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hv photonics Article Analyzing the Effects of a Basin on Atmospheric Environment Relevant to Optical Turbulence 1 , 2 , 3 4 1 , 3 , 1 , 2 , 3 1 , 3 Manman Xu , Liangping Zhou , Shiyong Shao * , Ningquan Weng and Qing Liu Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, Hefei Institute of Physical Science, Chinese Academy of Sciences, Hefei 230031, China; manxu@mail.ustc.edu.cn (M.X.); wnq@aiofm.cas.cn (N.W.); liuq@aiofm.ac.cn (Q.L.) Science Island Branch of Graduate School, University of Science and Technology of China, Hefei 230026, China Advanced Laser Technology Laboratory of Anhui Province, Hefei 230037, China Beijing Aviation Meteorological Institute, Beijing 100085, China; sa20168371@mail.ustc.edu.cn * Correspondence: shaoshiyong@aiofm.ac.cn Abstract: The performance of adaptive optics (AO) systems are highly dependent upon optical turbu- lence. Thus, it is necessary to have the appropriate knowledge of the spatiotemporal characteristics of optical turbulence strength. In this paper, the spatiotemporal distribution of meteorological param- eters (wind and temperature) and optical turbulence parameters (turbulence strength, temperature gradient, and wind shear) derived from pulsed coherent Doppler lidar, a microwave radiometer, and ERA5 reanalysis data are investigated, and the results show that the meteorological parameters in a basin develop independently, while the external influence will increase above the basin. By fitting radiosonde data, an existing parameterized model was improved to be more in line with the evolutionary properties of local optical turbulence. The development of temperature gradient and wind shear is influenced by the basin, which ultimately leads to an optical turbulence vertical profile that is discrepant at different altitude layers. The results indicate that temperature gradient plays a dominant role in turbulence generation below 2 km, and wind shear increases its impact significantly above 2 km. Furthermore, the optical turbulence parameters (outer scale, turbulence dif- fusion coefficient, and turbulence energy dissipation rate) and optical turbulence strength have good Citation: Xu, M.; Zhou, L.; Shao, S.; consistency, which might be derived from the combined effect of terrain and complex environment. Weng, N.; Liu, Q. Analyzing the Finally, the integrated parameters for astronomy and optical telecommunication were derived from Effects of a Basin on Atmospheric optical turbulence strength profiles. An appropriate knowledge of optical turbulence is essential for Environment Relevant to Optical improving the performance of adaptive optics systems and astronomical site selection. Turbulence. Photonics 2022, 9, 235. https://doi.org/10.3390/ photonics9040235 Keywords: Earth–atmosphere interaction; optical turbulence; temperature gradient; wind shear; outer scale Received: 1 March 2022 Accepted: 25 March 2022 Published: 2 April 2022 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in Optical turbulence within Earth’s atmosphere plays a significant role in electromag- published maps and institutional affil- netic radiation propagation, which degrades the image quality of adaptive optics (AO) iations. systems dramatically. The primary parameter used to characterize atmospheric optical turbulence is the index of refraction structure constant C [1–3]. It is important to charac- terize this parameter throughout the atmosphere, the boundary layer, and above for the evaluation of laser atmospheric transmission. Measurement techniques, such as radar, scin- Copyright: © 2022 by the authors. tillometers, ultrasonic anemometers, and micro-thermometers, have been used to measure Licensee MDPI, Basel, Switzerland. 2 2 C [4–9]. Since almost all methods of obtaining C using instruments are limited in the This article is an open access article n n spatial–temporal domains, developing a less expensive and more convenient alternative is distributed under the terms and necessary. conditions of the Creative Commons In order to accurately estimate the variation characteristics of the C profile, scientists Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ have developed many models. Firstly, the simplest empirical model can be derived from 4.0/). the use of segmented fitting on experimental data, requiring only the input of an altitude Photonics 2022, 9, 235. https://doi.org/10.3390/photonics9040235 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 235 2 of 15 variable, for example, the submission laser communication (SLC) model, the Air Force Geophysics Laboratory (AFGL) and Air Force Maui Optical Station (AMOS) model, and the Critical Laser Enhancing Atmospheric Research (CLEAR I) model [10–13]. Secondly, a parameterization model can be used based on the basic theory of turbulence to establish the relationship between conventional meteorological parameters and the profiles of C through the outer scale. The Tatarskii model took the important step of estimating C profiles through the use of routine meteorological parameters [1]. The Tatarskii model has been adopted to investigate the relative contributions of temperature and relative humidity to the refractive index gradient using three years of high-resolution radiosonde data over the tropical station Gadanki [14]. On the basis of the measured data, the upper altitude atmospheric turbulence parameter model was summarized by Hufnagel, whose model had a limited range of use [15]. Afterward, the National Oceanic and Atmospheric Administra- tion (NOAA) model was developed by VanZandt, which integrated the fine structure of wind shear and was relatively complex [16]. Furthermore, Abahamid conducted research on the behavior of atmospheric turbulence in the boundary layer and free atmosphere with the use of balloon-borne radiosondes from different sites, which was based on the optical turbulence parameterization of the Tatarskii model [17]. Even though there is considerable diversity among the reported results, the quasi-universality of the estimated C profiles with meteorological parameters is not clearly discernible [18]. Each existing approach has its own merits and limitations, but none of them are known to be superior [19]. In this paper, the spatiotemporal distribution of the optical turbulence strength was obtained with the use of high-resolution wind radar and microwave radiometer, which provided a new method for investigating optical turbulence. The affecting factors, in- dT cluding temperature gradient ( ) and wind shear (S), play important roles in different dh altitude layers during the development of optical turbulence. We found that temperature gradient played a dominant role in turbulence generation below 2 km, and wind shear had an important influence above 2 km, which provided support for studying the mechanism of optical turbulence generation. Furthermore, this is the first time that several complete 3D maps of optical turbulence parameters, from the ground up to 3 km, were obtained from the PCDL at the same time. The results provide further understanding of the diurnal variation law of optical turbulence, which is essential for improving the performance of AO systems. The remainder of this paper is organized as follows: Section 2 gives the experimen- tal detail including instruments and surrounding environment; Section 3 introduces the methodologies associated with the investigation of the C profiles; Section 4 provides the experimental results and discussion; finally, Section 5 concludes the paper. 