Assessment of Southern Ocean Clouds and Aerosols in the HadGEM3 Model using Multi-instrument Observations

The primary objective of this research is to investigate processes responsible for significant shortwave radiation biases over the Southern Ocean present in current general circulation models, which have been linked to deficiencies in representation of clouds and aerosols in this region. This is a subject of active research with multiple hypotheses being tested including cloud occurrence and types in cyclones, cloud microphysics, cloud-aerosol interaction and cloud horizontal homogeneity. Comparison with observations is necessary for identification and resolution of the deficiencies. Unfortunately, observations in the Southern Ocean are scarce, with satellites providing the most extensive spatial and temporal coverage, especially instruments/datasets such as MODIS and ISCCP and active instruments such as the radar on the lidar on the CloudSat and CALIPSO satellite. However, observations by these platforms are impeded when there is higher-level overlapping cloud.

We present details of a shipborne intensive observation period that included a ceilometer, micropulse lidar , weather radar and aerosol observations in the Southern Ocean, which allows the cloud to be examined ‘from below’ and assess the cloud-aerosol interaction. Additionally, balloon observations and novel observations made from a UAV and a tethered balloon are presented. We also discuss a modified version of the Cloud Feedback Model Intercomparison Project (CFMIP) Observation Simulator Package (COSP) to compare the ceilometer measurements with model simulations. The COSP simulator currently does not support ground-based lidars with a matching wavelength, but we have identified that the ACTSIM lidar simulator in COSP requires only a few modifications to support the analysis of ceilometer data. Using an instrument simulator such as COSP allows us to account for the limited view of the ceilometer and signal attenuation in the atmosphere. We therefore describe future efforts on extending the capability of the COSP simulator.