Evaluation of Southern Ocean cloud in the HadGEM3 general circulation model and MERRA-2 reanalysis using ship-based observations

Southern Ocean shortwave radiation biases of up to 40 Wm^-2 in summer are common in general circulation models, with misrepresentation of cloud identified as the major cause. We evaluate the atmospheric component GA7.0 and GA7.1 of the HadGEM3 general circulation model and the MERRA-2 reanalysis, and find that GA7.0 and GA7.1 underestimate the reflected top of atmosphere shortwave radiation, while MERRA-2 overestimates this quantity. Using a dataset of ship ceilometer and radiosonde observations we evaluate cloud cover and link it to the thermodynamic profile. We find low cloud below 2 km and fog predominant and cloud cover exceeding 90% in most regions. We show that this cloud is strongly linked to boundary layer stability and sea surface temperature. Using a ground-based lidar simulator we produce virtual ceilometer measurements along the voyage tracks for a 1:1 comparison with the ceilometer measurements. We find that GA7.0 and MERRA-2 underestimate cloud cover by 18–25%, especially cloud below 1 km and fog. While the boundary layer stability is well represented in GA7.0 and MERRA-2, the link between the boundary layer stability and cloud found in observations is not present in the models, pointing to deficiencies in the subgrid scale parametrisation of cloud.