Atmos. Chem. Phys., 19, 4659–4683, 2019 https://doi.org/10.5194/acp-19-4659-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. On the fine vertical structure of the low troposphere over the coastal margins of East Antarctica Étienne Vignon1, Olivier Traullé2, and Alexis Berne1 1Environmental Remote Sensing Laboratory (LTE), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland 2DSO-DOA, Météo France, Toulouse, France Correspondence: Étienne Vignon (etienne.vignon@epfl.ch) Received: 15 November 2018 – Discussion started: 2 January 2019 Revised: 25 March 2019 – Accepted: 26 March 2019 – Published: 8 April 2019 Abstract. In this study, 8 years of high-resolution radiosonde structure of the low troposphere over coastal East Antarctica, data at nine Antarctic stations are analysed to provide the this study gives insights into the reliability and accuracy of first large-scale characterization of the fine vertical structure two major reanalysis products in this region on the Earth. of the low troposphere up to 3 km altitude over the coastal The paper further underlines the difficulty of modeling the margins of East Antarctica. Radiosonde data show a large low-level flow over the margins of the ice sheet with a state- spatial variability of wind, temperature and humidity pro- of-the-art atmospheric model. files, with different features between stations in katabatic re- gions (e.g., Dumont d’Urville and Mawson stations), stations over two ice shelves (Neumayer and Halley stations) and re- gions with complex orography (e.g., McMurdo). At the Du- 1 Introduction mont d’Urville, Mawson and Davis stations, the yearly me- dian wind speed profiles exhibit a clear low-level katabatic The margins of East Antarctica are a region of great inter- jet. During precipitation events, the low-level flow generally est in meteorology particularly due to the fierce katabatic remains of continental origin and its speed is even reinforced winds that fascinated and severely tested the pioneering sci- due to the increase in the continent–ocean pressure gradient. entific expeditions in the far south. These so-called kata- Meanwhile, the relative humidity profiles show a dry low tro- batic winds that flow over the sloping surfaces of the ice posphere, suggesting the occurrence of low-level sublimation sheet can attain very high speeds in confluence regions such of precipitation in katabatic regions but such a phenomenon as the Adélie Land (Mawson, 1915; Wendler et al., 1993; does not appreciably occur over the ice shelves near Halley Parish and Walker, 2006) or the Lambert Glacier (Parish and and Neumayer. Although ERA-Interim and ERA5 reanalyses Bromwich, 1987). assimilate radiosoundings at most stations considered here, In winter, the strong radiative deficit of the surface leads substantial – and sometimes large – low-level wind and hu- to persistent, intense and directionally constant near-surface midity biases are revealed but ERA5 shows overall better per- winds from the interior of the continent. Beyond the coastal formance. A free simulation with the regional polar version slopes the atmospheric boundary-layer flow considerably of the Weather Research and Forecasting model (Polar WRF) thickens in response to a piling up of cold air downstream (at a 35 km resolution) over the entire continent shows too- over the sea ice or the ice shelves. This accumulation of cold strong and too-shallow near-surface jets in katabatic regions air is responsible for a pressure gradient force opposing the especially in winter. This may be a consequence of an under- katabatic wind that is particularly intense under weak synop- estimated coastal cold air bump and associated sea–continent tic forcing (Van den Broeke et al., 2002; Van den Broeke and pressure gradient force due to the coarse 35 km resolution of Van Lipzig, 2003). In some regions of the ice sheet, like in the Polar WRF simulation. Beyond documenting the vertical Adélie Land or in Coats Land, the flow regime transition can Published by Copernicus Publications on behalf of the European Geosciences Union. 4660 É. Vignon et al.: On the fine vertical structure of the low troposphere be abrupt (Pettré and André, 1991; Gallée et al., 1996; Gallée batic winds significantly diminish the precipitation amount and Pettré, 1998; Renfrew, 2004) and is therefore interpreted that actually reaches the ice sheet surface. As katabatic winds as a hydraulic jump, often referred to as a katabatic jump or are relatively dry, they sublimate an important part of the pre- Loewe’s phenomenon. Such jumps are however rarely im- cipitation before it reaches the ground surface. This dry layer portant in other sectors of the Antarctic periphery like Terra manifests with low values of relative humidity in the bound- Nova Bay (Parish and Bromwich, 1989). ary layer during snowfall events. From a model simulation In summer, the absorption of shortwave radiation by the with the Integrated Forecast Model (IFS), the authors