Timescale Matters to Aerosol Effects on Deep Convective Clouds
Modeling studies have indicated a wide range of potential responses of deep convective storms to increased aerosol loading. Some studies indicate that cloud systems will be invigorated under polluted conditions due to delayed warm-rain formation, greater lofting of cloud water in convective updrafts and enhanced freezing, latent heating, and buoyancy. However, these studies modeled cloud systems using short integration periods, often only a few hours, which are not necessarily applicable to the response of deep convection over large spatiotemporal scales because they neglect feedbacks between convection and its large-scale environment. To address this problem, Department of Energy researchers conducted simulations with a cloud-system-resolving model over a 7.5-day period of active monsoon convection, using environmental conditions observed during the Atmospheric Radiation Measurement (ARM) Tropical Warm Pool-International Cloud Experiment (TWP-ICE). To test the convective invigoration effect, heating perturbations were applied to convective updrafts above the freezing level to mimic the impact of enhanced freezing and ice processes on latent heating in polluted conditions. Unlike previous studies, heating perturbations were applied in updrafts and cooling in downdrafts to give no net change in moist static energy. This allows for an unambiguous evaluation of the impact of convective-scale heating perturbations in the context of feedback and adjustment with the larger-scale environment. In the perturbed simulations, there was an initial invigoration of convective updrafts and surface precipitation, but convection returned to its unperturbed state after about 24 hours because of feedback with the larger-scale environment. In contrast to recent studies, the researchers concluded that the invigoration effect is intimately coupled with larger-scale dynamics through a two-way feedback, and, in the absence of alterations in the larger-scale circulation, there is limited invigoration beyond the convective adjustment time scale. These results have important implications for understanding the effects of aerosol changes on tropical cloud systems; however, additional studies addressing midlatitude clouds and allowing feedbacks between the convection and large-scale forcing are needed before the question of aerosol impacts on deep convection can be fully resolved.
Morrison, H., and W. W. Grabowski. 2013. “Response of Tropical Deep Convection to Localized Heating Perturbations: Implications for Aerosol-Induced Convective Invigoration,” Journal of the Atmospheric Sciences 70, 3533-55. DOI:10.1175/JAS-D-13-027.1.