ABSTRACT In Antarctica, excellent astronomical observing conditions have been measured at Dome A during night-time (or polar winter). This study investigates the performance of the Polar-optimized version of the Weather Research and Forecasting (Polar WRF, PWRF hereafter) for simulating the night-time astronomical seeing at Dome A. The seeing values were estimated by a seeing model, which used the PWRF-simulated wind speed and temperature as inputs. Furthermore, three methods to obtain the boundary layer height in the seeing model have been examined. The estimated seeing agrees well with 50-d measurements from the KunLun Differential Image Motion Monitor at Dome A during the night-time of 2019; the correlation coefficients range from 0.62 to 0.71. The PWRF-simulated meteorological parameters indicate that low wind speed and strong temperature inversion (when a large gradient Richardson number always occurs) near the ground can lead to good seeing condition. The results suggest that the PWRF model could be a reliable tool for scheduling observational astronomy at Dome A during night-time.
Monitoring a wide range of atmospheric turbulence over the Antarctic continent is still tricky, while the atmospheric Richardson number (Ri ; a critical parameter determining the possibility of turbulence could be triggered) is easier to obtain. The Antarctic atmospheric Ri, calculated using the temperature and wind speed, was investigated using the daily results from the radiosoundings and forecasts of the Antarctic Mesoscale Prediction System (AMPS). Radiosoundings for a year at three sites (McMurdo, South Pole, and Dome C) were used to quantify the reliability of the AMPS forecasts. The AMPS-forecasted 1/Ri (inverse of the Richardson number) can identify the main characteristics of atmospheric turbulence over the Antarctic continent in terms of space and time. The correlation coefficients (Rxy) of 1/Ri at McMurdo, South Pole, and Dome C are 0.71, 0.66, and 0.68, respectively, where the performance gains during the warm seasons. In addition, a model to improve AMPS-forecasted 1/Ri has been presented. The monthly median at the three sites and the seasonal median throughout the two vertical cross-sections for the AMPS forecasts are presented. One can observe that the probability of triggering turbulence is primarily concentrated near the ground. In addition, strong wind shears near escarpment regions have been found in the range of 0–5 km above the ground, thus causing atmospheric instability (or a thick boundary layer). In addition, turbulent atmospheres are likely to be triggered over the ocean, moving toward the Antarctic Plateau and becoming stable. Finally, the 1/Ri at the planetary boundary layer height (PBLH), 1/RiPBLH, has been provided as a reference standard for judging atmospheric stability. The median value of 1/RiPBLH from the combined data of two vertical cross-sections was 0.55, which was used to calculate PBLH and agree well with the AMPS forecasts (Rxy >0.72).
By using time series Landsat TM satellite images and adopting GIS spatial analysis and landscape pattern analysis methods, this paper studied the spatiotemporal diversity of urban growth and the evolution of urban landscape pattern in Shenyang, and examined their driving forces. The results showed that in 1988-2004, the urban area in Shenyang increased persistently, and the growth intensity enhanced consistently, with the peaks occured in 2000-2004. The spatial differentiation of urban growth in the City was also distinct, with the southwest direction as the leading orientation, and the urban edges and different level economic development zones as the main growth areas. The urban landscape pattern became more and more complex, and the compactness index of urban development decreased. The evolution of urban landscape pattern was related to the characteristics of urban growth, which also showed spatiotemporal diversity. The urban growth and urban landscape pattern evolution in Shenyang were mainly attributed to the development of industrialization and the construction of different level economic development zones, the proper policies of local governments and the urban planning, as well as the development of traffic infrastructure.
Understanding turbulence in the free atmosphere is important for analyzing atmospheric pollution, forecasting weather, and light transmission. In this paper, we have tried to estimate the atmospheric refractive index structure constant Cn2 , the turbulent dissipation rate ε , and the turbulent diffusion coefficient K simultaneously during the experiment time over Lhasa, using the sounding data coupled with the Thorpe method. The result shows that the Cn2 estimation gives a better performance with the correlation coefficients and the average relative error when compared with Cn2 estimated by Dewan and HMNSP99. Besides this, the measured and estimated Cn2 , estimated ε , and K all show larger values in the troposphere, especially near the tropopause. It is worth noting that Cn2 and ε are similar in terms of height distribution. These attempts at estimation all suggest that the Thorpe method can be used to estimate the intensity of turbulence in the free atmosphere over Lhasa.