2. Experiments Details Based on pulsed coherent Doppler lidar (PCDL), microwave radiometer, micro- thermometer, and radiosonde, a field campaign was conducted by the Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, in 2021, to study the spa- tiotemporal evolutionary mechanism of optical turbulence in the northeast of the Sichuan Basin. The geographic location of the Sichuan Basin and the field campaign site is shown in Figure 1. The Sichuan Basin has a special geographical location, with the Qinghai–Tibet Plateau in the west and the middle and lower Yangtze plains in the east. In the northeast of the Sichuan Basin, there is a series of northeast–southwest valleys, and the test site was located in one of the wide valleys, where the altitude of the surrounding mountains was between 1500 and 2000 m. This altitude is within the range of the atmospheric boundary layer, which may have an important impact on the development of optical turbulence. With the unique topography and landforms, it will show different meteorological characteristics than other locations. The PCDL is a scanning and remote sensing instrument that measures the wind field along the lidar beam. The PCDL used here was from the Southwest Institute of Technical Physics, and its technical parameters are shown in Table 1. The laser pulse width can Photonics 2022, 9, 235 3 of 15 be tuned to 500 ns, corresponding to a spatial resolution of 30 m in the radial direction and a temporal sampling rate of 4 Hz, with significant advantages in studying optical turbulent characteristics. The experimental site was in complex terrain surrounded by mountains. To gain the temperature field of the same site, a microwave radiometer was installed 10 m away. The microwave radiometer was produced and developed by the Sun Photonics 2022, 9, x FOR PEER REVIEW 3 of 17 Create Electronics Co., Ltd. Hefei, China, which provides temperature and pressure profiles with high spatial resolution (i.e., 50 m) and high temporal resolution (i.e., 1 s). The main technical parameters of the microwave radiometer are described in Table 2. The above layer, which may have an important impact on the development of optical turbulence. two pieces of equipment are shown in Figure 2. Using routine meteorological parameters, With the unique topography and landforms, it will show different meteorological charac- the temporal and spatial variation characteristics of optical turbulence strength could be teristics than other locations. estimated according to the existing C model. Photonics 2022, 9, x FOR PEER REVIEW 3 of 17 layer, which may have an important impact on the development of optical turbulence. With the unique topography and landforms, it will show different meteorological charac- teristics than other locations. Figure 1. Geographic location of the Sichuan Basin and the field campaign site represented by the Figure 1. Geographic location of the Sichuan Basin and the field campaign site represented by the five-pointed star. five-pointed star. The PCDL is a scanning and remote sensing instrument that measures the wind field Table 1. The main technical parameters of the PCDL. along the lidar beam. The PCDL used here was from the Southwest Institute of Technical Physics, and its technical parameters are shown in Table 1. The laser p Wind ulse width can be Figure 1. Geographic location of the Sichuan Basin and the field campaign site represented by the Wind Speed five-tun poine ted d to sta 5 r.0 0 ns, corresponding to Detection a spatial resolution of 30 m in thDirection e radial direction a Vertical nd a Wavelength Detection Parameters Distance Detection Resolution temporal sampling rate of 4 Hz, with significant advantages in studying optical turbulent (nm) Range The PCDL is a scanning and remote sensing instrument that measures the wind field (m) Range (m) characteristics. The experimental site was in complex terrain surrounded by mountains. (m/s) along the lidar beam. The PCDL used here was from the Southwest Institute of Technical ( ) To gain the temperature field of the same site, a microwave radiometer was installed 10 Physics, and its technical parameters are shown in Table 1. The laser pulse width can be m away. The microwave radiometer was produced and developed by the Sun Create Elec- Value 1550 30–10,000 0–75 0–360 30/50/100/200 tuned to 500 ns, corresponding to a spatial resolution of 30 m in the radial direction and a tronics Co., Ltd. Hefei, China, which provides temperature and pressure profiles with temporal sampling rate of 4 Hz, with significant advantages in studying optical turbulent high spatial resolution (i.e., 50 m) and high temporal resolution (i.e., 1 s). The main tech- characteristics. The experimental site was in complex terrain surrounded by mountains. Table 2. The main technical parameters of the microwave radiometer. To gn aiic n t alhe p a tem ram per eta ers tur o e f f ie the ld o m f tih ce s row am ae s ve ra ite, a di m om icet roer a wavr e ra e des dioc m riet ber ed wia ns T inasb ta le 2 lled 1 . T0h e above two m away. The microwave radiometer was produced and developed by the Sun Create Elec- pieces of equipment are shown in Figure 2. Using routine meteorological parameters, the Temperature tronics Co., Ltd. Hefei, China, which provides temperature and pressure profiles with Detection Temporal Vertical temporal and spatial variation characteristics of optical turbulence strength could be esti- Detection high spatial resolution (i.e., 50 m) and high2 temporal resolution (i.e., 1 s). The main tech- Parameters Distance Resolution Resolution mated according to the existing C model. Range nical parameters of the microwave radiometer are described in Table 2. The above two (m) (s) (m) ( C) pieces of equipment are shown in Figure 2. Using routine meteorological parameters, the temporal and spatial variation characteristics of optical turbulence strength could be esti- Value 0–10,000 60–40 1 50 (b) (a) mated according to the existing C model. (b) (a) Figure 2. Experimental scenario of (a) the pulsed coherent Doppler lidar and (b) the microwave radiometer. Figure 2. Experimental scenario of (a) the pulsed coherent Doppler lidar and (b) the microwave Figure 2. Experimental scenario of (a) the pulsed coherent Doppler lidar and (b) the microwave radiometer. radiometer. During the campaign, a micro-thermometer, from the Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, was installed approximately 3 m over the During the campaign, a micro-thermometer, from the Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, was installed approximately 3 m over the Photonics 2022, 9, x FOR PEER REVIEW 4 of 17 Earth’s surface, and the daily variation in turbulence strength near the ground was ob- tained [20–23]. A comprehensive description of the micro-thermometer and its specifica- tions are given in [4,24,25]. The installation location of the instrument and the surrounding environment are shown in Figure 3a. In order to improve the turbulence strength estima- tion model to make it more suitable for the site, a balloon-borne radiosonde equipped with micro-thermometers and GPS was launched at the experimental site as shown in Figure 3b. The working status and available meteorological parameters provided by each instrument are described in Table 3. Table 1. The main technical parameters of the PCDL. Wind Direction Detection Detection Wind Speed Detection Vertical Wavelength Range Parameters Distance Range Resolution (nm) (°) (m) (m/s) (m) Value 1550 30–10,000 0–75 0–360 30/50/100/200 Table 2. The main technical parameters of the microwave radiometer. Vertical Photonics 2022, 9, 235 4 of 15 Detection Distance Temperature Detection Range Temporal Resolution Parameters Resolution (m) (°C) (s) (m) Value 0–10,000 −60–40 1 50 During the campaign, a micro-thermometer, from the Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, was installed approximately 3 m over Table 3. Working conditions of the instruments. the Earth’s surface, and the daily variation in turbulence strength near the ground was obtained [20–23]. A comprehensive description of the micro-thermometer and its specifica- Instruments Measurement Parameters Working Status tions are given in [4,24,25]. The installation location of the instrument and the surrounding PCDL Wind speed, wind direction Continuous environment are shown in Figure 3a. In order to improve the turbulence strength estimation Microwave radiometer Temperature, humidity Continuous model to make it more suitable for the site, a balloon-borne radiosonde equipped with Turbulence strength micro-thermometers and GPS was launched at the experimental site as shown in Figure 3b. Micro-thermometer Continuous (3 m above ground level) The working status and available meteorological parameters provided by each instrument Radiosonde Turbulence strength (0–30 km) Intermittent are described in Table 3. Figure 3. Photographs of (a) the micro-thermometer and (b) the balloon-borne micro-thermome- Figure 3. Photographs of (a) the micro-thermometer and (b) the balloon-borne micro-thermometer. ter. Table 3. Working conditions of the instruments. 3. Methodology of Estimating Optical Turbulence Instruments Measurement Parameters Working Status 3.1. Estimation Models PCDL Wind speed, wind direction Continuous Parameterization models of optical turbulence have been developed to convert Microwave radiometer Temperature, humidity Continuous standard radiosonde meteorologicaT l p urbulence arametestr rs in ength to vertical profiles of C , the refractive Micro-thermometer Continuous (3 m above ground level) index structure constant [26]. According to Kolmogorov’s theory, which was under the Radiosonde Turbulence strength (0–30 km) Intermittent assumption of local homogeneity and stationarity of the refractive index fluctuations, the parameterization model uses the Tatarskii model for estimating profiles as shown 3. Methodology of Estimating Optical Turbulence below [1]: 3.1. Estimation Models Parameterization models of optical turbulence have been developed to convert stan- dard radiosonde meteorological parameters into vertical profiles of C , the refractive index structure constant [26]. According to Kolmogorov’s theory, which was under the assumption of local homogeneity and stationarity of the refractive index fluctuations, the parameteriza- tion model uses the Tatarskii model for estimating C profiles as shown below [1]: 2 4/3 2 C = aL M (1) where a is a dimensionless constant that is mostly used at a value of 2.8 [17]; L is the outer scale, which is the largest scale of inertial range turbulence. The potential refractive index gradient, M, is expressed as: 79 10 P ¶ ln q M = (2) T ¶h 0.286 q = T (3) P Photonics 2022, 9, 235 5 of 15 where M is related to atmospheric temperature T , atmospheric pressure P , and po- ( ) ( ) tential temperature (q); h is the height above ground. In addition, Tatarskii introduced Equation (1), which can be rewritten as: 2 3 1/3 ¶V 2 2 2 4 5 C = a K /( ) M (4) ¶h where V is the horizontal wind velocity vector, and K is the turbulent diffusion coefficient. K can be expressed as: K = # / ¶V /¶h (5) TKE where # is the turbulence energy dissipation rate. TKE From Equations (1)–(3), it can be noted that the vertical temperature gradient can be estimated from sounding data; therefore, choosing the appropriate L is the key to estimation C . At present, the HMNSP99 model and Dewan model are the most widely used outer scale model [27–29]. With the basics of the Tatarskii equation, an outer scale model was proposed by Dewan considering the vertical shear of the horizontal wind speed, and two expressions for the troposphere and stratosphere were proposed [26]. Trinquetet’s research indicated that the model needs to be used with a vertical resolution of 300 m, and S must not exceed 0.04 s [30]. The outer scale is given as follows: 4/3 1.64+42S 0.1 10 , Troposphere 4/3 L = (6) 4/3 0.506+50S 0.1 10 , Stratosphere where S is the vertical shear of the horizontal velocity. 2 2 ¶u ¶v S = + (7) ¶h ¶h where u and v are two horizontal wind components. Another outer scale model was proposed by Ruggiero and DeBenedictis, called the HMNSP99 model, in which the outer scale is the function of S and temperature gradient dT ( ) simultaneously [31]. Due to the fact that it contains more atmospheric parameters, the dh model might be more in line with the nature of the actual development of turbulence: dT 4/3 0.362+16.728S 192.347 dh 0.1 10 , Troposphere 4/3 L = (8) 0 dT 0.757+13.819S 57.784 4/3 dh 0.1 10 , Stratosphere 3.2. Integrated Astroclimatic Parameters In AO applications, the laser beam does not always propagate to heights of tens of kilometers as is the case in astronomical applications and boundary-layer research. Quite often, the beam propagates through the atmosphere within several kilometers above the ground, a height for which the turbulence profile is not well behaved due to the ground effects. It is necessary to obtain integrated astroclimatic parameters including the atmospheric coherence length, r , and seeing # to design and evaluate the AO system. The integrated value of C allows us to predict the atmospheric optical quality in terms of seeing # as well as the coherence length,r , that depends on the local conditions at altitude h. The seeing # is a crucial parameter that leads us to distinguish the most effective windows in the AO systems, which can be used to retrieve r [32]. All the parameters are given as follows: " # 3/5 2p r = 0.423 C h dh (9) ( ) 0 