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In this paper, the statistics of cloud cover at night from 2016 to 2018 above the Ali observatory has been summarized, using the meso-scale numerical model and the cameras (Canon and AllSky-340 Camera). Above the western Tibetan Plateau, in the area of 200 km × 200 km centred at the Ali site, the average cloud cover during night was basically less than 30 per cent in 2016, and the Ali site was located in the zone with small value around 20 per cent; large cloud cover always occurred in summer, with mean value above 50 per cent in most parts of this area; while in winter, the smallest cloud cover was basically below 10 per cent; in other seasons, the cloud cover was mostly less than 30 per cent, and the Ali site located in the zone with cloud cover below 10 per cent in autumn and below 20 per cent in spring. At each monitored field of view from 90° to 45°, the cloud cover varied little; the proportion of clear nights with cloud cover below 10 per cent was close to 60 per cent of total observable nights per year, and the less cloudy nights with cloud cover below 40 per cent accounted for more than 70 per cent, besides, the observable nights with cloud cover below 70 per cent was around 80 per cent every year. Generally, the cloud cover condition at the Ali observatory above the western plateau is excellent, there are always abundant observable nights, while a bit worse just in July and August. Meanwhile, the numerical model has been proved to be a reliable tool to estimate cloud cover conditions above the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this study, we make use of a meso-scale numerical model to obtain the parameters of atmospheric optical turbulence at the Ali observatory above the Tibetan Plateau. These parameters are essential for ground-based optical observations and some adaptive optics techniques. We present the characterization of the vertical distributions of atmospheric optical turbulence Cn2 and the main integral parameters, such as coherence time τ0, isoplanatic angle θ0 and seeing ε0 derived from Cn2. Meanwhile, the measurement of Cn2 obtained with a radiosonde instrument is used to quantify the model performance on reconstructing the optical turbulence above the site. This shows a high level of agreement and provides a reliable tool for astronomical site testing. The results presented in this paper demonstrate the good behaviour of the atmospheric optical turbulence condition at Ali, particularly during the summer half of the year. We find yearly median values of seeing ε0 = 0.47 arcsec, isoplanatic angle θ0 = 4.35 arcsec and atmospheric coherence time τ0 = 5.52 ms in 2016, and we also provide independent confirmation of the potential of the Ali site in adaptive optics. Moreover, it has been proven that meso-scale models can provide reliable estimations of atmospheric optical turbulence conditions above an astronomical site on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]