The thermodynamic structure of the marine atmospheric boundary layer (MABL) over the Southern Ocean is examined using 109 dropsondes from 12 flights undertaken by the NSF/NCAR Gulfstream-V HIAPER aircraft between January 15 and February 24, 2018 during the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) campaign. The soundings are analysed to evaluate how well the dropsondes observation can be represented by a climatology of the MABL derived from 3,514 upper-air soundings at Macquarie Island as in Lang et al. 2018 (L2018).

L2018 applied a K-mean clustering algorithm (K=5) to the upper-air sounding and found 5 discrete clusters: pre-frontal and post-frontal, high pressure system, weak post-frontal and mixed conditions. In our work, fifteen low-level thermodynamic variables (as in L2018) are employed. The dropsondes are assigned to the nearest cluster centroid from L2018. The results show that the forced-to-5 cluster proportion of the dropsondes is relatively comparable to that in L2018 in which cluster 5 is most frequently present (36.7%). This cluster is characterised by westerly winds extending throughout the troposphere with a maximum wind speed at 480 hPa (15 m/s). The characteristics of pre-frontal and post-frontal conditions are well captured in Cluster 1 (18.3%) and 4 (15.6%), respectively. Cluster 2 (22%) represents an environment more closely associated with high pressure systems, whereas Cluster 3 exhibits a larger variability of the surface pressure. In general, the dropsondes are well represented the climatology at Macquarie Island.

Spatiotemporally collocated thermodynamic profiles from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis are compared with the observations to evaluate their representation of the MABL characteristics. The comparison shows that ERA5 reanalysis can capture thermodynamic profiles with some skill, except for the dew point temperature, where ERA5 profiles are moister than the dropsondes.