Regimes of the North Australian wet season

The North Australian Monsoon and its associated cloud systems are studied using satellite data (geostationary and TRMM) and wind and thermodynamic data from Darwin radiosonde ascents. A climatology of convection over Northern Australia and the surrounding oceans based on six wet seasons (September to April) is derived. The annual and diurnal cycle of cloudiness is studied, reflecting the changes in the global circulation and changes between active and break monsoon periods. Mesoscale Convective Systems (MCSs) are sorted into four objectively derived convective system classes using cluster analysis identified by their lifetimes and direction of zonal propagation. Short lived MCSs represent early and late monsoon convection with weak updrafts. Long lived MCSs represent break monsoon MCSs with strong initial updrafts as shown by remotely sensed rainfall and microphysical observations. Differences in the environments in which convection occurs are examined by applying cluster analysis to Darwin radiosonde data. Five objectively derived regimes of the monsoon are obtained and are found to differ significantly in their synoptic environment, cloud patterns and rainfall distributions. One of these regimes is clearly identified with the active monsoon at Darwin and is offered as a definition of monsoon onset. This regime captures the active monsoon environment associated with significant widespread rainfall. Another may be associated with the beginning of the wet season over northern Australia and the onset of significant convective rainfall.