Twenty years of precipitable water vapor measurements in the Chajnantor area

Abstract

Context. Interest in the use of the Chajnantor area for millimeter and submillimeter astronomy is increasing because of its excellent atmospheric conditions. Knowing the general site annual variability in precipitable water vapor (PWV) can contribute to the planning of new observatories in the area. Aims. We seek to create a 20-year atmospheric database (1997−2017) for the Chajnantor area in northern Chile using a single common physical unit, PWV. We plan to extract weather relations between the Chajnantor Plateau and the summit of Cerro Chajnantor to evaluate potential sensitivity improvements for telescopes fielded in the higher site. We aim to validate the use of submillimeter tippers to be used at other sites and use the PWV database to detect a potential signature for local climate change over 20 years. Methods. We revised our method to convert from submillimeter tipper opacity to PWV. We now include the ground temperature as an input parameter to the conversion scheme and, therefore, achieve a higher conversion accuracy. Reults. We found a decrease in the measured PWV at the summit of Cerro Chajnantor with respect to the plateau of 28%. In addition, we found a PWV difference of 1.9% with only 27 m of altitude difference between two sites in the Chajnantor Plateau: the Atacama Pathfinder Experiment and the Cosmic Background Imager near the Atacama Large Millimeter Array center. This difference is possibly due to local topographic conditions that favor the discrepancy in PWV. The scale height for the plateau was extracted from the measurements of the plateau and the Cerro Chajnantor summit, giving a value of 1537 m. Considering the results obtained in this work from the long-term study, we do not see evidence of PWV trends in the 20-year period of the analysis that would suggest climate change in such a timescale.

The Atacama Cosmology Telescope: Modeling Bulk Atmospheric Motion

“…The agreement between all four is quite good, suggesting that our physical picture of atmospheric emission resembles reality. Our investigation also shows good agreement of the PWV between ACT, APEX, ERA5, MERRA-2, and Cortés et al (2020). Further adaptations of the pair-lag method to telescopes with different optical characteristics located in different geographical sites will help to better understand the motion-driven emission fluctuations of the atmosphere.…”

“…Our analysis draws on a number of sources, including ACT (Thornton et al 2016), the ground-based weather station maintained by the Atacama Pathfinder EXperiment (APEX) collaboration (e.g., Schuller et al 2009), NASA's MERRA-2 database (Gelaro et al 2017), the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (Hersbach et al 2020), the Cortés et al (2020) synthesis of the precipitable water vapor (PWV) in the Cerro Chajnantor region, and the UdeC-UCSC 183 GHz radiometer next to ACT (Bustos et al 2014). The location of each is shown in Figure 3.…”

“…Using the pressure, temperature, and mass fraction of water vapor as determined by ERA5 and MERRA-2, we can obtain the mass density of water vapor as a function of height, and integrate over it to obtain an estimate for the total PWV. The median and distribution of PWV measurements in the Chajnantor region as measured by APEX, ERA5, MERRA-2, and Cortés et al (2020) are shown in Figure 4. We conclude that the PWV is generally consistent between different methods of measurement.…”

Cross-correlations between mm-wave line-intensity mapping and weak lensing surveys: preliminary consideration of long-term prospects

“…As for emissivity, we use expected atmospheric transmission based on the ATM model (Pardo et al 2001) to estimate the frequency-dependent 𝜖 sys as varying between 4% and 8% across the 200-300 GHz band from Chajnantor Plateau, assuming 0.6 mm precipitable water vapour (PWV) at zenith and source elevation of 45 degrees. 1 For reference, the first-quartile (median) PWV is 0.36 (0.67) mm across the year at the Cerro Chajnantor summit just above the FYST site (Cortés et al 2020). Note however that atmospheric transmission for a given PWV value depends strongly on site characteristics.…”

Calibration of TES bolometer arrays with application to CLASS

“…Nearby radiometer include the UdeC and UCSC 183 GHz radiometer located at the Atacama Cosmology Telescope site (Bustos et al 2014), and the Atacama Pathfinder Experiment radiometer (Güsten et al 2006;Cortés et al 2020) A . The detector optical time constant 𝜏 𝛾 is ∼ 2 ms and is measured to high accuracy by tracking the phase of the VPM synchronous signal (Harrington 2018).…”