Evidence of a Diurnal Cycle in Precipitation over the Southern Ocean as Observed at Macquarie Island

Due to a lack of observations, relatively large discrepancies exist between precipitation products over the Southern Ocean. In this manuscript, surface hourly precipitation observations from Macquarie Island (54.62 degrees S, 158.85 degrees E) are analysed (1998-2016) to reveal a diurnal cycle. The precipitation rate is at a maximum during night/early morning and a minimum in the afternoon at Macquarie Island station. Seasonally, the diurnal cycle is strongest in summer and negligible over winter. Such a cycle is consistent with precipitation arising from marine boundary layer clouds, suggesting that such clouds are making a substantial contribution to total precipitation over Macquarie Island and the Southern Ocean. Using twice daily upper air soundings (1995-2011), lower troposphere stability parameters show a stronger inversion at night, again consistent with precipitation arising from marine boundary layer clouds. The ERA-Interim precipitation is dominated by a 12 hourly cycle, year around, which is likely to be a consequence of the twice-daily initialisation. The implication of a diurnal cycle in boundary layer clouds over the Southern Ocean to derived A-Train satellite precipitation products is also discussed.