Aerosol, cumulus congestus, and Madden-Julian Oscillation

Cumulus congestus (convection with cloud top around 0 Celsius) contributes significantly to tropical precipitation. Recent observations show its importance in moistening and preconditioning atmosphere for the onset of deep convection in MJO events. The bottom heavy diabatic heating profile produced by congestus is also crucial in several theoretic studies of MJO. However, quantitative studies of how tropical atmosphere maintains its congestus population remain lacking. Here we study the role aerosol, serving as Cloud Condensation Nuclei (CCN), plays in modulating tropical cumulus congestus population using a cloud-resolving model with explicit bin microphysical scheme. Through case studies of aerosol indirect effects, we found that the low aerosol concentration is conducive to congestus formation in tropical atmosphere, where water vapor is abundant, atmosphere instability is low with a ubiquitous weak stable level near 0 Celsius. The mechanisms of aerosol modulating storm height and intensity are discussed through model sensitivity tests. We will also elaborate our postulation that marine aerosol and its lifecycle need to be included in MJO theories and in MJO modeling, especially in global models where cumulus parameterizations are used.