Understanding the Direct Effect of CO2 on Tropical Precipitation Extremes using Theory and High-Resolution Simulations

Communities worldwide are exposed to heavy rainfall events, with extreme tropical precipitation being particularly impactful. Climate change is expected to affect precipitation extremes, but climate models disagree on the response of tropical precipitation extremes to warming. To make progress on this, previous work has used high-resolution, cloud-resolving simulations to study the response of rainfall extremes to sea-surface temperature (SST) warming, but with fixed CO₂ concentrations. In contrast, this study examines how high percentiles of precipitation change with increasing CO₂, over fixed SSTs, and decomposes their response between thermodynamic and dynamic contributions. CO₂ concentrations are quadrupled using a cloud-resolving model, specifically the System for Atmospheric Modeling (SAM). These idealized, radiative-convective equilibrium simulations are contained in a 128 km² spatial domain over a tropical ocean and have a 1 km horizontal resolution, which resolves deep convection. High quantile fractional percentage change calculations show that tropical precipitation extremes (i.e., 99.9th percentiles of instantaneous rainfall) observe a 4-6% decrease in response to a quadrupling of CO₂, compared to a 5-6% increase per degree of SST warming, as found in previous work. The condensation rate equation used defines precipitation in an extreme event as proportional to the vertical velocity extremes (w_e) (dynamic contribution) and the saturation specific humidity vertical derivative (dq^*/dz) (thermodynamic contribution). The domain-averaged fractional percentage changes of both contributions result in a w_e decrease in the lower troposphere by a maximum of 3% and a dq^*/dz increase across most of the tropospheric column by approximately 1%. Per the condensation rate equation approach, these findings suggest that dynamics may play a more significant role in influencing the weakening of precipitation extremes over the tropics. Future work seeks to address the statistical significance of the examined contributions and assess the potential role of changes in precipitation efficiency.