Tropical free-tropospheric humidity differences and their effect on the clear-sky radiation budget in global strom-resolving models

Abstract

Free-tropospheric water vapor strongly impacts the Earth's outgoing longwave radiation (OLR) and therefore plays a key role in controlling the clear-sky response of the climate system to an increase in greenhouse gases. It is now widely accepted that this response is described by a warming and moistening of the atmosphere that is implied if the relative humidity (RH) and lapse rate were to depend on temperature alone, which corresponds to a warming at approximately constant RH (e.g., Held & Soden, 2000;Po-Chedley et al., 2019;Romps, 2014). This reduces the radiative response compared to a warming at constant absolute humidity, and can be described as a positive water-vapor-lapse-rate feedback. While general circulation models (GCMs) agree on this basic response (e.g., Bony et al., 2006;Soden & Held, 2006), there is still an appreciable inter-model spread in the magnitude of the water-vapor-lapse-rate feedback. This spread, which primarily originates from the tropics, contributes a non-negligible (about 30%) uncertainty to the climate sensitivity (Vial et al., 2013).