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Atmospheric turbulence and wind speed distributions are essential in determining the quality of astronomical sites and implementing adaptive optics systems. In this study, the optical turbulence with a general turbulence model and the wind speed characteristics at Lenghu are investigated by employing the 21-yr European Centre for Medium-Range Weather Forecasts' fifth set of reanalysis (ERA5) data. The vertical distributions and seasonal behaviours of the wind speed and atmospheric turbulence are analysed. The wind speed values at 200 hPa pressure level (V 200) are low in spring and summer except for June, and high in autumn and winter. The highest and lowest values of V 200 at Lenghu are 39.93 m s−1 in September and 27.48 m s−1 in April, respectively. Additionally, atmospheric conditions are relatively stable above 20 km throughout the year. The locations of the peaks in the |$C_n^2$| profiles correspond to the tropopause and the jet wind stream regions at the Lenghu site. Furthermore, the median seeing value from ERA5 is 0.72 arcsec, generally consistent with the statistically measured value of 0.75 arcsec. Moreover, the astronomical parameters (r 0, θAO, and τAO) were initially provided at the Lenghu site. The median values of r 0, ε, θAO, and τAO are 15.17 cm, 0.72 arcsec, 1.19 arcsec, and 2.83 ms, respectively, thus providing the potential reference for astronomical applications. [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]

The vertical profiles of wind speed and the optical turbulence are critical to the design and operation of a new generation of highly sophisticated astronomical telescopes and adaptive optics instrumentation. We present the first study of the temporal evolution behaviours and probability distributions of wind speed [ V (h) profiles, as well as the 200 hPa pressure level wind speed, V 200] and optical turbulence [ |$C_n^2(h)$| profiles, and the most relevant integrated astronomical parameters derived from |$C_n^2(h)$| profiles, i.e. the seeing ε, the isoplanatic angle θAO, the wavefront coherence time τAO, the average velocity of turbulence V AO, and the seeing layer height h AO] above the Dachaidan site of the Tibetan Plateau. The field campaigns of wind speed and optical turbulence were collected using the balloon-borne microthermal measurement system. From the whole field campaigns, the results are remarkable: The median V AO is 21.1 m s−1, the median V 200 is 32.5 m s−1, the median h AO is 7566 m, the median ε is 1.04 arcsec (below 1.00 arcsec 52 per cent of the time), the median θAO is 0.74 arcsec, and the median τAO is 1.33 ms; these conditions are comparable to some of the best astronomical observatories in the world. In particular, the linear relationship of average velocity and 200 hPa level wind at this site is V AO = 0.627 V 200. In this study, we flag the temporal evolution and probability distribution feature of wind speed, optical turbulence profile, and the relevant integrated astronomical parameters for astronomical applications. [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]

In this paper, we have analyzed the wind, temperature and optical turbulence index structure constant (C n 2) that are directly measured for the first time at Lhasa, Tibet. Unlike other mid-or-low latitude regions, the wind speed in Lhasa increases with the altitude according to the wind profiles, besides a strong wind speed near the tropopause. The temperature profiles indicate a large temperature change near the tropopause. The optical turbulence profiles show daily variation. When the altitude is below 12 km, except for individual points, the C n 2 values during morning and evening are basically in the same range of 10−17-10−15 m-2/3. When altitude is above 12 km, the intensity of turbulence during the morning in Lhasa is 1–2 orders of magnitude larger than that of the evening. In addition, we propose a turbulence profile model on the basis of Hufnagel-valley model to calculate the integrated values of the refractive index structure constants of the entire atmosphere, such as the Fried parameter, the seeing, the coherence time and the isoplanatic angle. The mean atmospheric coherence length r 0 is 5.77 cm. Differences in these parameters between morning and evening also illustrate the stronger turbulence in the morning than evening. • Wind, temperature, optical turbulence index structure constant (C n 2) are directly measured for the first time at Lhasa Tibet. • The integrated values of the refractive index structural constants for the entire atmosphere are calculated and analyzed. • The relation between turbulence and wind speed as well as that between turbulence and temperature in Lhasa is analyzed. • Turbulence profile model based on Hufnagel-valley model for Tibet is proposed in the paper to evaluate the turbulence. [ABSTRACT FROM AUTHOR]