The relationship between condensate lifetime and precipitating efficiency and their response to sea surface warming

Recent work has argued that sea surface warming causes an increase in the precipitating efficiency (PE) which leads to enhanced weakening of the tropical circulation and increased climate sensitivity 1. The argument however relies on two assumptions that we believe require further validation. The first is that enhanced PE weakens the strength of the overturning circulation. Using the analytical framework of Emanuel (2019) and the minimal recipe simulations of Jeevanjee & Zhou (2022), we show that enhanced PE weakens evaporation driven downdrafts but not radiatively driven subsidence. Since the latter is a key ingredient of tropical circulation weakening, we argue that this finding motivates continued process-based research on the relationship between the tropical circulation and PE. The second assumption made in Li et al. (2023) is that PE can be estimated from the inverse of condensate lifetime (τ^(-1)). Using a simple analytical argument, we show that precipitating efficiency only scales with τ^(-1)when evaporation of condensate is much larger than precipitation (i.e., PE is much less than 1). This assumption is not supported by existing published estimates of precipitating efficiency or estimates we derived from the Simple Cloud-Resolving-E3SM-Atmosphere model (SCREAM) which suggest precipitating efficiency to be between 0.1 and 1. We further show that due to the co-dependence of PE on τ^(-1)and the inverse evaporation timescale, PE can exhibit a very different SST scaling compared to τ^(-1). Our results highlight the limitations of using τ^(-1) as a proxy for PE.

Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52 07NA27344 IM Release number LLNL-ABS-861067. This work was supported by the LLNL-LDRD Program under Project No. 24-ERD-015.

References

1. Li, R. L., Studholme, J. H. P., Fedorov, A. V. & Storelvmo, T. Increasing Precipitation Efficiency Amplifies Climate Sensitivity by Enhancing Tropical Circulation Slowdown and Eastern Pacific Warming Pattern. Geophys. Res. Lett. 50, e2022GL100836 (2023).
2. Emanuel, K. Inferences from Simple Models of Slow, Convectively Coupled Processes. J. Atmospheric Sci. 76, 195–208 (2019).
3. Jeevanjee, N. & Zhou, L. On the Resolution-Dependence of Anvil Cloud Fraction and Precipitation Efficiency in Radiative-Convective Equilibrium. J. Adv. Model. Earth Syst. 14, e2021MS002759 (2022).