A Robust Constraint on the Response of Convective Mass Fluxes to Warming

A fundamental quantity in tropical dynamics is the “convective mass flux,” which measures the rate at which mass is transported upwards per unit area in convective updrafts. Convective mass flux encodes information about the frequency and intensity of thunderstorms, and has been linked to the strength of the large-scale tropical circulation. Changes in convective mass flux under warming are an important, but uncertain, aspect of climate change. Here we build off recent work linking changes in mass flux to the clear-sky energy budget to show that convective mass fluxes decrease along isotherms at around 3%–5% $K^{-1}$ under warming. We show that this constraint holds throughout the free-troposphere and across a hierarchy of models; from idealized radiative-convective equilibrium simulations to CMIP6 models. This decrease in convective mass flux with warming is driven by a stabilization of the lapse rate and can be captured with a simple analytical model. We also revisit previous work by Held and Soden (2006), https://doi.org/10.1175/jcli3990.1, who proposed a scaling for changes in the convective mass flux with warming. We show that the Held and Soden scaling does not capture inter-model spread in cloud-base mass flux changes under warming, and that their original verification was likely coincidental. Our work provides a quantitative constraint on changes in convective mass flux throughout the troposphere which can be derived from first principles, and which is verified across a hierarchy of models.