A Study of Multiscale Processes in Near-Global Aquaplanet Cloud-Resolving Models: From Shallow Cumulus Cloud Feedbacks to Tropical Cyclogenesis Predictability

Abstract

A near-global aquaplanet cloud-resolving models (NGAqua) has been used to study the responses of clouds and circulations to 4 K sea-surface temperature (SST) warming, CO2 quadrupling, and both combined. NGAqua spans a large tropical channel domain, with 4 km horizontal resolution, latitudinally dependent 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. Limited-area cloud-resolving model (CRM) simulations are used to reproduce and under- stand negative subtropical shallow cumulus cloud feedbacks in NGAqua. 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 as- pects 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. Furthermore, the analysis of the BL liquid virtual static energy suggests that if shallow cumulus clouds were deeper and rained more, warmer SST may result in increased precipitation rather than increased cloud cover. Topical cyclogenesis is a multiscale process that involves interactions between large-scale circulations and small-scale tropical convection. NGAqua simulations can produce tropical cyclones (TCs) when the SST distribution is shifted northward such that its maximum is off-equatorial. In this case, the TCs spin up spontaneously from the northern edge of the Intertropical Convergence Zone (ITCZ). The large-scale flows provide necessary conditions for barotropic instability and predetermine where the pre-TC vortices will be found. The predictability of this process is up to ten days. The ITCZ is also a rich source for atmo- spheric column-integrated moisture, which is important of the growth of tropical convection, a process that is less predictable. A Lagrangian-framework analysis shows that vertical stretching of absolute vorticity associated with moist convective updrafts contributes posi- tively to the absolute vorticity tendency of a TC. A compositing analysis suggests that the vorticity stretching is crucial for the spin up. But pre-TC vortices do not develop into TCs if they are situated over regions of strong horizontal moisture gradients, because the vertical stretching does not align with the centers of the cyclonic vorticity anomalies.