A Climatology of the Precipitation over the Southern Ocean as observed at Macquarie Island

The atmospheric environment over the Southern Ocean (SO) is unique: the lack of terrestrial and anthropogenic aerosols creates a pristine environment with few cloud condensation nuclei. Strong winds produce large waves that, when coupled together, generate large concentrations of sea spray. Recent satellite observations of the cloud-top thermodynamic phase suggest that vast fields of clouds composed of supercooled liquid water (SLW) are dominant over the region. Limited in-situ cloud observations have found that the SLW can exist throughout the entire depth of these clouds, which are often hundreds of meters thick. Chubb et al (2013) went further to reveal SLW existing at cloud-tops as cold as -22°C and even encountered, on occasion, supercooled precipitation processes existing below cloud base. This observed intermittency in the thermodynamic state of the precipitation highlights our very limited understanding in the nature and role of precipitation over the SO. Yet such an understanding is necessary to close both the hydrological and energy budgets over this region that covers 15% of the Earth's surface and is poorly represented in reanalysis and climate models. Unfortunately there is very little known about this precipitation given the inherent difficulties in making observations over the SO. Ellis et al (2009) employed the Cloud Profiling Radar (CPR) precipitation product aboard CloudSat to quantify the common occurrence of precipitation over the oceans; a peak in the frequency of precipitation occurrence was observed between 50 and 60°S. Further, they highlighted that at such high latitudes much of the CPR precipitation is actually classified as snow/ice or mixed phase. Macquarie Island (54.50°S 158.94°E) is an isolated island in the midst of the Southern Ocean and houses an Australian Bureau of Meteorology (ABoM) weather observation station. It has been in operation since 1948 and is maintained by the Australian Antarctic Division (AAD). In recent years these observations have become quite valuable in helping develop an understanding of the meteorology and climatology over the SO. For example, Adams (2009) examined the trends in the surface observations to highlight a 35% increase in the precipitation over a 38-year period from 1971-2008. This research explores the nature of the precipitation records over Macquarie Island with a focus is on the frequency, intensity, and on convective vs. non-convective classes. While these surface observations are not directly comparable to similar measures from either CloudSat or ERA-Interim due to differences in temporal and spatial scales, it is still enlightening to explore similarities and differences between them. The major conclusions are as follow. • The long-term precipitation records (1948-2011) display a peak in early autumn (March and April) and is then relatively flat for the rest of the year. ERA-I (1979-2011) records a peak in March, but shows much greater variability throughout the year. Annually averaged, ERA-I underestimates the precipitation over the SO as observed at Macquarie Island. • Light precipitation is most commonly observed. At an hourly time scale, no precipitation is encountered ~70% of the time while light precipitation (0 < P ≤ 0.5 mm hr-1) is encountered 12% of the time. Heavy precipitation (P > 2.0 mm hr-1) is encountered less than 1% of the time. Presenting the surface observations at a 3-hour time changes the frequency to 22%, 70% and 0.3% for clear, light and heavy precipitation, respectively. • Light precipitation was associated with fronts or cyclonic activity roughly half of the time for light precipitation and increased to roughly 80% of the time for heavy precipitation. It was further noted that there was little consistency in the definition of fronts out over the Southern Ocean. One case study illustrated a frontal passage that failed to be classified by all methods investigated. • At a very coarse time scale (6 hours), the precipitation rose from ERA-I is largely consistent with that from observations, although ERA-I overestimates the frequency at which light precipitation falls and underestimates the frequency at which heavy precipitation falls. This finding is largely consistent with the conclusions of Stephens et al (2010). • Precipitation from CloudSat (2.5° granule) of any magnitude was found to occur at a frequency consistent with the surface observations (3-hour time interval). The ERA-I recorded precipitation (3-hour time scale) still produces precipitation too frequently and at too weak of an intensity. • It was not possible to derive a meaningful precipitation rate from the CPR precipitation due to the frequent occurrence of missing data arising from the potential of mixed-phase precipitation. The missing data display a strong seasonal bias with more observations missing during winter. As highlighted in Huang et al (2012), it is quite common for the temperature over Macquarie Island to be between 0° and -20° C, making it difficult to interpret the thermodynamic phase of the precipitation. • A second case study illustrates a heavy precipitation event not associated with a frontal passage, confirming that there are other mechanisms for heavy precipitation over the Southern Ocean besides fronts and cyclonic lows. References Adams, N, 2009: Climate trends at Macquarie Island and expectations of future climate change in the sub-Antarctic. Papers and Proceedings of the Royal Society of Tasmania, 143, 1-8. Chubb, T. H., J. B. Jenson, S. T. Siems and M. J. Manton, 2013: In-situ observations of supercooled liquid clouds over the Southern Ocean during the HIAPER Pole-to-Pole Observations (HIPPO) campaigns. Geophys. Res. Letters, 40, 5280–5285, doi:10.1002/grl.50986. Ellis, T. D., T. L'Ecuyer, J. M. Haynes and G. L. Stephens, 2009: How often does it rain over the global oceans? The perspective from CloudSat. Geophys. Res. Letters, 36, L03815, doi:10.1029/2008GL03728. Huang, Y, S. T. Siems, M. J. Manton and L.B. Hande, 2012: The structure of low-altitude clouds over the Southern Ocean as seen by CloudSat, J. Clim., 25, doi: 10.1175/JCLI-D-11-00131.1. Stephens, G. L., T. L'Ecuyer, R. Forbes, A. Gettlemen, J.‐C. Golaz, A. Bodas‐Salcedo, K. Suzuki, P. Gabriel and John Haynes, 2010. Dreary state of precipitation in global models. J. Geophys. Res. 115, D24211, doi:10/1029/2010JD014532.