Climate Change Response of Tropical Atlantic Clouds in a Kilometer‐Resolution Model

Abstract

AbstractEstimates of the tropical cloud feedback of global climate models (GCMs) show a large inter‐model spread due to the small‐scale nature of convective cloud processes. Estimates from large‐eddy simulations (LES) are more consistent among themselves, but difficult to scale to the climate system. Here we consider a compromise between GCMs and LES, and study how tropical clouds over the Atlantic respond to a climate perturbation in a regional kilometer‐resolution model. We perform two 4‐year‐long simulations at 3.3 km horizontal grid spacing with the limited‐area model COSMO on a 9,000 × 7,000 km2 domain covering the tropical Atlantic: a control simulation and a climate change simulation using the pseudo‐global warming approach. In a previous publication we have demonstrated that this approach yields a credible representation of the tropical climate without the double‐ITCZ bias commonly seen in GCMs. Here we address the cloud feedback and find a reduction of ITCZ high‐cloud cover resulting in a negative longwave cloud feedback similar to the CMIP6 ensemble mean. We find a reduction of stratocumulus clouds, which we argue is primarily a thermodynamic response. More surprisingly, the shallow cumulus cloud cover over the West Atlantic increases, which we argue is due to increased stability resulting in weaker entrainment and a more humid boundary layer. The mean shortwave cloud feedback over the tropical Atlantic is positive, comparable to the CMIP6 ensemble mean. The emerging estimate of the total cloud‐radiative feedback over the Atlantic is slightly negative, consistent with CMIP6 GCMs, but it is substantially stronger than that of the driving GCM.