Understanding Negative Subtropical Shallow Cumulus Cloud Feedbacks in a Near-Global Aquaplanet Model Using Limited Area Cloud-Resolving Simulations

Limited area cloud-resolving model (CRM) simulations called LASAM are used to reproduce and understand negative subtropical shallow cumulus cloud feedbacks in a near-global aquaplanet CRM (NGAqua) with 4-K sea surface temperature (SST) warming. NGAqua spans a large tropical channel domain, with 4-km horizontal resolution, zonally symmetric equatorially peaked SST, and no cumulus parameterization. Prior work showed that its coarsely resolved shallow cumulus increases with warming. It was suggested that with warmer SST, the moister boundary layer is destabilized by more clear-sky radiative cooling, driving more cumulus convection. A small doubly periodic version of the same CRM is configured to analyze this low cloud increase in a simpler context. It is driven by steady thermodynamic and advective forcing profiles averaged over the driest subtropical column humidity quartile of NGAqua. Sensitivity studies separate effects of radiative cooling and free tropospheric relative humidity changes from other aspects of NGAqua's warmer climate. Enhanced clear-sky radiative cooling explains most of the cloud increase due to SST warming, regardless of CRM model resolution and advection scheme. A boundary layer energy budget shows that the downward entrainment heat flux strengthens to balance enhanced radiative cooling, carried by a stronger updraft cloud mass flux from a larger cumulus cloud fraction. In deeper trade cumulus layers, the enhanced radiative cooling in a warming climate may be balanced by increased precipitation warming, leaving the cloud coverage area almost unchanged. With larger domain sizes, shallow cumulus self-aggregates, especially with higher SST, marginally increasing domain-mean cloud fraction, but this is a secondary contributor to the cloud feedback. Plain Language Summary Cumulus clouds less than 2 km deep are widespread over the subtropical oceans. Hence, even small shallow cumulus cloudiness changes affect how much the underlying oceans are warmed by sunlight and the sensitivity of climate to greenhouse gas increases. Most climate models suggest slightly decreased shallow cumulus cloudiness in a warmer climate. However, a recent study using a near-global model with 4-km horizontal grid spacing, much finer than conventional climate models and capable of simulating individual cumuli, found increased subtropical shallow cumulus cloudiness when the surface is uniformly warmed. We use a simpler version of the same model with a much smaller domain to represent the drier parts of the full model's subtropical oceans, reproduce its cloudiness increase, isolate the possible causes, and show the robustness of this finding. Using this simpler model, we confirm an earlier hypothesis: The cloudiness increase is mainly from more infrared cooling of the cumulus layer that holds more water vapor in a warmer climate. Shallow cumuli are largely driven by this cooling, so more infrared cooling leads to more cumuli. An important caveat is that if the cumuli were deeper so they rained more, warming could lead to more rain rather than more cloud.