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In support of characterization of potential sites for the European Extremely Large Telescope (E-ELT) the European
Southern Observatory (ESO), the Institute for Space Imaging Science (ISIS) and the astrometeorology group of the
Universidad Valparaiso have jointly established an improved understanding of atmospheric precipitable water vapour
(PWV) above ESO's La Silla Paranal Observatory. In a first step, 8 years worth of high resolution near-IR spectra taken
with VLT-UVES have been statistically analysed to reconstruct the PWV history above Paranal. To this end a radiative
transfer model of Earth's atmosphere (BTRAM) developed by ISIS has been used. A median PWV of 2.1 mm is found
for Paranal based on UVES data covering the period 2001-2008. Furthermore we conclude that Paranal can serve as a
reference site for Northern Chile due to the stable atmospheric conditions in the region. The median offset between
Paranal and Armazones is derived to be 0.3 mm, but local arbitrary variations of a few tenths of a mm between the sites
have been found by measurement. In order to better understand the systematics involved two dedicated campaigns were
conducted in August and November 2009. Several methods for determining the water column were employed, including
radiosonde launches, continuous measurements by infrared radiometer, and VLT instruments operating at various
wavelengths: CRIRES, UVES, VISIR and X-shooter. In a first for astronomical instruments all methods have been
evaluated with respect to the radiosondes, the established standard in atmospheric research. Agreement between the
radiosondes and the IR radiometer (IRMA) is excellent while all other astronomical methods covering a wavelength
range from 700 - 20000 nm have also been successfully validated in a quantitative manner. All available observations
were compared to satellite estimates of water vapour above the observatory in an attempt to ground-truth the satellite
data. GOES can successfully be used for site evaluation in a purely statistical approach since agreement with the
radiosondes is very good on average. For use as an operational tool at an observatory GOES data are much less suited
because of significant deviations depending on atmospheric conditions. We propose to routinely monitor PWV at the
VLT and to use it as an operational constraint to guide scheduling of IR observations at Paranal. For the E-ELT we find
that a stand-alone high time resolution PWV monitor will be essential for optimizing the scientific output.
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