The surface layer observed by a high-resolution sodar at DOME C , Antarctica

A one-year field experiment started on December 2011 at the French Italian station of Concordia at Dome C, East Antarctic Plateau. The objective of the experiment was the study of the surface layer turbulent processes under stable/very stable stratifications, and the mechanisms leading to the formation of the warming events. A sodar was improved to achieve the vertical/temporal resolution needed to study these processes. The system, named surface layer sodar (SL-sodar), may operate both in high vertical resolution (low range) and low vertical resolution (high range) modes. SL-sodar observations were complemented with in situ turbulence and radiation measurements. A few preliminary results, concerning the standard summer diurnal cycle, a summer warming event, and unusually high frequency boundary layer atmospheric gravity waves are presented.


I. INTRODUCTION
t Dome C, light wind and clear sky favor weak turbulence and mixing, and strong temperature gradients near the surface.The boundary layer height varies depending on the relative contribution of the mechanical and thermal generation of turbulence.Because of the extremely low temperature and humidity, and the high elevation, Dome C is a potentially ideal site for astronomical observations.For this reason, the optical turbulence over the Antarctic plateau has been a subject of studies by astronomers [Lawrence et al. 2004, Aristidi et al. 2005, Agabi et al. 2006, Lascaux et al. 2009].
At Dome C, Ricaud et al. [2012] made an experiment to monitor the vertical evolution of the planetary boundary layer (PBL) temperature and humidity in the transition from winter to summer, by using a microwave radiometer operating at 60 GHz and 183 GHz.Due to the instrument low vertical resolution, the fine structure of the thermal turbulence could not be evidenced.

* Corresponding author:
Giampietro Casasanta, g.casasanta@isac.cnr.itA Ghenton et al. [2010] analyzed the Dome C 45 m meteo tower measurements (temperature, humidity, wind speed and direction measurements) for a three-week period in summer 2008.The main task of their work was to compare these measurements with the 6hourly European Center for Medium-Range Forecasts (ECMWF) analyses and the daily radiosoundings.Pietroni et al. [2012], using the temperature profiles measured with a passive microwave radiometer [Kadygrov andPick 1998, Argentini et al. 2004], characterized the behavior of the surface-based temperature inversions over the course of a year.They found that during the winter and the summer "nights" strong temperature inversions allow for a mixing depth of a few tens of meters with a quiescent layer above, decoupled from the surface layer.During the summer, despite the low surface temperatures, weak convection generates the development of a mixed layer characterized by a maximum depth of 200-400 m [Argentini et al. 2005].The diurnal behavior of this mixed layer, monitored with a sodar, was described by Mastrantonio et al. [1999], Argentini et al. [2005], and King et al. [2006].The sodar measurements, because of the membrane ringing just after the tone burst emission, allowed for a first echo recording starting at 20-30 m, depending on the membrane ringing time.Because of this limitation, those measurements did not allow to study the surface turbulent layer under stable conditions, neither in summer nor in winter.
An advanced high-resolution sodar named surface-layer sodar (hereafter SL-sodar), allowing for the lowest observation height at ≈ 2 m and a vertical resolution of ≈ 2 m, was developed by the ISAC-CNR [Argentini et al. 2011].The SL-sodar was deployed at Concordia station after a preliminary test period at the ISAC-CNR research centre of Rome [Argentini et al. 2011].In this paper, a few preliminary results from the summer season are shown.

II. SITE AND INSTRUMENTATION
Concordia is a permanent station located at Dome C (75.1° S, 123.3°E, 3233 m a.s.l.), on the East Antarctic plateau, at approximately 1000 km from the nearest coast.One-year in situ turbulence and radiation measurements, as well as SL-sodar observations, were carried out at Concordia station from December 2011 up to December 2012.The can change according to the two modes listed in Table 1."Mode 1" allows to monitor the convective mixed layer, while "Mode 2", with higher vertical resolution, is used to investigate the near-surface stable layer.Measurements of turbulence were made with a Metek USA-1, a three-axes sonic thermoanemometer (sampling frequency of 10 Hz) installed on a 3.5 m mast.The heat and momentum fluxes are estimated using the eddy covariance method [Lee et al. 2004].The longwave and shortwave radiation components (up and down) are measured using Kipp & Zonen CNR1 pyrgeometers and pyranometers, installed at 1.5 m above the snow surface.
In this paper, unless told otherwise, the local standard time (LST) is used.

