Characteristics of the Marine Atmospheric Boundary Layer Over the Southern Ocean in Response to the Synoptic Forcing

Abstract

The characteristics of the marine atmospheric boundary layer (MABL) in relation to synoptic meteorology over the Southern Ocean are examined using upper-air soundings and surface precipitation at Macquarie Island (54.62 ∘ S, 158.85 ∘ E), with a primary focus 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. Observations confirm that boundary layer clouds over the Southern Ocean commonly reside within a shallow MABL under the influence of frequent midlatitude cyclones and fronts. The evaluation of MABL height shows that, for both observations and reanalysis, the inversion is higher northward of the low center and under postcold 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 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%.

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

“…These results suggested that the light and moderate precipitation and associated diurnal cycle generated by MABL clouds (shallow convection) were likely to make a significant contribution to the precipitation amount over the SO. This supported the conclusion that a significant portion of the precipitation over the SO came from non-frontal clouds [6,7], perhaps more so than suggested by [8] using the GPCP precipitation product.…”

“…The smoothed profiles were then interpolated onto standard pressure levels (37 pressure levels) for a comparison with ERA-Interim. The five-point smoothing was found to have a minor impact relative to the interpolation on to the 37 pressure levels [7]. No smoothing or interpolation was required for the ERA-Interim profiles.…”

“…For comparison purposes with ERA-Interim, MAC observations were accumulated into 3 h intervals. Using typical surface wind speeds of 12-16 m s −1 , a 3 h window of the observed surface precipitation spanned ∼140 km, which was on the same order of magnitude as ERA-Interim 0.75 • [6,7]. Consistent with [6], we defined three categories for the precipitation records: "light" precipitation as 0.2 ≤ P < 0.5 mm h −1 , "moderate" precipitation as 0.5 ≤ P < 1.5 mm h −1 , and "heavy" precipitation as P ≥ 1.5 mm h −1 .…”

“…The precipitation, correspondingly, is a major component of the water budget over the SO and has large uncertainties [2,3]. The precipitation uncertainty over the SO can be broken into frequency, intensity, e.g., [5,6], and location relative to synoptic conditions, e.g., [7]. The work in [8] coupled the daily Global Precipitation Climatology Project (GPCP) [9] precipitation product with a climatology of fronts [10] to conclude that 70-90% of the precipitation across the SO was associated with frontal passages.…”

“…Heavy precipitation, which is more likely associated with cold fronts, contributes to more than 50% of the precipitation above 1.5 mm h −1 . The work in [7] found that the precipitation intensity unrelated to cold fronts was underestimated in ERA-Interim reanalysis by 11%.…”