Fixed anvil temperature feedback, positive, zero, or negative?

Abstract:

Cloud feedback remains to be the largest source of uncertainty in estimating the climate sensitivity. While low-cloud response is responsible for the greatest part of this uncertainty, contribution by high clouds is not negligible, particularly in the longwave (LW) component. In this study, we highlight two seemingly different views on the fixed anvil temperature (FAT) feedback that is considered to be a robust theory constraining the high-cloud response to tropospheric warming. First, we evaluate the sign and magnitude of FAT-induced cloud feedback by using the partial radiative perturbation (PRP) method with two different general circulation models. Second, we examine the co-dependency of LW feedback components by conducting sensitivity experiments with idealized lapse-rate changes. Third, we propose a new LW feedback framework, Thermal radiative damping with Fixed RH and Anvil Temperature (T-FRAT), by emphasizing the zero-cloud-emission-change aspect in the FAT feedback. It is shown that FAT leads to positive cloud feedback in the PRP framework due to the altitude effect. It is argued that this altitude effect is cancelled by the cloud component of the temperature feedback, and thus the FAT feedback does not contribute to the model spread in climate sensitivity. This study constitutes a part of the following paper: Yoshimori, M., F. H. Lambert, M. J. Webb, and T. Andrews (2020): Fixed anvil temperature feedback - positive, zero or negative? J. Climate, 33, 2719-2739. https://doi.org/10.1175/JCLI-D-19-0108.1.