Atmospheric Turbulence Profiling in the Lenghu Site Area Using MERRA-2 Data
The Lenghu site, in the Saishiteng Mountains of the northern Tibetan Plateau, has been identified as an excellent astronomical site from preliminary statistics of clear nights, sky background brightness, precipitable water vapour, total seeing and weather. Since 2022, several projects with apertures larger than 4 m have started construction or are being planned. Given the use of adaptive optics (AO) on these projects, in-depth measurements and analysis of the atmospheric turbulence above the site are required: the distribution and variation of the turbulence profiles provide a large-scale, climatological basis for site planning and for comparing candidate locations, complementing rather than replacing the on-site measurements needed for instrument design and operations. Predicting the atmospheric profiles of the site and its surroundings from satellite data, combined with on-site measurements, can help to optimise the site planning. This paper uses MERRA-2 reanalysis data to study the atmospheric profiles of the site and its environment, revealing the seasonal and long-term variations of the observing conditions. The reanalysis results are cross-checked against on-site ground-level instruments serving the same purpose. MERRA-2 overestimates the median free-atmosphere seeing by +0.22 arcsec (+50 %) while keeping the night-to-night correlation weak (r ~ 0.2), and underestimates the median total seeing by -0.24 arcsec (-27 %). It captures only ~22 % of the ground-layer turbulence and little of the observed seasonal amplitude of the free atmosphere. MERRA-2 is therefore suited to relative comparisons of spatial variability between locations and to climatological characterisation, not to quantitative AO applications or night-by-night assessment of profiles.