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

Abstract

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 • S, 158.85 • E) are analysed (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(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)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(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.

Southern Ocean Precipitation Characteristics Observed From CloudSat and Ground Instrumentation During the Macquarie Island Cloud & Radiation Experiment (MICRE): April 2016 to March 2017

“…However, in their examination of the longer Macquarie tipping bucket record from 1998 to 2016, Lang et al ( 2020) find a minimum in the mean (3-hourly) precipitation rate near 14:00 LT and a maximum in the mean rate in the early morning near 05:00 LT, with this pattern being most prominent for "moderate" precipitation (meaning when the 3-hourly mean rate was between 0.5 and 1.5 mm/hr; see their Figure 3). We also find a similar magnitude in the diurnal cycle to that reported in Lang et al (2020), with the size of the variation (defined as the daily maximum minus minimum) being about 0.01 mm/hr (for both light rates <0.5 mm/hr and heavier rates >0.5 mm/hr). While not definitive, this suggests that at least the presence of an early afternoon minimum is characteristic for Macquarie Island.…”

“…The diurnal cycle at heavier rates (>0.5 mm/hr) does appear to be strongest in summer, but the statistical confidence is low. Using the longer Macquarie Island tipping-bucket-only data set, Lang et al (2020) do conclude that there is a stronger, more pronounced diurnal cycle with a larger difference between the daily maximum and minimum mean rates during the summer compared to other seasons.…”

“…Previous studies have documented that SO clouds, including the wide-spread shallow (low altitude cloud top height) clouds that dominate total cloud cover, are frequently precipitating with liquid, frozen, and mixed phase precipitation (Ahn et al, 2017;Chubb et al, 2013;Lang et al, 2018Lang et al, , 2020Mace & Protat, 2018a, 2018bWang et al, 2015). The primary addition that MICRE measurements and the present analysis provide to previous data sets and studies comes from having a year of W-band radar, ceilometer, and disdrometer measurements that allow an examination of seasonal, diurnal, and synoptic variations in surface precipitation type and particle size (in addition to accumulation), though interannual variability may impact each of these to varying degrees as discussed in the previous section.…”

“…Using the longer Macquarie Island tipping‐bucket‐only data set, Lang et al. (2020) do conclude that there is a stronger, more pronounced diurnal cycle with a larger difference between the daily maximum and minimum mean rates during the summer compared to other seasons.…”

Shallow Convection and Precipitation Over the Southern Ocean: A Case Study During the CAPRICORN 2016 Field Campaign

“…Figure 2e shows the time series of reflectivity profiles from the Radar‐lidar merged product, where it is possible to observe cloud‐free areas and intermittent open MCC clouds with high reflectivities (>10 dBZ) associated with precipitation that reached the sea surface, as observed by the disdrometer. 95% of the heavy precipitation, defined as >1.5 mm hr −1 over the SO (Lang et al., 2020; Z. Wang, Siems, et al., 2015), is associated with profiles of mixed‐ or ice‐phase clouds. The disdrometer measures the particle diameter for recorded precipitation.…”