Exploring Impacts of Size-Dependent Evaporation and Entrainment in a Global Model

Abstract

While most observations indicate well-buffered clouds to aerosol perturbations, global models do not. Among the suggested mechanisms for this discrepancy is the models' lack of connections between cloud droplet size and two processes that can contribute to reduced cloudiness when droplets become more numerous and smaller: evaporation and entrainment. In this study, we explore different implementations of size-dependent evaporation and entrainment in the global atmospheric model CAM5.3-Oslo.We study their impact on the preindustrial-to-present day change in liquid water path (LWPPD-PI) and the corresponding aerosol indirect effect (AIEPD-PI). Impacts of the 2014–2015 fissure eruption in Holuhraun, Iceland, are also presented. Our entrainment modifications only have a moderate effect on AIEPD-PI (changes from −1.07Wm−2 to −0.98Wm−2), and a small impact on the signal from the Holuhraun eruption compared to other suggested compensating mechanisms. Simulations with added size-dependent evaporation in the top of the stratiform clouds also show small evaporation differences between PI and PD. Moderate changes in AIEPD-PI were achieved when also including an entrainment feedback to the evaporation changes, mixing air between the cloudtop layer and the layer above. These changes were not associated with the size dependency, but changes in the cloud susceptibility to aerosols in both PI and PD when adding evaporation.We find that increased evaporation of smaller droplets at stratiform cloud tops can reduce LWP, but can increase LWP in some areas due to enhanced shallow convection caused by destabilization.