Wd(°) Ws(m/s) Photonics 2022, 9, 235 6 of 15 3/5 1/5 2 # = 5.25l C (h)dh (10) 4. Results and Discussion 4.1. Characteristics of Basic Meteorological Parameters Figure 4 shows the temporal and spatial characteristics of the wind speed and wind direction data from the PCDL at the experimental site and the ERA5 reanalysis data of the grid points near the experimental site. A comprehensive description of the ERA5 reanalysis data is given in [33]. Comparing the wind speed and direction data from the PCDL and ERA5, the results show that their diurnal variation with height distribution had good consistency. From the side, it is reasonable to use wind speed and wind direction data from the PCDL. As shown in Figure 4 (top), this may be due to the influence of the topography of the basin, where wind speed is relatively stable (i.e., below 1.5 km), and wind speed values were between 0 and 6 m/s. In contrast, high wind speeds began to appear above 1.5 km, with the maximum wind speed reaching 12 m/s. In addition, compared to the wind direction below 1.5 km, the wind direction above 1.5 km was relatively stable, where there was a wide range of relative fixed wind directions, since it was under the influence of the westerly zone as shown in Figure 4 (middle). The differences in wind direction may be attributed to the surrounding mountains that have a mutual effect on atmospheric conditions. Due to the influence of the special terrain, it can be predicted that there is a large wind shear near the peaks around the basin. It should be noted that there is a difference between the location of the grid point area obtained by satellite data and the site where the Photonics 2022, 9, x FOR PEER REVIEW 7 of 17 measuring equipment was located; thus, the two data were not exactly the same, but their trend was very close. Wind speed Wind speed Wind speed 05-21 05-25 05-23 05-21 05-25 Wind Wind Wind 05-23 direction direction direction 05-21 05-23 10 m/s 05-25 F Figure igure 4 4.. S Spatiotemporal patiotemporal d distribution istribution of of w wind ind s speed peed a and nd w wind ind d dir irec ect tiion on f fr rom om t the he PC PCDL DL ( (to top p a and nd middle) and the ERA5 reanalysis data (bottom). middle) and the ERA5 reanalysis data (bottom). To better explain the vertical distribution of wind speed and direction, Figure 5 shows the corresponding changes at 12:00 on 21 May at different AGL heights (106.15, 897, 1566, 2300, 3000, and 3700 m). The white arrows represent the magnitude and direc- tion of the wind speed and the yellow five-pointed stars the campaign site. In the Sichuan Basin, the wind speed at heights of 106.15 and 897 m was slow, and there was no uniform wind direction. At a height of 1566 m, the wind speed over the basin is still very small, but the wind direction tended to be consistent. In contrast, the wind speed increased, and the wind direction was consistent as the height increased. After the height reached to 3700 m, the wind direction over the entire basin was consistent with the outside of the basin. The spatial difference in wind speed and wind direction could be the result of the terrain, which had good agreement with the results obtained by PCDL. The Sichuan Basin can be regarded as a semi-enclosed space. Inside the space, the wind speed and direction of each location developed independently, so the wind speed was different and showed different wind directions. Outside the space, due to the influence of the westerly zone, the wind speed increased overall, and the wind direction tended to be consistent. Photonics 2022, 9, 235 7 of 15 To better explain the vertical distribution of wind speed and direction, Figure 5 shows the corresponding changes at 12:00 on 21 May at different AGL heights (106.15, 897, 1566, 2300, 3000, and 3700 m). The white arrows represent the magnitude and direction of the wind speed and the yellow five-pointed stars the campaign site. In the Sichuan Basin, the wind speed at heights of 106.15 and 897 m was slow, and there was no uniform wind direction. At a height of 1566 m, the wind speed over the basin is still very small, but the wind direction tended to be consistent. In contrast, the wind speed increased, and the wind direction was consistent as the height increased. After the height reached to 3700 m, the wind direction over the entire basin was consistent with the outside of the basin. The spatial difference in wind speed and wind direction could be the result of the terrain, which had good agreement with the results obtained by PCDL. The Sichuan Basin can be regarded as a semi-enclosed space. Inside the space, the wind speed and direction of each location developed independently, so the wind speed was different and showed different wind Photonics 2022, 9, x FOR PEER REVIEW 8 of 17 directions. Outside the space, due to the influence of the westerly zone, the wind speed increased overall, and the wind direction tended to be consistent. Date: 21,May CST: 12:00 altitude:106.15m Date: 21,May CST: 12:00 altitude:897m Date: 21,May CST: 12:00 altitude:1566m 5 m/s -500 Date: 21,May CST: 12:00 altitude:2300m Date: 21,May CST: 12:00 altitude:3000m Date: 21,May CST: 12:00 altitude:3700m 35 4000 10 m/s 10 m/s 10 m/s -500 103 104 105 106 107 108 109 110 103 104 105 106 107 108 109 110103 104 105 106 107 108 109 110 Figure 5. Wind speed and direction from the ERA5 reanalysis at different heights; the yellow five- Figure 5. Wind speed and direction from the ERA5 reanalysis at different heights; the yellow pointed star represents the campaign site. five-pointed star represents the campaign site. Similarly, the spatiotemporal distribution of the temperature from the microwave Similarly, the spatiotemporal distribution of the temperature from the microwave ra- radiometer and ERA5 reanalysis data are shown in Figure 6. The temperature had an ob- diometer and ERA5 reanalysis data are shown in Figure 6. The temperature had an obvious vious diurnal variation trend, which reached its maximum between 15:00 and 16:00. diurnal variation trend, which reached its maximum between 15:00 and 16:00. Weather Weather had an obvious effect on the vertical distribution of temperature; for example, on had an obvious effect on the vertical distribution of temperature; for example, on 25 May, 25 May, after a drizzle, there was a heavy rain between 7:00 and 9:00, and then the sun after a drizzle, there was a heavy rain between 7:00 and 9:00, and then the sun was shining. was shining. Compared with the pictures obtained from the microwave radiometer, the Compared with the pictures obtained from the microwave radiometer, the ERA5 data did ERA5 data did not accurately reflect the real-time changes in the temperature due to the not accurately reflect the real-time changes in the temperature due to the fact of its lower fact of its lower resolution, but they still displayed the same temperature change trends. resolution, but they still displayed the same temperature change trends. T(℃) T(℃) Photonics 2022, 9, 235 8 of 15 Photonics 2022, 9, x FOR PEER REVIEW 9 of 17 2021-05-21 2021-05-23 2021-05-25 2021-05-21 2021-05-23 2021-05-25 Figure 6. Spatiotemporal distribution of temperature from microwave radiometer (top) and ERA5 Figure 6. Spatiotemporal distribution of temperature from microwave radiometer (top) and ERA5 reanalysis data (bottom). reanalysis data (bottom). 