Dependence of Convective Precipitation Extremes on Near-Surface Relative Humidity

Abstract Precipitation extremes produced by convection have been found to intensify with near-surface temperatures at a Clausius–Clapeyron rate of 6%–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 lifting condensation level is higher, leading to a small thermodynamic weakening of precipitation extremes. Second, the higher lifting 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. Significance Statement Thunderstorms and other types of convection produce heavy rainfall in many regions on Earth. In this paper, we ran a computer model to show that when relative humidity near the surface is reduced, convection produces weaker rainfall rates. This happens for three reasons: The updrafts in the storms are weaker, there is less cloud water because the cloud base is higher, and less of that cloud water makes it to the surface as rainfall. This is an important finding because we expect that relative humidity will decrease over many land regions as the climate warms, possibly seasonally offsetting some of the impact on rainfall extremes of increases in the absolute amount of water vapor.