Abstract Precipitation extremes produced by convection have been found to intensify with near-surface temperatures at a Clausius-Clapeyron rate of 6 to 7% K −1 in simulations of radiative-convective equilibrium (RCE). However, these idealized simulations are typically performed over an ocean surface with a high near-surface relative humidity (RH) that stays roughly constant with warming. Over land, near-surface RH is lower than over ocean and is projected to decrease by global climate models. Here, we investigate the dependence of precipitation extremes on near-surface RH in convection-resolving simulations of RCE. We reduce near-surface RH by increasing surface evaporative resistance. Simultaneously, we hold free-tropospheric temperatures fixed by increasing surface temperatures. This “top-down” approach produces an RCE state with a deeper, drier boundary layer, which weakens convective precipitation extremes in three distinct ways. First, the lifted condensation level is higher, leading to a small thermodynamic weakening of precipitation extremes. Second, the higher lifted condensation level also reduces positive buoyancy in the lower troposphere, leading to a dynamic weakening of precipitation extremes. Third, precipitation re-evaporates more readily when falling through a deeper, drier boundary layer, leading to a substantial decrease in precipitation efficiency. These three effects all follow from changes in near-surface relative humidity and are physically distinct from the mechanism that underpins the Clausius-Clapeyron scaling rate. Overall, our results suggest that changes in relative humidity must be taken into account separately from specific humidity when seeking to understand and predict changes in convective precipitation extremes over land.
Drift et al. (Mon,) studied this question.