4.2. Modification of HMNSP99 Outer Scale Model 4.2. Modification of HMNSP99 Outer Scale Model To better investigate the optical turbulence spatiotemporal characteristics, two outer To better investigate the optical turbulence spatiotemporal characteristics, two outer scale models are chosen here to estimat2e C profiles, including Dewan model and scale models are chosen here to estimate C profiles, including Dewan model and HMNSP99 model HMN as SPmentioned 99 model as me above. ntione The d ab comparison ove. The com of paC rison of profiles C betw profeen iles b measur etween ements meas- and n n calculation from models is depicted in Figure 7. The red line is the measured optical urements and calculation from models is depicted in Figure 7. The red line is the measured turbulence optical tustr rbu ength lence st obtained rength ob by tain the ed b micr y tho-thermometer e micro-thermomcarried eter carrby ied b meteor y meteo ological rologica balloon, l others balloar on e, o derived thers are deri by the ved PCDL by th and e PCmicr DL aowave nd micrradiometer owave radiobased meter b on ased models. on model It is s. I obvious t is obvious that the Dewan model is likely to overestimate the turbulence strength espe- that the Dewan model is likely to overestimate the turbulence strength especially at high cially at high altitudes. Compared to Dewan model, the estimated profile values using the altitudes. Compared to Dewan model, the estimated profile values using the HMNSP99 HMNSP99 model are more consistent with the measured values. However, the error be- Photonics 2022, 9, x FOR PEER REVIEW 10 of 17 model are more consistent with the measured values. However, the error between the tween the calculated value and the measured value is still large. calculated value and the measured value is still large. Figure 7. Comparison of the C profiles between measurements conducted by the meteorological Figure 7. Comparison of the C profiles between measurements conducted by the meteorological bal- loonsbal and loocalculations ns and calcuby latio dif nsfer by ent dif outer ferentscale outemodels r scale m including odels incHMNSP99, luding HMN the SP9modified 9, the moHMNSP99, dified HMNSP99, and Dewan. and Dewan. In order to better reconstruct the optical turbulence strength of the experimental site, a new statistical outer scale model was improved based on the HMNSP99 model using the meteorological balloons’ data. The statistical average of the vertical profiles of , dT S , and were acquired with a vertical resolution of 30 m, and the exponent of the dh formula was refitted by the least squares method. The modified HMNSP99 model was characterized as Equation (11). The profiles estimated by the modified HMNSP99 model are also illustrated in Figure 7. Although it is difficult to describe the optical turbu- lence’s fine structure precisely, the model can reflect the trend in the optical turbulence’s variation to some extent. dT 1.242+− 8.663S 86.732 4/3 4/3 dh (11) L 0.1 ×10 4.3. Spatiotemporal Characteristics and Effect Factors of Optical Turbulence Using basic meteorological parameters from the PCDL and microwave radiometer, the spatiotemporal variation of C estimated by the modified HMNSP99 model is illus- trated in Figure 8. Both 21 and 25 May were selected to display the spatiotemporal varia- tion in the optical turbulence and corresponding affecting factors. It is noted that in the –2/3 two figures, the values of logC were basically above −16.5 m , and the values seem to be consistent with Masciadri’s studies [34]. In order to investigate the factors affecting the development process of turbulence, Figure 8 also shows the temporal and spatial diurnal dT variation characteristics of temperature gradient ( ) and wind shear ( S ). It is obvious dh that the optical turbulence strength and temperature gradient were in good agreement under 2 km, that is, the greater the temperature gradient, the stronger the turbulence strength. Moreover, the strong turbulent layer above 2 km, especially from 2.5 to 3 km, was related more to wind shear, where there was an increase in the influence of wind shear and a decrease in the influence of temperature gradient. = Photonics 2022, 9, 235 9 of 15 In order to better reconstruct the optical turbulence strength of the experimental site, a new statistical outer scale model was improved based on the HMNSP99 model using the meteorological balloons’ data. The statistical average of the vertical profiles of L , S, and dT were acquired with a vertical resolution of 30 m, and the exponent of the formula was dh refitted by the least squares method. The modified HMNSP99 model was characterized as Equation (11). The C profiles estimated by the modified HMNSP99 model are also illustrated in Figure 7. Although it is difficult to describe the optical turbulence’s fine structure precisely, the model can reflect the trend in the optical turbulence’s variation to some extent. dT 4/3 4/3 1.242+8.663S 86.732 dh L = 0.1 10 (11) 4.3. Spatiotemporal Characteristics and Effect Factors of Optical Turbulence Using basic meteorological parameters from the PCDL and microwave radiometer, the spatiotemporal variation of C estimated by the modified HMNSP99 model is illustrated in Figure 8. Both 21 and 25 May were selected to display the spatiotemporal variation in the optical turbulence and corresponding affecting factors. It is noted that in the two figures, 2 –2/3 the values of log C were basically above 16.5 m , and the values seem to be consistent with Masciadri’s studies [34]. In order to investigate the factors affecting the development process of turbulence, Figure 8 also shows the temporal and spatial diurnal variation dT characteristics of temperature gradient ( ) and wind shear (S). It is obvious that the dh optical turbulence strength and temperature gradient were in good agreement under 2 km, that is, the greater the temperature gradient, the stronger the turbulence strength. Moreover, Photonics 2022, 9, x FOR PEER REVIEthe W strong turbulent layer above 2 km, especially from 2.5 to 3 km, was related mor 11 o ef to 17 wind shear, where there was an increase in the influence of wind shear and a decrease in the influence of temperature gradient. -15.4 05-21 05-25 -15.6 -15.8 -16 -16.2 -16.4 -12 05-21 -10 05-25 2000 -8 -6 -4 -2 0.05 2500 05-21 0.04 05-25 0.03 0.02 0.01 0 0 00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 24:0000:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 24:00 2 dT dT Figure Figure 8. 