maritime mesoscale convective system (MCS) and a continental MCS in the inactive phase of a large-scale disturbance associated with Madden-Julian Oscillation (MJO) were studied mainly using dual-Doppler radar data around Darwin, Australia during the southern summer in 1998-1999. The maritime MCS (15 January 1999) formed in a west to southwesterly flow of moist air at the lowmid levels during the monsoon active regime, and had several convective lines oriented parallel to the mid-level shear in 1-2 hours cycle within an extensive stratiform region. Radar analyses revealed that a new convective line formed on a convergence between the outflow of a weak downdraft within the stratiform precipitation and the moist westerly in the low-level, and developed more deeply within a mid-level moist rear inflow. A cyclonic circulation (mesovortex) in the mid-level was subsequently found in the stratiform region. The analysis of heating profiles is indicative of a convective heating maximum at 6 km and a stratiform region with large heating and relatively small cooling above and below the melting level, respectively. The developments of the convective line and mesovortex are attributed to the small low-level cooling and convergence of moist air with the large mid-level heating, respectively, and both are considered to play a significant role in the maintenance of the MCS. The continental MCS (21 January 1999) formed in a strong mid-level east-southeasterly flow of dry air that was influenced by tropical depressions during the break regime. Radar analyses showed that the MCS formed a number of convective lines oriented parallel to low- and mid-level shears with relatively strong negative vortices and convergence in the low-mid level. Positive vorticity were found with a descending rear inflow in the rear of the lines and the stratiform region of the MCS below the mid-level. The heating profiles represented a convective heating maximum at 5.5 km, and a moderate stratiform cooling below the melting level. It is suggested that the stratiform cooling due to the mid-level dryness made a contribution to the advection of positive vorticity below about 5 km, resulting in the development of convective lines with a intensive vortex couplet. A comparison of the heating profiles in the stratiform parts of two MCSs indicated a larger low-level cooling in the continental MCS and a larger heating at the mid-upper level in the maritime MCS. It was suggested that the different stratiform heating of two MCSs in the inactive phase of the MJO played significant roles in their structures and development processes.