Improved simulation of clouds over the Southern Ocean in a General Circulation Model

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 micro-physics 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 Wm−2 , and seasonally much larger reductions. By slowing the growth of the ice phase, the model simulates substantially more supercooled liquid cloud. We hypothesise that such abundant supercooled liquid cloud is the result of a paucity of ice nucleating particles in this part of the atmosphere.