8. Spatiotemporal Spatiotemporal d distribution istribution o off CC , , , and , aS nd in two S in days. two days. ndh dh Overall, in the process of optical turbulence development, temperature gradient plays Overall, in the process of optical turbulence development, temperature gradient a dominant role in turbulence generation below 2 km, and wind shear plays an important plays a dominant role in turbulence generation below 2 km, and wind shear plays an im- role above 2 km. Optical turbulence is also affected by the basin terrain. Due to the low portant role above 2 km. Optical turbulence is also affected by the basin terrain. Due to altitude of the field campaign site, it was almost surrounded by mountains, which isolated the low altitude of the field campaign site, it was almost surrounded by mountains, which isolated the turbulence inside it; thus, optical turbulence developed independently and was mainly affected by the temperature gradient. Over the basin, the strong wind in the westerly zone and the weak wind at the top of the basin formed a large wind shear, the influence of which far exceeded the temperature gradient and played a leading role in the development of optical turbulence. Based on the PCDL and microwave radiometer data, the diurnal variation curves of the turbulence strength at different altitudes were obtained, and they are depicted in Fig- ure 9. As can be seen, for turbulence mainly deduced by temperature gradient, its strength gradually decreases with the increase in altitude. The refractive index structure constant had a relatively typical “Sombrero” feature at different altitudes, which gradually disap- pear with the increase in altitude. Notably, there was a little more disagreement between the two days’ diurnal variation curves of turbulence strength, where the typical “Som- brero” feature disappeared at 704 and 2055 m, respectively. The reason for this was that on 21 May it was cloudy, while on 25 May it was mainly sunny. The development of op- tical turbulence in the boundary layer is closely related to air temperature. Similarly, the diurnal variation curves of the surface layer, C , from the micro-thermometer are clearly illustrated in Figure 9c. Compared with the turbulence strength on 21 May, the strength on 25 May was significantly enhanced, and strong turbulence existed for a longer time. From Figure 6, the air temperature on 21 May was lower than that on 25 May. Only the air temperature near the ground reached 25 °C at 15:00 on 21 May. Compared with 21 May, the high temperature lasted longer on 25 May, and the maximum height reached 0.5 km. Differences in the air temperature resulted in the strength of the optical turbulence and limited the maximum height of turbulence development. Photonics 2022, 9, 235 10 of 15 the turbulence inside it; thus, optical turbulence developed independently and was mainly affected by the temperature gradient. Over the basin, the strong wind in the westerly zone and the weak wind at the top of the basin formed a large wind shear, the influence of which far exceeded the temperature gradient and played a leading role in the development of optical turbulence. Based on the PCDL and microwave radiometer data, the diurnal variation curves of the turbulence strength at different altitudes were obtained, and they are depicted in Figure 9. As can be seen, for turbulence mainly deduced by temperature gradient, its strength gradually decreases with the increase in altitude. The refractive index structure constant had a relatively typical “Sombrero” feature at different altitudes, which gradually disappear with the increase in altitude. Notably, there was a little more disagreement between the two days’ diurnal variation curves of turbulence strength, where the typical “Sombrero” feature disappeared at 704 and 2055 m, respectively. The reason for this was that on 21 May it was cloudy, while on 25 May it was mainly sunny. The development of optical turbulence in the boundary layer is closely related to air temperature. Similarly, the diurnal variation curves of the surface layer, C , from the micro-thermometer are clearly illustrated in Figure 9c. Compared with the turbulence strength on 21 May, the strength on 25 May was significantly enhanced, and strong turbulence existed for a longer time. From Figure 6, the air temperature on 21 May was lower than that on 25 May. Only the air temperature near the ground reached 25 C at 15:00 on 21 May. Compared with 21 May, the high temperature lasted longer on 25 May, and the maximum height reached 0.5 km. Photonics 2022, 9, x Dif FOR P fer Eences ER REVIin EWthe air temperature resulted in the strength of the optical turbulence 12 and of 17 limited the maximum height of turbulence development. Figure 9. Diurnal profiles of C at different altitudes, (a) May 21; (b) May 25; (c) surface. Figure 9. Diurnal profiles of C at different altitudes, (a) May 21; (b) May 25; (c) surface. 4.4. Spatiotemporal Characteristics of Turbulence Basic Parameters According to Equations (1) and (4), we can calculate the optical turbulence strength if given the outer scale , turbulence diffusion coefficient K , turbulence energy dissi- pation rate ε , and the corresponding conventional meteorological parameters. It is TKE necessary to obtain the spatiotemporal characteristics of basic turbulence parameters. Fig- ure 10 presents the results derived from conventional meteorological parameters of the experimental site. It is apparent that agrees well with C in the temporal and spatial 0 n characteristics, where turbulence strength increases as the outer scale increases. The rela- L L tionship between C and accords with our earlier observations that showed that n 0 0 has the biggest influence weight among the turbulence influence factors [22]. In addition, L L the value of agrees with Takato’s (1995) findings that found that is basically in the 0 0 range of 1–10 m [35]. K , ε , and C in this study also had good consistency, where TKE K and ε could reflect the strength of the turbulence from the side. The results, as TKE shown in the middle and bottom of Figure 10, indicate that the values of K and ε TKE 2 −1 −6 −3 2 −3 were basically in the range of 0–1 m ·s and 10 –10 m ·s , respectively. There was a Photonics 2022, 9, 235 11 of 15 4.4. Spatiotemporal Characteristics of Turbulence Basic Parameters According to Equations (1) and (4), we can calculate the optical turbulence strength if given the outer scale L , turbulence diffusion coefficient K, turbulence energy dissipation rate # , and the corresponding conventional meteorological parameters. It is necessary to TKE obtain the spatiotemporal characteristics of basic turbulence parameters. Figure 10 presents the results derived from conventional meteorological parameters of the experimental site. It is apparent that L agrees well with C in the temporal and spatial characteristics, where turbulence strength increases as the outer scale increases. The relationship between C and L accords with our earlier observations that showed that L has the biggest n 0 0 influence weight among the turbulence influence factors [22]. In addition, the value of L agrees with Takato’s (1995) findings