Mechanisms for convection initiation over North-East Australia

The mechanisms for the initiation of convection over the Cape York Peninsula in North-East Australia are poorly understood. Several previous studies have documented the daily development of sea breezes on the west and east coast of the peninsula within the prevailing easterly flow. These collide during the late afternoon and are responsible for the formation of atmospheric phenomena such as the Morning Glory and the North Australian cloud line. During the pre and early monsoon period (November to mid-January) the initiation of deep convection over the peninsula coincides with the collision of the sea breezes. Although the sea breezes form every day, the convective storms occur much less frequently and it is unknown what additional factors are necessary for their formation.

This study uses year-long global Met Office Unified Model simulations run with 12 km grid-spacing, satellite retrievals of wind and rainfall and ERA-Interim data to investigate this problem. The model simulations are unique in that they are run with a particularly small grid-spacing for a global simulation and there are three configurations which vary only in their representation of convection; the first has conventional parameterised convection, the second only parameterises shallow convection and the third allows all convection to develop explicitly. This set-up allows analysis of a model over the entire pre-monsoon period that is able to resolve the peninsula adequately. It appears that the collision of the sea breezes provides the necessary uplift to trigger deep convection. This however only occurs when the low-level winds to the west of the peninsula are north-westerly, which strengthens the west coast sea breeze and transports moisture into the region from the north. The model configurations with explicit deep convection produce a much more realistic diurnal cycle of convection, which interacts with other aspects of the model, such as the sea-breeze. These differences are also investigated.