Atmospherical mesoscale models can offer unique potentialities to characterize and discriminate potential astronomical
sites. Our team has recently completely validated the Meso-Nh model above Dome C (Lascaux et
al. 2009, 2010). Using all the measurements of C<sup>2</sup><sub><i>N</i></sub>
profiles (15 nights) performed so far at Dome C during
the winter time (Trinquet et al. 2008) we proved that the model can reconstruct, on rich statistical samples,
reliable values of all the three most important parameters characterizing the turbulence features of an antarctic
site: the surface layer thickness, the seeing in the free atmosphere and in the surface layer. Using the same
Meso-Nh model configuration validated above Dome C, an extended study is now on-going for other sites above
the antarctic plateau, more precisely South Pole and Dome A. In this contribution we present the most important
results obtained in the model validation process and the results obtained in the comparison between different
astronomical sites above the internal plateau. The Meso-Nh model confirms its ability in discriminating between
different optical turbulence behaviors, and there is evidence that the three sites have different characteristics
regarding the seeing and the surface layer thickness. We highlight that this study provides the first homogeneous
estimate, done with comparable statistics, of the optical turbulence developed in the whole 20-22 km above the
ground at Dome C, South Pole and Dome A.
In this contribution we present how the mesoscale model Meso-Nh model together with the Astro-Meso-Nh
and a set of diagnostic tools allow for a full 3D investigation of the C<sup>2</sup><sub><i>N</i></sub> and derived integrated astroclimatic
parameters. The possibility to study simultaneously 3D fields of meteorological and astroclimatic parameters
permits us to provide a comprehensive understanding of the characteristics of the atmospheric flow as well as
the physical mechanisms that produce them. To illustrate the different diagnostics and their potentialities, we
investigated one night and looked at instantaneous fields of meteorologic and astroclimatic parameters. To show
the potentialities of this tool for applications in an Observatory we ran the model above sites with very different
optical turbulence (OT) distributions: the antarctic plateau (Dome C, Dome A, South Pole) and a mid-latitude
site (Mt. Graham, Arizona). We put particular emphasis on the 2D maps of integrated astroclimatic parameters
(such as seeing, isoplanatic angles, ...) and to the possibility to calculate them in different slabs (having what
ever thickness) at different heights in the troposhere. The latter option is particularly useful for the seeing. For
this reason this is an useful tool of prediction and investigation of the turbulence structure and it can support
the optimization of the AO, GLAO and MCAO systems running at the focus of the ground-based telescopes.
The examples we provide put clearly in evidence that different astroclimatic sites present different OT behaviors.
Besides, our tool allowed us for discriminating these sites.
A characterization of the optical turbulence vertical distribution and all the main integrated astroclimatic parameters
derived from the C<sup>2</sup><sub><i>N</i></sub> and the wind speed profiles above Mt. Graham is presented. The statistic includes
measurements related to 43 nights done with a Generalized Scidar (GS) used in standard configuration with a
vertical resolution of ~1 km on the whole 20-22 km and with the new technique (HVR-GS) in the first kilometer.
The latter achieves a resolution of ~ 20-30 m in this region of the atmosphere. Measurements done in different
periods of the year permit us to provide a seasonal variation analysis of the C<sup>2</sup><sub><i>N</i></sub>. A discretized distribution of the
profiles useful for the Ground Layer Adaptive Optics (GLAO) simulations is provided and a specific
analysis for the LBT Laser Guide Star system ARGOS case is done including the calculation of the 'gray zones'
for J, H and K bands. Mt. Graham confirms to be an excellent site with median values of the seeing without
dome contribution equal to 0.72", the isoplanatic angle equal to 2.5" and the wavefront coherence time equal to
4.8 msec. We provide a cumulative distribution of the percentage of turbulence developed below H* where H*
is included in the (0,1 km) range. We find that 50% of the whole turbulence develops in the first 80 m from the
ground. The turbulence decreasing rate is very similar to what has been observed above Mauna Kea.
