Precipitable Water Vapor: Considerations on the water vapor scale height, dry bias of the radiosonde humidity sensors, and spatial and temporal variability of the humidity field

The Thirty Meter Telescope (TMT) and the European Extremely Large Telescope (E-ELT) site testing teams have recently finalized their site testing studies. Since atmospheric water vapor is the dominant source of absorption and increased thermal background in the infrared, both projects included precipitable water vapor (PWV) measurements in their corresponding site testing campaigns. TMT planned to monitor PWV at the sites of interest by means of using infrared radiometers. Technical failures and calibration issues prevented them from having a sufficiently long PWV time-series to characterize the sites using this method. Therefore, for the sites in Chile TMT used surface water vapor density measurements, which taken together with an assumed water vapor scale height, allowed for the estimation of PWV. On the other hand, the E-ELT team conducted dedicated PWV measurement campaigns at two of their observatory sites using radiosonde soundings to validate historical time-series of PWV reconstructed by way of a spectroscopic analysis of astronomical standard sources observed at the La Silla and the Paranal sites. The E-ELT also estimated the median PWV for the Armazones site from extrapolation of their Paranal statistics accounting for the difference in elevation between the two places; and also from archival analysis of radiosonde data available from the city of Antofagasta by integration of the humidity profile starting from 3000 m altitude. In the case of the Armazones site, the published median of PWV by both groups differ by about 1 mm with the E-ELT values being drier than those estimated by the TMT group. This work looks at some of the reasons that could explain this difference, among them the water vapor scale height, the horizontal variability of the water vapor field, and an unaccounted correction due to a dry bias known to affect the radiosondes relative humidity sensors.