Warming from cold pools: A pathway for mesoscale organization to alter Earth's radiation budget

TL;DR

This study demonstrates that mesoscale processes, especially cold pools, significantly influence Earth's radiation budget by affecting cloud organization and moisture distribution, which has implications for climate modeling accuracy.

Contribution

It reveals how the balance between mesoscale moisture aggregation and cold pools impacts radiative budgets, emphasizing the importance of mesoscale dynamics in climate models.

Findings

Cold pools reduce mesoscale ascent and moisture aggregation.

Suppressing cold pools increases net rainfall and boundary layer moistening.

Warming effect of approximately 1.88 W/m2 due to altered mesoscale processes.

Abstract

Marine shallow cumulus clouds have long caused large uncertainty in climate projections. These clouds frequently organize into mesoscale (10-500 km) structures, through two processes that couple the clouds to shallow mesoscale circulations: (i) mesoscale moisture aggregation, and (ii) cold pools, driven locally from rain-evaporation. Since global climate models do not capture these mesoscale processes, while the degree of mesoscale organization is observed to correlate to shortwave cooling, it has been suggested that mesoscale processes modulate the cloud response to global warming. Here, we show that introducing mesoscale dynamics can indeed substantially alter top-of-the-atmosphere radiative budget, if the balance between the two circulations is upset. By homogenizing rain-evaporation across the horizontal domain, we suppress the cold-pool-driven circulations in a large ensemble of large-domain, large-eddy simulations. We find that cold pools reduce mesoscale ascent, thereby arresting a runaway self-aggregation of moisture into very moist regions. This reduces the net rainfall of the cumulus fields, moistens the boundary layer and thus reduces the emission of clear-sky longwave radiation to space, giving an ensemble-averaged warming of 1.88 W/m2. Our results highlight that the proper interplay between mesoscale processes is critical for capturing radiative budgets-especially in kilometer-scale climate models that only partially resolve aggregation and cold pools.