Aerosol Effects on Cumulus Congestus Population over the Tropical Pacific: A Cloud-Resolving Modeling Study

Abstract

 This study examines the significance of aerosol serving as cloud condensation nuclei (CCN) in modulating strengths of tropical maritime convection. Through a Tropical Ocean Global Atmosphere Couple Ocean-Atmosphere Response Experiment (TOGA COARE) case study using a cloud-resolving model (the Goddard Cumulus Ensemble Model with a horizontal mesh interval of 750 m) and a detailed spectral bin microphysical scheme, it is found that low aerosol concentration acts to reduce convection strengths. Over the tropical western Pacific where low-level water vapor is abundant and a ubiquitous weak stable level exists near 0°C, the low background maritime aerosol concentration is conducive for forming cumulus congestus. Sensitivity tests show that the main mechanism of convection damping in a clean maritime environment is through reduced condensational growth, although the freezing of supercooled water, cloud top evaporation, and rain evaporation also contribute to the simulated effects. Considering the importance of congestus in tropical dynamics and the Madden-Julian oscillation (MJO) lifecycle, we further propose a hypothesis that aerosol-cloud-precipitation interactions in an ultraclean marine environment may serve as a damping mechanism for tropical convection.