Cloud-circulation coupling in convection-permitting simulations

Understanding of cloud-circulation coupling in a warming world is underpinned by global climate models (GCMs) with coarse horizontal resolutions necessitating the use of convective parameterizations. Global convection-permitting models are now emerging, but their high computational cost is a barrier to their use for studies of climate change. 

Here, we present results from 2-year atmosphere-only, limited domain simulations at a horizontal resolution of ~4.5km over a region of the west Pacific using the Met Office Unified Model. The limited-area model is driven at the lateral boundaries by an existing ~25km global model. Two simulations are analysed: a control run with present-day SSTs and a perturbed run with a prescribed SST warming of approximately 4K. 

There are substantial differences in cloud-circulation coupling between the high-resolution simulations and the global driving model. In particular, we find – on average – large cloud radiative effects (CREs) associated with strongly subsiding gridpoints, in comparison to the relatively small CREs for this circulation regime as expected from previous GCM studies and indeed in the global driving model. We demonstrate that this systematic difference in subsiding CRE between models arises from the existence of complex circulation structures in the high-resolution simulations, which are absent in the global simulations. For the highest (>80%) percentiles of column relative humidity, subsiding gridpoints have O(5 Wm^-2) weaker cooling compared to ascending gridpoints with similar column relative humidity. We discuss how this strong subsidence regime responds to warming, and potential implications for cloud feedbacks.