Summertime ABL diurnal behavior
The facsimile recording of the vertical/temporal variation of the acoustic backscattering (sodargram) "depicts" the thermal structure of the atmosphere.The scattering elements producing the change of echo intensity are the small-scale temperature inhomogeneities due to thermal turbulence.
Temperature fluctuations are usually associated with the convective plumes originating from the surface, or with potential temperature gradients and wind shear usually occurring in the inversion layers.During the summer, the boundary layer at Dome C can reach a depth of 200-400 m.Therefore, the "Mode 1" setting (see Table 1) was used to catch the whole vertical evolution during the daily cycle.timated as the height at which an elevated secondary maximum occurs (i.e. the height of the turbulent zone characterizing the top of the mixing layer).For the same day, Figure 1b shows the downward longwave radiation  ↓ and the sonic temperature   , Figure 1c the wind speed and direction.The heat turbulent flux  0 and friction velocity  * are plotted in Figure 1d. ↓ ranges between 100 and 150 W m -2 , while   reaches its minimum (-35°C) at 0300 LST, and its maximum (-25°C) between 1200 and 1500 LST.The direction indicates a wind from the continent persisting the whole day.The maximum wind speed (6 ms -1 ) occurs because of the momentum transfer from the free atmosphere to the surface layer, as a consequence of the turbulent mixing (confirmed by the positive and increasing values for  0 and  * ) during convective hours.

Gravity waves in the ABL
Between 2 and 5 February 2012, waves with periods of a few minutes are observed under stable stratification for more than 35% of the time.The resolution achieved by the SL-sodar with the "Mode2" setting (Table 1) allowed to visualize the fine structure of these wave patterns.At the transition time from the stable to the unstable boundary layer (between 0800 and 1000 LST) the capping inversion layer oscillates with an amplitude that reaches 70 m (Figure 2).The apparent period of these oscillating structures was estimated through the spectral analysis of the sonic temperature and the wind components of the sonic anemometers, installed at 3 different levels (7.0, 22.8, and 37.5 m) on a 45-m meteorological tower [Genthon et al. 2009] located at ≈ 1 km from the SLsodar.
In Figures 3a and 3b 0930 LST, gives an apparent period ranging between 90 and 120 s.The apparent period remains approximately the same also when the inversion strength and height change.This behavior indicates that the origin of these waves might be a disturbance (probably the wind shear) originating between the inversion layer and the free atmosphere.A similar behavior was observed during other days.
A summer warming event Warming events of particular intensity were regularly observed at Dome C during the winter [Argentini et al. 2001, Petenko et al. 2007, Ghenton et al. 2013].During these events the surface temperature sometimes has a sharp increase of 20-40 °C [Argentini et al. 2001], reaching then the typical summer values.
Studies carried out at South Pole [Carroll 1982, Stone et al. 1990, Stone and Kahl 1991, Stone 1993] have evidenced that these warming events are generally observed in presence of clouds.Neff [1999], analyzing the particles trajectories across Antarctica, found that these warming processes are mostly due to warm and moist air intrusion and to the condensation of nuclei originating from the Weddell Sea, producing a wide variety of cloud types.
Carroll [1982] suggested two possible mechanisms of this phenomenon: the advection of warm air, and/or the vertical mixing of air from different layers.Schwerdtfeger and Weller [1977] related the surface warming to the variation of long-wave radiation emitted by the clouds associated to the moist air in the upper part of the atmosphere.The measurements collected during a summer warming event observed between 8 and 17 January 2012 have been analyzed.Figure 4 shows the time series of the downwelling longwave radiation  ↓ and the sonic temperature   (Figure 4a), the wind speed and direction (Figure 4b), the heat flux  0 and the friction velocity  * (Figure 4c) during the selected period.the behavior is again the typical summer one, with a peak in the wind speed at 1200 LST.
The sodargram for 10 January, with the mixing height estimate superimposed (dots), confirms this hypothesis.A clear and intense convective activity is observed during the whole day even in the nighttime between 9 and 10 January.

IV. SUMMARY
The main results of this observational study can be summarized as follows: • during the summer, under steady weather conditions, the atmospheric boundary layer thermal structure is characterized by the alternation of a stable stratified layer with a convective boundary layer, following a behavior similar to that observed at mid-latitudes.
• A regular wave activity was observed within the inversion layer.The time period of these waves ranges between 90 s and 120 s, and their origin can be attributed to the wind shear across the inversion layer.
• The summer warming events take origin from the presence of clouds advected from the coast toward the Dome C area.Clouds modify the surface radiation budget by increasing the downward longwave radiation, which in turn produces an increase of surface temperature, leading to convection.Due to the decrease of the temperature inversion strength, the vertical mixing, combined with the wind shear, allows the transport of warm air from the upper parts of the atmosphere towards the surface, further contributing to the surface warming.

Figure 2 .
Figure 2. Sodargram for 5 February 2012, the full dots represent the mixing height estimate.High amplitude waves are observed between 0800-0930 LST.