Improving the Southern Ocean cloud albedo biases in a general circulation model

Vidya Varma,Kalli Furtado,Paul Field,Olaf Morgenstern,Jonny Williams,Patrick Hyder

doi:10.5194/acp-20-7741-2020

Abstract

Abstract. The present generation of global climate models is characterised by insufficient reflection of short-wave radiation over the Southern Ocean due to a misrepresentation of clouds. This is a significant concern as it leads to excessive heating of the ocean surface, sea surface temperature biases and subsequent problems with atmospheric dynamics. In this study, we modify cloud microphysics in a recent version of the Met Office's Unified Model and show that choosing a more realistic value for the shape parameter of atmospheric ice crystals, in better agreement with theory and observations, benefits the simulation of short-wave radiation. In the model, for calculating the growth rate of ice crystals through deposition, the default assumption is that all ice particles are spherical in shape. We modify this assumption to effectively allow for oblique shapes or aggregates of ice crystals. Along with modified ice nucleation temperatures, we achieve a reduction in the annual-mean short-wave cloud radiative effect over the Southern Ocean by up to ∼4 W m−2 and seasonally much larger reductions compared to the control model. By slowing the growth of the ice phase, the model simulates substantially more supercooled liquid cloud.

Highlights

One of the major known problems in present-day global climate models is an excess in the absorbed short-wave (SW) radiation over the Southern Ocean (SO) (Trenberth and Fasullo, 2010; Ceppi et al, 2012; Hwang and Frierson, 2013; Williams et al, 2013; Hyder et al, 2018)
We reduce the annual-mean SW radiation biases over SO in a recent version of the UK Met Office’s Unified Model by up to ∼ 4 W m−2 and seasonally up to ∼ 8 W m−2 compared to the control model
This and other contemporary climate models are characterised by excess cloud ice causing biases in SW radiation which are especially pronounced over the SO

Summary

Introduction

One of the major known problems in present-day global climate models is an excess in the absorbed short-wave (SW) radiation over the Southern Ocean (SO) (Trenberth and Fasullo, 2010; Ceppi et al, 2012; Hwang and Frierson, 2013; Williams et al, 2013; Hyder et al, 2018). By modifying the shallow convection detrainment in their global climate model, Kay et al (2016) showed that the resultant increase in the supercooled liquid clouds enable large reductions in longstanding climate model SW radiation biases. In another study by Furtado and Field (2017), the importance of ice microphysics parameterisation in determining the phase composition, and the liquid water content of the SO clouds is highlighted

Results

Discussion

Conclusion