Statistics of precipitable water vapour above the sites of the 6-m Big Telescope Alt-azimuthal and new 3-m Large Solar Telescope using ERA5 data

Abstract

ABSTRACTAtmospheric conditions have a major impact on the quality of ground-based astronomy observations. Among atmospheric molecular gases, water vapour presents a significant challenge in ground-based astronomical observations in spectral bands ranging from radio to infrared. The impact of water vapour on astronomical observations is expressed in terms of precipitable water vapour (PWV). Water vapour is a greenhouse gas on Earth that plays an important role in various atmospheric processes, including global climate change. In this work, we analyse the long-term (1975–2021) statistics of PWV above two mid-latitude sites: the 6-m Big Telescope Alt-azimuthal in the Caucasus Mountains and the new 3-m Large Solar Telescope in the Sayan Mountains. Both astronomical sites were established over 45 years ago and are located at an altitude of 2000 m above sea level. We use ERA5 – a new-generation European Centre for Medium-Range Weather Forecasts (ECMWF) retrieved atmospheric reanalysis product. First, we evaluate the reliability of the PWV variable provided in the ERA5 data using global navigation satellite system (GNSS) measurement data. We present the analysis of PWV statistics, including seasonal behaviour above the sites in the infrared astronomical observations context. Then we provide long-term changes of PWV for the period 1975–2021 and examine the data set for the statistical significance of trends. Finally we discuss PWV correlation with other atmospheric parameters: surface temperature and total cloud cover in the regions of both sites.