Statistical Characteristics of Convective Storms in Darwin, Northern Australia

Abstract: This M. Sc. thesis studies the statistical characteristics of convective storms in a monsoon regime in Darwin, northern Australia. It has been conducted with the use of radar. Enhanced knowledge of tropical convection is essential in studies of the global climate, and this study aims to bring light on some special characteristics of storms in a tropical environment. The observed behaviour of convective storms can be implemented in the parameterisation of these in cloud-resolving regional and global models. The wet season was subdivided into three regimes; build-up and breaks, the monsoon and the dry monsoon. Using a cell tracking system called TITAN, these regimes were shown to support different storm characteristics in terms of their temporal, spatial and height distributions. The build-up and break storms were seen to be more vigorous and particularly modulated diurnally by sea breezes. The monsoon was dominated by frequent but less intense and vertically less extensive convective cores. The explanation for this could be found in the atmospheric environment, with monsoonal convection having oceanic origins together with a mean upward motion of air through the depth of the troposphere. The dry monsoon was characterised by suppressed convection due to the presence of dry mid-level air. The effects of wind shear on convective line orientations were examined. The results show a diurnal evolution from low-level shear parallel orientations of convective lines to low-level shear perpendicular during build-up and breaks. The monsoon was dominated by complex orientations of convective lines. The thesis includes a study of merged and splitted cells, which have been separated from other storms, and mergers were shown to support more vigorous convection in terms of height distribution and reflectivity profiles. They were also seen to be the most long-lived category of storms as well as the most common type. Split storms were generally weaker, indicative of their general tendency to decay shortly after the split occurred.