How Convective Mass Flux Responds to Environmental Humidity

Abstract: Our goal in this study is to characterize the relationship between lower tropospheric environmental humidity and convective mass flux in the tropics. To do so, we have created gridded convective mass flux datasets from five global storm-resolving models (GSRMs). We have three principal findings. First, in humid environments, mass flux increases with height from the surface through the depth of the lower free troposphere, forming a ``deep-inflow". In dry environments, mass flux decreases with height through the lower free troposphere. Second, mid-tropospheric mass flux increases nonlinearly with increasing lower tropospheric humidity, resembling a widely reported pickup in tropical precipitation. Third, increased lower tropospheric humidity is associated with reduced deep convective updraft buoyancy. To interpret these findings, we employ a simple three-equation parcel model with stochastic entrainment. The parcel model suggests that the response of convective mass flux to lower tropospheric humidity is governed by two mechanisms: (1) survival, in which a greater share of entraining parcels ascend rather than detrain with greater humidity; and (2) dilution, in which the average entrainment rate among surviving parcels increases with environmental humidity. Together, these two mechanisms account for the three mass flux responses to humidity.