Convective aggregation in idealised models: moving towards the real world

Thesis

Great advances in our understanding of the climate have been made through utilising a hierarchy of models. In idealised models of the tropical climate, deep convection can spontaneously cluster together, a phenomenon termed convective self-aggregation. This clustering impacts the moisture and energy budgets and thus significant work has been undertaken to gain a more complete understanding of self-aggregation and its impacts on the climate. In particular, self-aggregation and convective organisation in the real world present quite differently. Thus, a key open question is how self-aggregation manifests in the real atmosphere. This thesis probes this question by incrementally increasing the complexity of a general circulation model in a radiative-convective equilibrium configuration to investigate the impact of an aerosol plume, a land continent, and a meridional sea surface temperature (SST) gradient on self-aggregation.

The diabatic heating from the aerosol plume generates a large-scale, overturning circulation, which forces convection to aggregate. However, despite the aggregation being forced, self-generated feedbacks between clouds and longwave radiative fluxes are essential for amplifying the aggregation. Thus, I postulate that self-aggregation feedbacks could contribute to real-world convective organisation, even when aggregation is initially driven by external forcing. This hypothesis is reinforced when investigating the impacts of land. A land-sea thermal contrast triggers a circulation which forces convection to aggregate, but as before, self-aggregation feedbacks are necessary for this to be maintained. Finally, I develop a novel budget for the variance in transient integrated frozen moist static energy. This is used to show that self-aggregation feedbacks also drive zonal organisation over a meridional SST gradient. This thesis consistently highlights that self-aggregation feedbacks could be contributing to real-world convective organisation, whilst also showing that the inclusion of more realistic processes can fundamentally alter the geometry, spatial and temporal scales of self-aggregation.

Authors: Dingley, B

• DOI: 10.5287/ora-kzzvwqrpb
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: aerosols; idealised modelling; convective organisation; climate model hierarchy; clouds; convective aggregation; tropical convection
• Subjects: Atmospheric models; Clouds