Convective Systems of the North Australian Monsoon

Abstract

The climatology of convection over northern Australia and the surrounding oceans, based on six wet seasons (September?April), is derived from the Japanese Meteorological Agency Geostationary Meteorological Satellite-5 (GMS-5) IR1 channel for the years from 1995/96 to 2000/01. This is the first multiyear study of this kind. Clouds are identified at two cloud-top temperature thresholds: 235 and 208 K. The annual cycle of cloudiness over northern Australia shows an initial (October?November) buildup over the Darwin region before widespread cloudiness develops over the entire region during the monsoon months (December?February), followed by a northward contraction during March and April. Tracking mesoscale convective systems (MCSs) reveals that both the size of the cloud systems and their lifetimes follow power-law distributions. For short-lived MCSs (less than 12 h), the initial expansion of the cloudy area is related to the lifetime, with mergers important for long-lived MCSs (greater than 24 h). During periods of deep zonal flow, which coincide with the active phase of the monsoon, the number of convective elements in the Darwin region peaks in the early afternoon, which is characteristic of the diurnal cycle over land. In contrast, when the zonal flow is deep and easterly and the monsoon is in a break phase, the areal extent of the convective elements in the Darwin region is greatest in the late morning, which is more typical of maritime convection.