Diurnal cycles of cloud and rainfall over North-East Queensland during the coral bleaching season

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Abstract
Mass coral bleaching events are becoming increasingly frequent over the Great Barrier Reef (GBR), posing a critical risk to Australia's marine ecosystems and the broader global ocean environment. These events are primarily driven by anomalously warm water temperatures, but their severity is strongly influenced by local cloud cover, which controls the amount of solar radiation reaching the ocean surface (including ultra-violet radiation which exacerbates bleaching). This study presents a characterization of the cloud and rainfall diurnal cycles over north-east Queensland during the coral bleaching season, providing a foundational step to untangling the complex relationships between clouds, rainfall, local-scale processes and the surface energy budget for this climate-sensitive region. Leveraging high-resolution Himawari-8 satellite brightness temperature data, C-band radar observations, and BARRA-R2 regional reanalysis, a multi-year analysis is conducted across three representative zones: coastal land, coastal ocean and open ocean. Results show that diurnal cycles vary distinctly by region and are strongly modulated by prevailing wind regimes. In general, westerly regimes are associated with clear skies and stronger daytime heating over the coastal ocean with enhanced convection over coastal land. In contrast, the frequently observed southeasterlies lead to relatively weaker development over the land and ocean. Cloud and rainfall maxima exhibit out-of-phase behavior between land and ocean, with rainfall often preceding cold cloud tops, indicative that cold brightness temperatures frequently correspond to decaying anvils rather than active convection. Latitudinal and topographic differences contribute to more intense convection near Cairns than Townsville. Variations in inland versus offshore propagation speeds further highlight regional complexity. Our findings emphasize the need for high-resolution simulations to better understand the convection initiation and propagation processes that shape cloud cover and rainfall variability over the GBR during the coral bleaching season, when cloud-radiation interactions may influence reef heat stress.
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Funding Information
  • Australian Research Council (DP230100639)
  • Australian Research Council (CE170100023)
  • Australian Research Council (CE230100012)

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