Optical turbulence in the atmospheric surface layer at the Ali Observatory, Tibet

Abstract

ABSTRACT In this paper, we make use of the ultrasonic anemometers placed on a 30-m high tower at the Ali Observatory, and we obtain a preliminary set of data covering 2017 April 29 to August 18. We sample quantities at heights of 6, 12 and 18 m above the ground. This is the first analysis from the site-testing campaign to report the detailed optical turbulence in the atmospheric surface layer (ASL) at Ali. The behaviour of optical turbulence parameters (Cn2 and seeing ε) in the ASL have been investigated. The Cn2 decreases with height exponentially, with the power series mainly between 10−15 and 10−13 throughout the day, and, especially, as low as 10−17 at night. Meanwhile, the h−1.1 dependency of Cn2 varying with height in the ASL is obtained, and this is especially close to h−1.3 after midday, which is similar to the h−4/3 dependency and the dependency of h−1.27 above Dome C. We also present statistics of the contribution of the ASL to the total seeing in the whole atmosphere at Ali: the median seeing above 6 m is about 0.86 arcsec; at night, the seeing values are almost below 1.0 arcsec; the seeing below 1.2 arcsec takes an account of nearly 98 per cent of the time. On the whole, the initial calculated and observed results, with the high altitude and unique longitudinal location, show that the Ali Observatory could prove to be a promising site for large optical telescopes.