The high elevation, complex topography, and unique atmospheric circulations of the Tibetan Plateau (TP) make its optical turbulence characteristics different from those in low-elevation regions. In this study, the characteristics of the atmospheric refractive index structure constant (Cn2) profiles in the Lhasa area at different strength states of the Asian summer monsoon anticyclone (ASMA) are analyzed based on precious in situ sounding data measured over the Lhasa in August 2018. Cn2 in the upper troposphere–lower stratosphere fluctuates significantly within a few days during the ASMA, particularly in the upper troposphere. The effect of the ASMA on Cn2 varies among the upper troposphere, tropopause, and lower stratosphere. The stronger and closer the ASMA is to Lhasa, the more pronounced is the “upper highs and lower lows” pressure field structure, which is beneficial for decreasing the potential temperature lapse rate. The decrease in static stability is an important condition for developing optical turbulence, elevating the tropopause height, and reducing the tropopause temperature. However, if strong high-pressure activity occurs at the lower pressure layer, such as at 500 hPa, an “upper highs and lower highs” pressure field structure forms over the Lhasa, increasing the potential temperature lapse rate and suppressing the convective intensity. Being almost unaffected by low-level atmospheric high-pressure activities, the ASMA, as the main influencing factor, mainly inhibits Cn2 in the tropopause and lower stratosphere. The variations of turbulence intensity in UTLS caused by ASMA activities also have a great influence on astronomical parameters, which will have certain guiding significance for astronomical site testing and observations.
ABSTRACT A worldwide search is currently being conducted to determine the most appropriate sites for the next generation of large optical and infrared telescopes. Here, we report a global map of atmospheric optical turbulence-associated parameter (atmospheric coherence length) based on the fifth generation of European Centre for Medium Range Weather Forecasts re-analysis data set and results for atmospheric coherence lengths at European Southern Observatory and Tibetan Plateau locations (all at mid-latitudes). These confirm the accuracy of the global atmospheric optical turbulence model, but also the simulation result reveals that an excellent atmospheric calm site exist at the Ethiopian Plateau (at a latitude of ∼10°N) outside of the rainy season. The Ethiopian Plateau is the highest plateau in Africa, known as the ‘Roof of Africa’, with an average elevation of over 2500 m, and thus potentially provides good opportunities for astronomy and astrophysics. The median atmospheric coherence length at the Ethiopian Plateau was 17.7 cm and approximately 75 per cent of the time above 15.6 cm for dry conditions, indicating a very calm atmosphere. Although favourable atmospheric turbulence conditions at the Ethiopian Plateau were found from the global turbulence model, due to the complete lack of infrastructure it has never been visited, meaning that additional validation experiments are required.
ABSTRACT 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 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, V200] 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 VAO, and the seeing layer height hAO] 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 VAO is 21.1 m s−1, the median V200 is 32.5 m s−1, the median hAO 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 VAO = 0.627V200. 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.
Since systematic direct measurements of refractive index structure constant ( Cn2) for many climates and seasons are not available, an indirect approach is developed in which Cn2 is estimated from the mesoscale atmospheric model outputs. In previous work, we have presented an approach that a state-of-the-art mesoscale atmospheric model called Weather Research and Forecasting (WRF) model coupled with Monin-Obukhov Similarity (MOS) theory which can be used to estimate surface layer Cn2 over the ocean. Here this paper is focused on surface layer Cn2 over snow and sea ice, which is the extending of estimating surface layer Cn2 utilizing WRF model for ground-based optical application requirements. This powerful approach is validated against the corresponding 9-day Cn2 data from a field campaign of the 30th Chinese National Antarctic Research Expedition (CHINARE). We employ several statistical operators to assess how this approach performs. Besides, we present an independent analysis of this approach performance using the contingency tables. Such a method permits us to provide supplementary key information with respect to statistical operators. These methods make our analysis more robust and permit us to confirm the excellent performances of this approach. The reasonably good agreement in trend and magnitude is found between estimated values and measurements overall, and the estimated Cn2 values are even better than the ones obtained by this approach over the ocean surface layer. The encouraging performance of this approach has a concrete practical implementation of ground-based optical applications over snow and sea ice.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(60)'/%3e%3ccircle r='17' fill='%23000095'/%3e%3ccircle r='15' fill='%23fff'/%3e%3c/svg%3e)