Since November 2004 we measured the optical turbulence (C<sup>2</sup><sub>N</sub>
profiles) with a Generalized Scidar (GS) placed at
the focus of the Vatican Advanced Technology Telescope at Mt.Graham, Arizona. The present statistic consists
in measurements related to 43 nights covering different periods of the solar year. In this paper we calculate the
statistics of the astroclimatic parameters
(C<sup>2</sup><sub>N</sub>, seeing ε , isoplanatic angle θ<sub>0</sub>, wavefront coherence time τ<sub>0</sub>) and
we compare these values with those measured above other top level astronomic sites. All profiles are reduced
into a form suitable to be used as inputs for adaptive optics point spread function simulations for the conceptual
design of the Laser Guide Star Facility supported by a GLAO system of the Large Binocular Telescope. With
GS measurements done observing wide binaries (30-35 arcsec), the turbulence in the first kilometer above the
ground is characterized with the vertical resolution (200-250 m) required for the optimization of a 4 arcmin
field of view AO system. It is the first time that are published measurements of the optical turbulence vertical
distribution above a mid-latitude site with such a high vertical resolution and such a high statistical reliability.
On 8 of those nights, employing cross-correlation scintillation maps of wide binaries and the method described
in Ref. we characterize the distribution of the optical turbulence in the first kilometer at the extremely high
vertical resolution of 20-30 meters.
Mesoscale model such as Meso-Nh have proven to be highly reliable in reproducing 3D maps of optical turbulence
(see Refs. 1, 2, 3, 4) above mid-latitude astronomical sites. These last years ground-based astronomy has
been looking towards Antarctica. Especially its summits and the Internal Continental Plateau where the optical
turbulence appears to be confined in a shallow layer close to the icy surface. Preliminary measurements have
so far indicated pretty good value for the seeing above 30-35 m: 0.36" (see Ref. 5) and 0.27" (see Refs. 6, 7) at
Dome C. Site testing campaigns are however extremely expensive, instruments provide only local measurements
and atmospheric modelling might represent a step ahead towards the search and selection of astronomical sites
thanks to the possibility to reconstruct 3D C<sup>2</sup><sub>N</sub>
maps over a surface of several kilometers. The Antarctic Plateau
represents therefore an important benchmark test to evaluate the possibility to discriminate sites on the same
plateau. Our group<sup>8</sup> has proven that the analyses from the ECMWF global model do not describe with the required
accuracy the antarctic boundary and surface layer in the plateau. A better description could be obtained
with a mesoscale meteorological model. In this contribution we present the progress status report of numerical
simulations (including the optical
turbulence - C<sup>2</sup><sub>N</sub>) obtained with Meso-Nh above the internal Antarctic Plateau.
Among the topic attacked: the influence of different configurations of the model (low and high horizontal resolution),
use of the grid-nesting interactive technique, forecasting of the optical turbulence during some winter
The atmospheric properties above three sites on the Internal Antarctic Plateau are investigated for astronomical
applications calculating the monthly median of the analysis-data from ECMWF (European Centre for Medium-Range Weather Forecasts) for an entire year (2005) thus covering all seasons. Radiosoundings extended on a
yearly time scale from Dome C and the South Pole are used to verify the reliability of the analyses in the
free atmosphere and to study the wind speed in the first 100 m as the analysis-data are not optimized for this
altitude-range. The wind speed in the free atmosphere is obtained from the ECMWF analyses from all three
sites. It appears that the strength of the wind speed in the upper atmosphere in winter is correlated to the
distance of the site from the centre of the polar high.
The Richardson number is employed to investigate the stability of the free atmosphere and, consequently,
the probability to trigger thermodynamic instabilities above the three sites. We find that, in a large majority of
the cases, the free atmosphere over the Internal Antarctic Plateau is more stable than at mid-latitude sites.
Given these data we can obtain a ranking of the three sites both with respect to wind speed, in the free
atmosphere as well as in the surface layer, and with respect to the stability of the atmosphere, using the
Dome C is considered a site particularly suited for wide-field imaging thanks to its shallow surface turbulent
layer and its weak turbulence in the free atmosphere. What is the quantitative gain one can hope to achieve
at Dome C with respect to a mid-latitude site? With the point spread function model defined analytically in
the spatial frequency domain we are better able to connect the morphological and statistical behaviour of the
turbulence profile to the trade-off between the adaptive telescope's field of view and a figure of merit for survey
rate. A familiar image quality figure of merit is the radius of 50% encircled energy, and for J-band images it
quickly identifies the requirement that will make a Dome C telescope, 8 meters above the ice, competitive with
a mid-latitude one. From the radius of 50% encircled energy we derive the wide-field survey rate equation to
estimate the impact of uncertainty in the vertical distribution of ground-layer turbulence on the trade-off between
field of view (in the domain 10-20 arcminutes) and their survey rate.