esti- surface diminishes the katabatic forcing and the large-scale mate that sublimation corresponds to 17 % of the precipita- pressure gradient force dominates the overall momentum tion over the entire continent. This term reaches up to 35 % budget of the boundary layer (Van den Broeke et al., 2002; when considering only the margins of the ice sheet. Parish and Cassano, 2003). The lower sea-ice concentration The reliable representation of the Antarctic climate in re- and sea-ice extent generally diminish the offshore extent of gional and global climate models as well as atmospheric re- the land flow due to the development of diurnal sea breezes analyses therefore strongly depends on their ability to repro- (Pettré et al., 1993) and because of the thermal and mechan- duce the low-level atmospheric flow at the Antarctic periph- ical erosion of the flow at the ocean surface. In Adélie Land ery. A significant body of literature has focused on the near- and in Queen Maud Land, a typical summertime diurnal cy- surface atmosphere in Antarctica (Parish and Bromwich, cle of the low-level flow has been evidenced. Nocturnal kata- 2007; Nicolas and Bromwich, 2014; Bracegirdle and Mar- batic forcing alternates with a combination of thermal wind shall, 2012) and its representation in meteorological reanal- forcing and surface radiative heating that leads to a weaken- yses and models. In particular, Wille et al.(2017) have high- ing of downslope diurnal flow or even to a diurnal anabatic lighted an excessive wind speed and a dry bias in the bound- flow (Gallée and Pettré, 1998; Parish et al., 1993; Bintanja, ary layer over the Ross Ice Shelf in the Antarctic Mesoscale 2000). Prediction System (AMPS; http://www2.mmm.ucar.edu/rt/ The interactions between the low-level atmospheric flow amps, last access: 1 April 2019), which is based on simu- from the interior of the ice sheet and the oceanic air masses lations from the polar version of the Weather Research and over – or coming from – the austral ocean are varied and Forecasting model (Polar WRF). Sanz Rodrigo et al.(2013) complex. For instance, it has been shown that katabatic have further stressed that the near-surface wind speed in es- winds are stronger when an anticyclone sets over the plateau carpment areas is strongly underestimated in ERA40 and or when the pressure over the ocean is low as during the ERA-Interim reanalyses, and to a lesser extent in model approach of deep cyclones (Parish and Bromwich, 1998; simulations with the Regional Atmospheric Climate Model Naithani et al., 2003; Orr et al., 2014). On the other hand, (RACMO). The simulations with the EC-Earth global cli- katabatic winds have been shown to be a key driver of the mate model in Bintanja et al.(2014) and in the LMDZ gen- mesoscale cyclogenesis off Adélie Land and the Ross Sea eral circulation model in Vignon et al.(2018) concur with (Gallée and Schayes, 1994; Bromwich et al., 2011) or over these conclusions, especially at low horizontal resolutions the Weddell Sea (Carrasco et al., 2003). due to the coarse representation of terrain slopes. From a meteorological and climate perspective, the low- The vertical structure of the atmosphere over the coastal level atmospheric dynamics over the coastal margins of regions of the ice sheet and its representation in models Antarctica plays a key role for the energy and mass bud- have been less documented. Using radiosonde data, Streten gets of the atmosphere over the ice sheet. The low-level (1990) and König-Langlo et al.(1998) study the climatolog- horizontally diverging and northward drainage flow from ical structure of the whole troposphere and low stratosphere Antarctica drives a thermally direct zonal circulation. Sub- at the Mawson and Dumont d’Urville (hereafter DDU), Neu- sidence – and associated upper cyclonic vorticity – takes mayer and Halley stations, respectively. However, they do place over the central ice sheet, while rising motions occur not give specific details on the structure of the boundary layer over the ocean, leading to an active mass exchange between or on the low troposphere. Significant advances in our un- Antarctica and subpolar latitudes (Parish and Bromwich, derstanding of the low-level flow have been achieved due to 1998, 2007). Moreover, the low-level circulation over coastal the case studies – often in summer – using a combination of Antarctica is critical for the surface mass balance of the ice tethersonde and radiosonde observations (e.g., Sorbjan et al., sheet. While transient eddies are responsible for the mois- 1986; Bintanja, 2000) and thanks to the deployment of so- ture transport towards the continent, the export of moisture dars in Coats Land (Renfrew and Anderson, 2007), in Adélie by the mean circulation mostly occurs in the low troposphere Land (Argentini et al., 1996; Gera et al., 1998) and in the (Connolley and King, 1993; Van Lipzig and Van Den Broeke, Terra Nova Bay area (Argentini and Mastrantonio, 1994).