that found that L is basically in the range of 0 0 Photonics 2022, 9, x FOR PEER REVIEW 13 of 17 1–10 m [35]. K, # , and C in this study also had good consistency, where K and # TKE TKE could reflect the strength of the turbulence from the side. The results, as shown in the middle and bottom of Figure 10, indicate that the values of K and # were basically in the TKE 2 1 6 3 2 3 range of 0–1 m s and 10 –10 m s , respectively. There was a significant positive significant positive correlation between these basic turbulence parameters. Moreover, no correlation between these basic turbulence parameters. Moreover, no report was seen that report was seen that a complete spatiotemporal distribution of , K , and ε was TKE a complete spatiotemporal distribution of L , K, and # was obtained from the PCDL at 0 TKE obtained from the PCDL at the same time. the same time. 05-25 05-21 0 0 05-21 0.8 05-25 0.6 0.4 0.2 -2 05-21 05-25 -3 -4 1000 -5 -6 00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 24:00 24:00 Figure 10. Spatiotemporal distribution of , K , and ε in two days. Figure 10. Spatiotemporal distribution of L , K, and # in two days. 00 TKE TKE 4.5. Spatiotemporal Characteristics of Astronomical Parameters 4.5. Spatiotemporal Characteristics of Astronomical Parameters Figure 11 provides the temporal and spatial characteristics of integrated astronomical Figure 11 provides the temporal and spatial characteristics of integrated astronomical parameters (the atmospheric coherence length r , seeing #) at the experimental site. It parameters (the atmospheric coherence length r , seeing ε ) at the experimental site. It was apparent that r had relative typical diurnal variation characteristics and, on the whole, increased with height at different times. The coherence length was smaller between was apparent that r had relative typical diurnal variation characteristics and, on the approximately 07:00 and 20:00 and larger at other times. Comparing the results of the whole, increased with height at different times. The coherence length was smaller between two days, r on 21 May was larger than that on 25 May after 7:00 in the daytime. As approximately 07:00 and 20:00 and larger at other times. Comparing the results of the two Figure 11 shows, there was an opposite diurnal characteristic between r and #. The days, r on 21 May was larger than that on 25 May after 7:00 in the daytime. As Figure whole trend of # decreased with height at different times, which had good agreement with 1 Mchugh’s 1 shows, t investments here was an [ 36 op ].pMor osite eover diur , n#a was l cha lar ra ger cter between istic betw appr eenoximately r and 07:00 . Th and e w20:00, hole trend of ε decreased with height at different times, which had good agreement with Mchugh’s investments [36]. Moreover, ε was larger between approximately 07:00 and 20:00, and smaller at other times. The ε from May 25 can be compared with ε on the May 21, which shows that there was a strong seeing layer below 2 km during the daytime. It is difficult to explain this result, but the different diurnal characteristics might be related to the season and weather conditions. Photonics 2022, 9, 235 12 of 15 Photonics 2022, 9, x FOR PEER REVIEW 14 of 17 and smaller at other times. The # from May 25 can be compared with # on the May 21, which shows that there was a strong seeing layer below 2 km during the daytime. It is Photonics 2022, 9, x FOR PEER REVIEW 14 of 17 difficult to explain this result, but the different diurnal characteristics might be related to 05-21 05-25 the season and weather conditions. 05-21 05-25 05-21 05-25 1.2 0.8 05-21 05-25 1.2 0.6 0.4 0.8 0.6 r ε Figure 11. Spatiotemporal distribution of and during the two days. 0.4 According to Equations (9) and (10), the diurnal variation curves of the integrated atmospheric parameters of the two days are presented in Figure 12. The and ε on Figure 11. Spatiotemporal distribution of r and # during the two days. Figure 11. Spatiotemporal distribution of 0 and ε during the two days. 25 May had typical diurnal characteristics. The general trend for was that it was According to Equations (9) and (10), the diurnal variation curves of the integrated smaller during the daytime and larger at nighttime, reaching a minimum value of 5 cm. According to Equations (9) and (10), the diurnal variation curves of the integrated atmospheric parameters of the two days are presented in Figure 12. The r and # on 25 May On the contrary, was larger during the daytime and smaller at night. A positive cor- r ε athad mostypical pheric p diur aranal metcharacteristics. ers of the two da The ys a general re prestr en end ted i for n F r ig was ure 1 that 2. Tit he was smaller and during on ε C the daytime and lar relger ation i at s nighttime, found betw reaching een a aminimum nd , and value the rof esu 5lcm. t seeOn ms t the o be contrary consist,ent # with Tian’s 25 May had typical diurnal characteristics. The general trend for was that it was was larger during the daytime and smaller at night. A positive correlation is found between research that suggested that such a seeing minimum had a good temporal association with smaller during the daytime and larger at nighttime, reaching a minimum value of 5 cm. # and C , and the result2seems to be consistent with Tian’s research that suggested that such the minimum simultaneously observed at ATS [37]. Compared with the results ob- n 2 Oa nseeing the con minimum trary, had waa s good largertemporal during th association e daytime a with nd sm the alle C r a minimum t night. A simultaneously positive cor- observed at ATS [37]. Compared with the results obtained on r 25 Mayε , the diurnal variation tained on 25 May, the diurnal variation of and on 21 May was weakened. There ε C relation is found between and , and the result seems to be consistent with Tian’s of r and # on 21 May was weakened. There was not much difference between r and # 0 0 r ε was not much difference between and for the two days at before approximately research that suggested that such a seeing minimum had a good temporal association with for the two days at before approximately 07:00 and after approximately 20:00. A possible 07:00 and after approximately 20:00. A possible explanation for these differences may be explanation for these differences may be that 21 May can be regarded as close to stable the minimum simultaneously observed at ATS [37]. Compared with the results ob- that 21 May can be regarded as close to stable atmospheric conditions, since it was over- atmospheric conditions, since it was overcast, and on 25 May it was sunny. The above cast, and on 25 May it was sunny. Tε he above results provide further knowledge for the tained on 25 May, the diurnal variation of and on 21 May was weakened. There results provide further knowledge for the space–time 0 characteristics of the astronomical space–time characteristics of the astronomical parameters of the basin. parameters of the basin. r ε was not much difference between and for the two