Most of the present-day global climate models is identified by insufficient reflection of short-wave radiation over the Southern Ocean due to a misrepresentation of clouds. In this study, we present the results from a recent version of the Met Office's Unified Model (the base model for the atmospheric component of the New Zealand Earth System Model). By modifying the cloud micro-physics parametrisation and choosing a more realistic value for the shape parameter of atmospheric ice-crystals, in agreement with theory and observations, we suggest that the simulation of short-wave radiation is significantly improved. 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. We also examine the impact of changing other temperature thresholds in the cloud micro-physics scheme for the onset of heterogeneous ice production. As a result, we achieved a reduction in the annual-mean short-wave cloud radiative effect over the Southern Ocean by up to 4 W/m², and much larger seasonal reductions as well. The base model had an excess of atmospheric ice. Thus, by slowing the growth of the ice phase, the model simulates substantially more super-cooled-liquid cloud. We hypothesize that such abundant super-cooled-liquid cloud is the result of a paucity of ice nucleating particles in this part of the atmosphere.