OPTICAL TURBULENCE AND ENVIRONMENT CONDITIONS

The optical turbulence directly depends on basically all the meteorological parameters even if mainly on the wind speed and the potential temperature. The optical turbulence is a random phenomenum, highly non linear and, for this reason, it is a paramater that has to be characterized in statistical terms from a qualitative as well as quantitive point of view. The wind speed and the potential temperature can be measured and/or calculated in a much easier way and with higher level of confidence than the optical turbulence because they changes on spatial and temporal scales larger than those characterizing the optical turbulence fluctuations. For this reason, measurements and predictions with classical General Circulation Models and mesoscale model can can achieve realistic estimates of these parameters and provide useful insights on the mechanisms producing and triggering the optical turbulence. In some contexts it can be, therefore, very useful to study directly the wind speed, the potential temperature and derived parameters such as the Richardson number, to reitreve information on the optical turbulence characteristics.

The Antarctica continent has been studied in a detailed way in the context of the ForOT actvities. Here a few examples showing how the optical turbulence features/characteristics can be inferred from studies on meteorological parameters.




Fig. 1: (from Hagelin et al. 2008): Monthly median wind speed for 2005. Dome A (blue line), Dome C (green line), Dome F (blue light), South Pole (red line). Data are analyses from the European Centre for Medium-Range Weather Forecasts (http://www.ecmwf.int). The wind speed is extremely weak all along the 20 km in the summer period (December-March) but, above 10 km, it ncreases monotonically. Moreover, the increment in the wind speed is more important for some sites (Dome C) and less for others (South Pole). n this paper it has been argued that the high values of the wind speed in the high part of the atmosphere is due ot the polar vortex circulation and it has been proposed that the farther is the site from the centre of the polar vortex, the higher is the wind speed in the high part of the atmosphere.


Fig. 2: (from Lascaux et al. 2009): Median wind speed above the Antarctic continent (May-Spetember 2005) at 15 km and 20 km above sea level. Black dots indicate Sout Pole, Dome C, Dome A and Dome F. Data are analyses from the European Centre for Medium-Range Weather Forecasts (http://www.ecmwf.int). Left: from 6 ms-1 to 30 ms-1 with step of 1 ms-1. Right: from 6 ms-1 to 30 ms-1 with step of 1ms-1, the two more external isolines indicate the wind speed at 35 ms-1 and 40 ms-1.
With Fig.2 it has been confirmed the thesis presented in Hagelin et al. (2008) (see caption Fig.1). The polar vortex centre (corresponding to the minimum of the wind speed) is affectd by small fluctuations during the winter time but it remains confined in the region between South Pole and Dome A, at a more or less equal distance between the two sites as indicated in Fig.2. Dome C is the farthest from the polar vortex centre and, by consequence, the site with the highest wind speed in the high part of the atmosphere.

What about the wind speed near the ground ?


Fig. 3:
(from Hagelin et al. 2008): Radiosoundings median wind speed values near the ground at Dome C (red line) and South Pole (blue line). From April 2006 up to November 2006. The wind speed at the ground is much weaker at Dome C than at South Pole but, due to the strong wind shear in the first tens of meters, the wind speed at 10-30 m is higher at Dome C than at South Pole.
Data of Dome C are from the Osservatorio Meteo Climatologico (http://www.climantartide.it) of the Concordia Station at Dome Coperated by the Programma Nazionale Ricerche Antartide (PNRA) and the Institut Polaire Paul Emil Victor (IPEV) and from the Antarctic Research Centre, University of Winsconsin, Madison.


Which are the consequences for the optical turbulence ?

- Near the ground, the strong thermal stability (see Hagelin et al. (2008) - Fig.5) and the strong wind speed shear induce a turbulence surface layer that is typically thin and characterized by strong optical turbulence. This feature is more evident during the winter when both the thermal stability and the wind speed shear are more important.

- Using the wind speed and potential tempearture vertical profiles, it is possibile to retrive the Richardson number and, following Masciadri & Garfias (2001), the probability to trigger the optical turbulence in the 20 km from the ground. In Hagelin et al. (2008) it is proved that such a method is not suggested to reitrieve information near the ground  if we use the analyses from the General Circulation Models.


Fig. 3: (from Hagelin et al. 2008): Monthly median of the inverse of the Richardson number (1/Ri) for Dome A (blue line), Dome C (green Line) and South Pole (red line). Data are analyses from the European Centre for Medium-Range Weather Forecasts (http://www.ecmwf.int). The higher the (1/Ri), the higher is the probability to trigger the optical turbulence. The reader has to pay attention because, a higher probability to trigger optical turbulence does not mean a higher probability that the optical turbulence is sustained.

Acknowledgments: This work is funded by the Marie Curie Excellence Grant ForOT - MEXT-CT-2005-023878

  E.Masciadri, 3/2010