Increase in Precipitation Efficiency With Surface Warming in Radiative‐Convective Equilibrium. (28th November 2018)

• Record Type: Journal Article
• Authors: Lutsko, Nicholas J.; Cronin, Timothy W.
• Is Part Of: Journal of advances in modeling earth systems. Volume 10:Number 11(2018)
• Page Start: 2992
• Page End: 3010
• Publication Date: 2018-11-28
• Subjects: precipitation efficiency -- tropical convection -- cloud microphysics -- Walker circulation -- climate sensitivity; Geological modeling -- Periodicals; Climatology -- Periodicals; Geochemical modeling -- Periodicals; 551.5011
• DOI: 10.1029/2018MS001482
• Languages: English
• ISSNs: 1942-2466

Abstract

Abstract: The precipitation efficiency of convection ( ε ) plays an important role in simple models of the tropical atmosphere as well as in global climate models' projections of future climate changes, but remains poorly understood and poorly constrained. A particularly urgent question is how ε will change in warmer climates. To address these issues, this study investigates the precipitation efficiency in simulations of radiative‐convective equilibrium with a cloud‐resolving model forced by a wide range of sea surface temperatures (SSTs). Two different domains are considered: a small, doubly periodic domain, and a 2‐D ( x ‐ z ) "mock‐Walker" domain with a sinusoidal SST profile that resembles the equatorial Pacific, and the sensitivities of the results to the microphysical scheme and to the horizontal resolution are also explored. It is found that ε generally increases with warming in the small domain simulations because of increases in the efficiency with which cloud condensate is converted into precipitation, with changes in the re‐evaporation of falling precipitation playing a secondary role. This picture is complicated in the 2‐D simulations by substantial changes in the degree of convective organization as the underlying SSTs are varied. ε is found to decrease as convection becomes more organized, because convective organization results in relatively more low clouds, which have small (≤0.1) precipitation efficiencies, and relatively less high clouds, which have larger (∼0.4) precipitation efficiencies. Plain Language Summary: The precipitation efficiency of convection ( ε ) quantifies the fraction of water that condenses in a cloud that reaches the surface as precipitation. Recent work has shown that changes in ε can play an important role in determining the warming of climate models in response to increases in atmospheric carbon dioxide concentrations, and ε is also a key factor in theories for the dynamics of the tropical atmosphere. Despite this importance, however, ε is poorly understood and poorly constrained. In this study, we take a first step to addressing this issue by investigating how precipitation efficiency behaves in idealized simulations of the tropical atmosphere, in which the underlying sea surface temperature is varied across a wide range of values. We find that ε generally increases with warming because clouds become denser and so form precipitation more easily, though in some cases ε decreases because of changes in the large‐scale flow of the tropical atmosphere. Key Points: Precipitation efficiency is investigated in RCE simulations with a CRM forced by a wide range of SSTs Precipitation efficiency generally increases with warming because of increased cloud density Changes in the large‐scale circulation can cause precipitation efficiency to decrease