days at before approximately 07:00 and after approximately 20:00. A possible explanation for these differences may be that 21 May can be regarded as close to stable atmospheric conditions, since it was over- cast, and on 25 May it was sunny. The above results provide further knowledge for the space–time characteristics of the astronomical parameters of the basin. Figure 12. Diurnal profiles of r and # over the two days. r ε Figure 12. Diurnal profiles of and over the two days. 5. Conclusions 5. Conclusions Since the late 1960s, many research communities have shown an interest in measuring and estimating optical turbulence profiles. Most of the profiles are measured at different r ε Figure 12. Diurnal profiles of and over the two days. 5. Conclusions Photonics 2022, 9, 235 13 of 15 time periods or at a single measuring point, for example, using a radiosonde or an ultra- sonic anemometer, which cannot reflect the spatiotemporal variation of optical turbulence. Appropriate knowledge of the spatiotemporal characteristics of C is important, since laser transmission and astronomical observation are severely affected by optical turbulence. In this investigation, the spatiotemporal variation of C was estimated using basic meteo- rological parameters derived from PCDL and a microwave radiometer. Moreover, the integrated parameters for astronomy and optical telecommunication were derived from optical turbulence strength profiles at the experimental site. The corresponding conclusions are as follows. (1) Due to the fact that temperature gradient and wind shear play important roles in the development of turbulence, they are applied to various parameterized models. However, the proportion of temperature gradient and wind shear in the model will be affected by time, altitude, environment, and other factors. In this paper, in order to obtain the temporal and spatial variation characteristics of turbulence strength over the basin, on the basis of the original HMNSP99 model, a new parametric model was obtained through fitting the C profile measured by balloon-borne micro-thermometer. Compared with the original HMNSP99 model, the modified model had better environmental adaptability; (2) Usually, C has a large value near the ground and decreases with height. Turbulence strength was stronger during the daytime and weaker at nighttime. The temperature gradient and wind shear together affected the development of optical turbulence. However, due to the existence of the basin, the development of optical turbulence was affected. The basin can be regarded as a semi-enclosed space, where meteorological parameters in the basin develop independently and avoid outside factors, while external affecting will increase above the basin. The lower wind speed in the basin directly weakened the influence of wind shear, which increased the effect of temperature gradient. There was a strong turbulence layer above the basin, especially between 2.5 and 3 km, which may mainly be caused by the combined action of the westerly zone and topography. Moreover, the differences in the optical turbulence strength between the two days in Figure 8 were relatively large, from 0.5 to 2 km, which may be related to the generation and elimination mechanism of turbulence; (3) Under the main influence of the temperature gradient inside the basin, the C had a relatively typical “Sombrero” feature at different altitudes, which gradually disappeared with the increase in altitude. Over the basin, although there was also a strong turbulence layer under the main influence of wind shear, the diurnal variation in turbulence also did not appear. Under the influence of temperature gradient and wind shear, optical turbulence exhibited different temporal and spatial characteristics. The results of this study indicate that in the process of optical turbulence development, temperature gradient plays a dominant role in turbulence generation below 2 km and wind shear has an important influence above 2 km. (4) This is the first time that several complete 3D maps of optical turbulence parameters, from the ground up to 3 km, were obtained from PCDL at the same time. Unlike other previous attempts, our results cover a wide variety of time, space, and parameters. The consistency of variations in multiple parameters increases the reliability of the results. From the results, we can conclude that in the basin environment, the approximate ranges of the outer scale, turbulence diffusion coefficient, and turbulent energy dissipation rate were 2 1 6 3 2 3 1–10 m, 0–1 m s , and 10 –10 m s , respectively. Moreover, the spatiotemporal characteristics of the astronomical parameters derived from the optical turbulence profiles are present in this work. The integrated astronomical parameters had typical diurnal characteristics, where the general trend for r was that it was smaller during the daytime and larger at night, while # was larger during the daytime and smaller at night. This leads to a better understanding of the behavior of these parameters relevant to the optical quality of the atmosphere from the ground up to 3 km. These findings enhance our understanding of the spatiotemporal characteristics of C under the influence of complex terrain. However, the findings in this investigation n Photonics 2022, 9, 235 14 of 15 are subject to at least two limitations. First, the current study was limited by the size of experimental data set. A number of possible future studies using the same experimental setup are apparent, which will increase the persuasiveness of the experimental conclusions. Second, due to the limitations of the experimental equipment’s parameters, the current research did not specifically indicate the relationship of wind shear related to optical turbulence strength over the basin. It is suggested that the association of these factors be investigated in future studies. Notwithstanding these limitations, the findings are of significance for laser transmission and astronomical observation. Author Contributions: Conceptualization, M.X. and S.S.; software, M.X.; validation and formal analysis, M.X., S.S., and N.W.; investigation, M.X.; resources, L.Z.; data curation, L.Z. and Q.L.; writing—original draft preparation, M.X.; writing—review and editing, M.X.; visualization, M.X.; supervision, S.S. and N.W.; project administration and funding acquisition, S.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key Research and Development Program (grant no: 2018YFC0213102), the Open Project of Equipment Pre-Research Fund (grant no: 6142404180302), the National Natural Science Foundation of China (grant no: 41475024 and 42027804). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data are contained within the article. 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Photonics – Multidisciplinary Digital Publishing Institute
Published: Apr 2, 2022
Keywords: Earth–atmosphere interaction; optical turbulence; temperature gradient; wind shear; outer scale
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