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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]

This paper presents a comprehensive site survey of the Delingha site on the northern Tibetan Plateau from 2012 to 2014. In this campaign, we investigated 24 h continuous astronomical seeing, meteorological properties, and |$C_{n}^{2}$| in the ground layer (GL). The results show that the median seeing is 3.46 arcsec during the day and 1.80 arcsec during the night. The best nighttime seeing occurs in summer (1.66 arcsec), while the worst occurs in autumn (1.96 arcsec). Seeing exhibits obvious diurnal behaviour, with the worst seeing occurring around mid-day (11:00 to 14:00, Beijing time), and the best seeing just before sunset (18:00 to 20:00, Beijing time). Moreover, the Delingha site has good meteorological properties for astronomical observations during the measurement period, especially the low median wind speed (2.8 m s−1) and great temperature inversion (day: −0.3 K, night: 0.6 K). As for the vertical distribution of |$C_{n}^{2}$|⁠ , the exponential h −0.46 dependence of |$C_{n}^{2}$| varying with height in the GL is also obtained, and its power index is significantly related to the seasons. Furthermore, we calculated the contribution of optical turbulence in the GL to the whole atmosphere. Turbulence within the GL contributes significantly in summer, and its contribution can reach more than 35 per cent. Finally, we present the complete dependence analysis of seeing on meteorological parameters at the Delingha site. The results show that the best seeing tends to occur when the temperature inversion is sufficiently large, but the dependence of seeing on wind speed is not significant. [ABSTRACT FROM AUTHOR]

In this paper, we present comprehensive night-time seeing statistics at the Lenghu site using data collected between 2018 and 2021 using a differential image motion monitor. The results show that the median seeing is 0.80 arcsec, which slightly larger than that previously reported due to climate change. Additionally, we investigated the annual, seasonal, and monthly seeing statistics, as well as possible hourly seeing trends in each season. The results demonstrate that the seeing at the Lenghu site is very stable, with monthly median seeing varying from 0.6 to 0.95 arcsec. The seasonal dependence of seeing shows obvious variations over different years. The best year of seeing was 2019 (0.74 arcsec), and the best seeing occurred in spring (0.68 arcsec) to early summer (June, 0.63 arcsec), while the worst seeing occurred in winter (0.80 arcsec). The minimum seeing value throughout the night is at approximately three hours before dawn. Furthermore, wind speed profiles, especially V 200 (particularly the wind speed at 200 mbar), are important parameters for evaluating astronomical sites in terms of their suitability for adaptive optics. We first show the complete monthly average behaviours of the wind speed profile and V 200 using the European Center for Medium-Range Weather Forecast reanalysis data sets. The results demonstrate the annual periodicity and variability of V 200, and the yearly V 200 modulation function is also provided. V 200 maintains large values throughout the autumn (average: 35.01 m s−1) and winter (average: 34.33 m s−1), while the minimum V 200 occurs in spring (average: 27.93 m s−1). [ABSTRACT FROM AUTHOR]

Appropriate knowledge of wind-speed distributions and optical turbulence at existing and potential astronomical observatories is crucial for siting ground-based telescopes and applying adaptive optics (AO) systems. In this paper, the wind-speed and optical-turbulence characteristics above Gaomeigu and the Tibetan Plateau are studied by employing the 20-yr (1999–2018) European Centre for Medium-Range Weather Forecasts' fifth set of reanalysis data (ERA5). First, the meteorological parameters derived from ERA5 data are evaluated with coinciding radiosonde measurements. Results show that the meteorological parameters of ERA5 data in the free atmosphere have quite good reliability, with bias and root mean square error basically lower than 1.2 K in temperature and basically smaller than 2 m s−1 for wind speed. Then, vertical distributions and seasonal behaviour of the wind speed at Gaomeigu and Lhasa station above the Tibetan Plateau are analysed. Thirdly, the Richardson number (Ri) in the free atmosphere is calculated to provides us with a map of relative probability of different periods and regions of optical turbulence being developed above the two sites. In general, the atmospheric stability of Gaomeigu is higher than that of Lhasa station. Particularly in June, for Gaomeigu, the atmospheric stability within 6–30 km a.s.l. is basically superior or equal to the stable condition found at two mid-latitude sites: Oukaimeden and La Palma. Moreover, Lhasa station has a relative higher stability during June–September than other months. Furthermore, we provide the |$({C_n^2})$| profiles using ERA5 data at Gaomeigu and Lhasa. The results indicate that the choice of an appropriate outer-scale model is crucial for revealing local turbulence characteristics. [ABSTRACT FROM AUTHOR]