The influence of convective parameterisation on sea breeze dynamics and convection initiation over the Maritime Continent

The Maritime Continent consists of a collection of complex islands that span the equator, including Malaysia, Indonesia and the north coast of Australia (90-160°E, 15°S-15°N). It has become known as the “boiler-box” due to the warm sea surface temperatures, the frequent and intense deep convection and its remote influences through mechanisms such as the Madden-Julian Oscillation. The complex coastlines and orography of the islands are known to play a significant role in the large-scale organisation of the convective storms. Despite the region's global importance, climate and NWP models struggle to resolve mechanisms such as the diurnal variation of convection, which are key for adequate representations of the region.

This study uses three multi-year global Met Office Unified Model simulations run with 12 km grid-spacing to investigate how the parameterisation of convection influences sea breeze dynamics and convection initiation over the Maritime region. 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 model statistics over the entire monsoon period in a model that is more able to resolve the coastlines and orography of the islands than a standard-resolution climate model. The diurnal cycle of rainfall in the model configuration with parameterised convection peaks too early in the day over land compared to the simulations with explicit deep convection, in agreement with previous studies. The formation of strong sea breezes requires a strong temperature contrast between adjacent land and ocean. In the simulation with parameterised convection the early peak in rainfall wets and cools the land surface during the late morning, decreasing the land-sea contrast and thus the sea breeze strength relative to the other two model configurations. Reducing the sea breeze strength reduces the likelihood of convection at the correct time of day in the simulation with parameterised convection. This result has implications for all regions of the world where sea breeze dynamics are important.