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

Abstract

Macquarie Island (54.50°S, 158.94°E) is an isolated island with modest orography in the midst of the Southern Ocean with precipitation records dating back to 1948. These records (referred to as MAC) are of particular interest because of the relatively large biases in the energy and water budgets commonly found in climate simulations and reanalysis products over the region. A basic climatology of the surface precipitation P is presented and compared with the ERA-Interim (ERA-I) reanalysis. The annual ERA-I precipitation (953 mm) is found to underestimate the annual MAC precipitation (1023 mm) by 6.8% from 1979 to 2011. The frequency of 3-h surface precipitation at MAC is 36.4% from 2003 to 2011. Light precipitation (0.066 ≤ P < 0.5 mm h−1) dominates this dataset (29.7%), and heavy precipitation (P ≥ 1.5 mm h−1) is rare (1.1%). Drizzle (0 < P < 0.066 mm h−1) is commonly produced by ERA-I (43.9%) but is weaker than the detectable threshold of MAC. Warm rain intensity and frequency from CloudSat products were compared with those from MAC. These CloudSat products also recorded considerable drizzle (16%–30%) but were not significantly different from MAC when P ≥ 0.5 mm h−1. Heavy precipitation events were, in general, more commonly associated with fronts and cyclonic lows. Some heavy precipitation events were found to arise from weaker fronts and lows that were not adequately represented in the reanalysis products. Yet other heavy precipitation events were observed at points/times not associated with either fronts or cyclonic lows. Two case studies are employed to further examine this finding.

Understanding Orographic Effects on Surface Observations at Macquarie Island

“…Four case studies (Table 1)-namely, a cold front, a warm front, drizzle, and a midlatitude cyclone-have been chosen to represent the common synoptic meteorology/ precipitation types (Wang et al 2015). Because of limited space, only the cold-and warm-frontal cases are presented in detail, as they are often associated with strong precipitation (Catto et al 2012).…”

“…The temperature ranges from 58 to 138C in January and from 18 to 98C in July, and snow is common in winter (Streten 1988;Wang et al 2015). There is no diurnal cycle or sea-breeze signature given the strong westerly winds.…”

“…Midlatitude cyclones or fronts pass across this island approximately every five or six days through the year; more frequently in the winter and less frequently in the summer (Streten 1988). According to Wang et al (2015), cold fronts are found to contribute more than 40% of the precipitation above 1 mm h 21 , and warm fronts contribute about 20%.…”

“…Huang et al (2012) employed the MAC soundings to infer the common presence of boundary layer clouds. Recently Wang et al (2015) compared the precipitation records from MAC with those from the reanalysis product [ERA-Interim (ERA-I); Dee et al 2011], and found that ERA-I underestimates the average annual precipitation by approximately 6.8% from 1979 to 2011. These in situ observations will be of increasing value for many future field activities over the Southern Ocean, such as the proposed Southern Ocean Cloud Radiation and Aerosol Transport Experimental Studies (SOCRATES) campaign, where additional surface observations at Macquarie Island, including microwave radiometers and surface broadband radiometers, ground-based lidar, and boundary layer observations from unmanned aerial systems have been proposed (http://www.atmos.…”

“…Wang et al (2015) detailed that from 1979 to 2011 the average annual MAC precipitation (1023 mm) was 6.8% greater than that from ERA-I (953 mm). This underestimate is consistently found in the eight surrounding ERA-I grids (0.758 3 0.758) for 2005-10 (Fig.…”