Precipitation over the Southern Ocean As Observed at Macquarie Island

Large uncertainties exist in the amount and characteristics of precipitation over the Southern Ocean (SO), especially for shallow convective clouds between frontal passages. Satellite products have suggested that supercooled liquid water is prevalent across the SO throughout the year, while recent in-situ observations suggest that open mesoscale cellular convection and secondary ice-production may be commonplace across the lower latitudes of the SO, particularly during the winter season. Better quantitative precipitation estimates are necessary to help close the energy budget over the SO, which continues to display relatively large biases in climate models and reanalysis products. The characteristics of the marine atmospheric boundary layer (MABL), including its precipitation, is examined in relation to synoptic meteorology over the Southern Ocean (SO) using upper-air soundings and surface precipitation at Macquarie Island (MAC) (54.62 S, 158.85 E). The primary focus is on the post-cold-frontal environment, where large cloud and radiative biases are presented in a multitude of climate models. Thermodynamic profiles from the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalyses are compared with the observations to evaluate their representation of the MABL characteristics. These observations confirm that boundary layer clouds over the SO commonly reside within a shallow MABL under the influence of frequent mid-latitude cyclones and fronts. The evaluation of MABL height shows that, for both observations and reanalysis, the MABL is higher northward of the low center and under post-cold front conditions. Under cold frontal passages, however, the main inversions are not well represented by ERA-Interim, which is featured by an underestimate of the MABL height by 22%. Significant differences are found in the moisture profiles within the MABL between the observations and ERA-Interim soundings within the context of cold frontal passages. The moisture in the ERA-Interim is found to be too confined to the surface layer, which is consistent with the shallower MABL represented by the ERA-Interim. Analysis of the MAC surface precipitation shows that ERA-Interim overestimates the amount of precipitation over Macquarie Island in the vicinity of cyclone cores but underestimates the precipitation not immediately associated with cold fronts, leading to an overall underestimate of the annual precipitation by 11%. A further analysis of the MAC surface precipitation finds that a strong (~15%) diurnal cycle is evident during the summer season, further suggesting that precipitation generated from shallow clouds makes a significant contribution to the overall total.