Long-term variations in precipitable water vapor and temperature at Lenghu Site

Abstract

Context. A high-quality astronomical observing site, the Lenghu site, was recently discovered on the Tibetan Plateau. The statistical analysis of site quality monitor data collected so far have indicated that the precipitable water vapor (PWV) is lower than 2 mm for 55% of the night. The nighttime temperature is also very stable; the median of the intranight variation amplitude is only 2.4 C. Aims. The long-term trend of the PWV and temperature variations, which is essential for future facilities operating at infrared, millimeter, and submillimeter wavelengths, is investigated in this work. Methods. Here we used the atmospheric reanalysis datasets of the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) and ERA5, the fifth major atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF), as well as the measurements from the weather station at the site to conduct a long-term (22 yr) comparative analysis of PWV and temperature at the Lenghu site. Results. The weighted annual mean nighttime temperature and PWV increase at rates of 0.17 C decade-1 and 0.12 ∼ 0.13 mm decade-1, respectively. The nighttime temperature and PWV slightly both decrease during the winter with rates of-0.04 C decade-1 and-0.05 ∼-0.07 mm decade-1, respectively. Conclusions. These results indicate that the variation patterns of PWV and temperature at the Lenghu site are quite stable, especially during the winter; it is projected that the nighttime average PWV will be below 1 mm and the nighttime average temperature will be below-13 C toward the end of this century. These conditions are ideal for large optical, infrared, millimeter, and submillimeter facilities where great scientific discoveries will be made that address the ultimate questions of humankind.