Cold pools mediate mesoscale adjustments of trade-cumulus fields to changes in cloud-droplet number concentration

Abstract. The mesoscale self-organization of trade-cumulus cloud fields is a major cloud-climate uncertainty. Cold pools, i.e. pockets of cold, dense air resulting from rain evaporation, are a key mechanism in shaping these dynamics and are controlled by the large-scale forcing. We study the microphysical sensitivity of cloud-field self-organization through cold pools by varying cloud-droplet number concentration N_c from 20 to 1000 /cm³ in large-eddy simulations on large 154×154 km²-domains. We find that cold pools exhibit two distinct regimes of mesoscale self-organization. In very low-N_c conditions, cold pools transition from a stage where they are small and randomly distributed to forming large, long-lived structures that perpetuate due to the collisions of cold pools at their fronts. Under high-N_c conditions, cold pools display strongly intermittent behaviour and interact with clouds through small, short-lived structures. While N_c thus influences the number of cold pools and, in turn, mesoscale organization, cloud depth, and cloud albedo, we find its effect on cloud cover to be minimal. Comparing the microphysical sensitivity of cold-pool-mediated mesoscale dynamics to the external, large-scale forcing shows that N_c is as important as horizontal wind and large-scale subsidence for trade-cumulus albedo. Our results highlight that cold pools mediate adjustments of trade-cumulus cloud fields to changes in N_c. Such mesoscale adjustments need to be considered if we are to better constrain the effective aerosol forcing and cloud feedback in the trade-wind regime.