Site selection for Extremely Large Telescopes using the FriOWL software and global re-analysis climate data

FriOWL is a site selection tool for large or extremely large telescope projects. It consists of a graphical user interface and a large global climatic and geophysical database, and is directly accessible on the world wide web. A new version (version 3.1) of the software has recently been developed by scientists at the University of Bern (Switzerland) and European Southern Observatory (Germany). The main feature of the new FriOWL database is the inclusion of ERA40 re-analysis data, giving access to over 40 years of long-term climate data. New software tools, programmed in the style of a Geographical Information System, include the capability of resampling layers and time series extraction. A new global seismic hazard layer has been introduced, as well as very high resolution (1km) topographic tiles. Reclassification and overlaying of layers is also possible. Although FriOWL is primarily designed for site selection projects, it can equally be used in other climate studies. It is especially important in the determination of the climatic stability of a potential site, and in the analysis of climatic anomalies and trends. The long-term astroclimatological seeing and photometric statistics for the Paranal and La Silla observatories can be used to validate FriOWL. A case study of ESO Paranal using FriOWL reveals that the deterioration in seeing conditions since 1998 is co-incident with a strong increase in 1000 hPa geopotential height to the south-east of the observatory; there may be a link with the Interdecadal Pacific Oscillation.