Convective Dynamics and the Response of Precipitation Extremes to Warming in Radiative–Convective Equilibrium

Abstract

Tropical precipitation extremes are expected to strengthen with warming, but quantitative estimates remain uncertain because of a poor understanding of changes in convective dynamics. This uncertainty is addressed here by analyzing idealized convection-permitting simulations of radiative–convective equilibrium in long-channel geometry. Across a wide range of climates, the thermodynamic contribution to changes in instantaneous precipitation extremes follows near-surface moisture, and the dynamic contribution is positive and small but is sensitive to domain size. The shapes of mass flux profiles associated with precipitation extremes are determined by conditional sampling that favors strong vertical motion at levels where the vertical saturation specific humidity gradient is large, and mass flux profiles collapse to a common shape across climates when plotted in a moisture-based vertical coordinate. The collapse, robust to changes in microphysics and turbulence schemes, implies a thermodynamic contribution that scales with near-surface moisture despite substantial convergence aloft and allows the dynamic contribution to be defined by the pressure velocity at a single level. Linking the simplified dynamic mode to vertical velocities from entraining plume models reveals that the small dynamic mode in channel simulations (≲2% K−1) is caused by opposing height dependences of vertical velocity and density, together with the buffering influence of cloud-base buoyancies that vary little with surface temperature. These results reinforce an emerging picture of the response of extreme tropical precipitation rates to warming: a thermodynamic mode of about 7% K−1 dominates, with a minor contribution from changes in dynamics.

Response of precipitation extremes to warming: what have we learned from theory and idealized cloud-resolving simulations, and what remains to be learned?

“…Similar increases in precipitation extremes, close to low-tropospheric moisture, have been found in another idealized study using a different CRM (Romps 2011), suggesting robustness to the model used, although more CRM studies beyond those two are needed to assess robustness with confidence. Abbott et al (2019) further confirm the dominance of the thermodynamic contribution to changes in instantaneous precipitation extremes, following near-surface moisture, across a wide range of climates. In the tropics, GCMs predict an increase of water vapor larger than 8% K −1 (O'Gorman and Muller 2010).…”

Response of extreme precipitation to uniform surface warming in quasi-global aquaplanet simulations at high resolution

“…In the tropics, the thermodynamic contribution is close to Clausius-Clapeyron scaling with near-surface temperature. This scaling with near-surface temperature has been noted before but it is somewhat surprising since much of the convergence of water vapour occurs higher in the atmosphere, and the cause for this scaling has recently been found to be the universal shape of vertical velocity profiles conditioned on extreme precipitation events when expressed in a moisture vertical coordinate [54].…”

“…Our aim in making this decomposition is to better understand the variation in sensitivity across latitudes and the differing sensitivities when 3 h and daily coarse-grained precipitation extremes are considered. The scaling approximation was derived assuming saturated moist adiabatic ascent [1] or using a dry static energy budget in the tropics without assuming saturated ascent [32], and it has been used in a number of studies to relate the precipitation rate to the vertical profiles of temperature and vertical velocity in extreme precipitation events [3,36,51–54]. We refer to it as a scaling approximation because it estimates the vertically integrated net condensation rate and thus only predicts the precipitation rate up to a factor that can be thought of as a precipitation efficiency.…”

“…For 3 h precipitation extremes, the dynamic contribution is positive at low latitudes and negative at high latitudes. The positive dynamic contribution at low latitudes differs from a negative dynamic contribution to changes in precipitation extremes in small-domain RCE [32,54] and this discrepancy may relate to the presence of organized convection in our simulations (see also [7]). For daily coarse-grained precipitation extremes, the dynamic contribution is mostly negative in the tropics and positive at high latitudes, but there is not as clear a poleward shift of the vertical velocities as was found in previous studies of daily precipitation extremes in aquaplanet climate models [24,25].…”

“…In particular, there is little to no increase in moist static stability and no corresponding decrease in upward motion in the lower troposphere at high latitudes in contrast to the changes for 3 h extremes. An upward shift of the circulation and tropopause with climate warming is likely also contributing to the response of vertical velocities in the middle and upper troposphere [54,57].…”