2 Feasibility study

The results reported in this article were produced by a collaboration between the DA (Astrophysical Departement of Nice University) and the CNRM (Centre National des Recherches Meteorologiques) to provide ESO (European Southern Observatory) with an assessment of the potential and limitations of seeing prediction in the context of a flexible operation mode for the VLTI. In order to verify the technical feasibility, we compared the MESO-NH model (mesoscale, non-hydrostatic) simulations with the optical measurements obtained during the PARSCA93 campaign at Paranal (Fuchs & Vernin 1994) by the team of Nice University of J. Vernin and by M. Sarazin of ESO. During this campaign both the seeing and $C_{\rm N}^2$ profiles were assessed. To our knowledge, it is the first time that such a detailed comparison has been attempted. A forthcoming article will be devoted to a complete statistical comparison of the PARSCA93 campaign and our simulations. In this paper we intend to present a detailed description of the Meso-Nh model and its performance related to the astronomical application of seeing forecasting. The astronomical code has now been adapted to simulate all the principal atmospheric parameters that are necessary for the exploitation of a telescope and the selection of different observing modes: Direct Imaging, Spectroscopy and Interferometry (Site selecting workgroup 1990).

In order to better study the model performances we used a simple model configuration. We first initialize the numerical model with orographically unperturbed meteorological data provided by either Meteorological Station radiosounding (Antofagasta station in our case) or ECMWF (European Center for Medium Weather Forecast) data analysis computed in the nearest grid point to the meteorological station. Both the station and the analysis grid point are located north-west of the Paranal mountain. Knowing that the prevailing wind blows in a NW-SE direction, we can reasonably consider that the atmosphere above the Antofagasta station is unperturbed by orography. We start the numerical simulations, at the initial instant t₀, with a uniform distribution of meteorological parameters inside each horizontal plane. Then the model simulates the effects of the adjustment to orographic forcing in a realistic way. After a certain lapse of time, the system converges to a stable solution or it oscillates around an equilibrium condition. We validate the model results by comparing with optical measurements (realized with a SCIDAR and DIMM techniques) related to a precise night, hour and for a selected time interval.