Title: The temperature dependence of moist convection: changes in updrafts and precipitation statistics in radiative-convective equilibrium

Abstract: Moist convection is important for the energy and water budget of the Earth, but, because of the small spatial scales of convective clouds, is not well represented in global climate models. Basic questions such as what sets the updraft velocity and precipitation intensity in convective clouds, and how these quantities change as the surface temperature is varied, are not well settled. Recent studies have shown that a measure of convective instability, the convective available potential energy (CAPE), increases with warming in global climate simulations as well as idealized simulations of radiative-convective equilibrium. The implications of this result for convective precipitation intensity or cloud updraft velocity are, however, unclear.

Here we investigate the CAPE, updraft velocity and precipitation extremes in an idealized cloud-system resolving model run in radiative-convective equilibrium over a wide range of surface temperatures. We find a large increase in the CAPE, and weaker increases in the strength of updrafts with warming. These results are explained through a conceptual model of convection as a spectrum of entraining plumes. The increases in updraft strength occur primarily in the upper troposphere, and thus affect precipitation rates only weakly. Instead, the fall speed of hydrometeors emerges as a key determinant of the intensity of precipitation extremes. The increase in hydrometeor fall speed accompanying the transition from snow to rain induces an increase in precipitation extremes with warming that, for accumulation periods shorter than one hour, is larger than predicted from thermodynamics alone. Possible implications of these results for future climate are discussed.