| Atmospheric optical turbulence is a small-scale turbulence,which is closely related to atmospheric circulation.It is necessary to analyze regional atmospheric temperature,vertical structure of wind speed,dynamic and thermodynamic processes and other comprehensive factors.At present,there is no report on the statistical analysis of astronomical and meteorological parameters over different observatories near the same latitude.The purpose of this paper is to study the spatial-temporal distribution and seasonal characteristics of optical turbulence-prone areas by comparing the similarities and differences of statistical characteristics of astronomical and meteorological parameters at several observatories near 30 north latitude,which is of great significance to the understanding of the law of atmospheric turbulence,the site selection of ground-based telescopes,the site evaluation and the application of adaptive optical systems.Gaomeigu Station is the location of the 2.4-meter telescope.Because of its unique latitude and longitude advantages,Gaomeigu Station plays an indispensable role in international astronomical observation.The Tibetan Plateau has attracted the attention of astronomers all over the world because of its average altitude of over 4,500 meters and unique climatic advantages.Ali Station is located in the west of Tibetan Plateau,and it is likely to be built into a world-class observatory in the future.At present,research on turbulence measurement and astronomical observation has been carried out in Ali.Lhasa Station is located in the south of Tibetan Plateau,with perennial meteorological observation data and convenient observation conditions,which provides help for comparing and testing turbulence profile models over Tibetan Plateau.Therefore,it is very important to study the characteristics of optical turbulence parameters at Gaomeigu station,Ali station and Lhasa station.In this paper,the characteristics of optical turbulence parameters,such as wind speed profiles and 1/Ri,at Gaomeigu Station,Lhasa station and Ali station over Tibetan Plateau,and Mt Graham Station in recent 20 years(1999-2018)are explored by using European Centre for Medium-Range Weather Forecasts' fifth set of reanalysis data(ERA5).We studied the vertical wind speed distribution of Gaomeigu Station,Lhasa Station,Ali Station,and Mt Graham in recent 20 years(1999-2018)and the seasonal variation characteristics of average wind speed at 200 hPa.At the same time,we compared the stability of free atmosphere over Gaomeigu Station and Lhasa Station by using the sum profile,and also compared the stability of free atmosphere over Ali Station and Mt Graham Station near the same latitude.In addition,based on the turbulent meteorological sounding data obtained at Lhasa station for the first time,the characteristics of atmospheric optical turbulence in Lhasa were investigated,and a model for the Lhasa was built based on HV 5/7 model and HMN model,using the sounding measurements.The main contents and results of this thesis are organized as follows:(1)The reliability of temperature and wind speed obtained from ERA5 data was evaluated,compared with the sounding data of IGRAv2.Overall statistical results show that the temperature and wind speed of free atmosphere obtained from ERA5 data were credible,the BIAS and RMSE of temperature are basically lower than 1.2 K,and the BIAS and RMSE of wind speed are less than 2 m/s.Therefore,the ERA5 data set can be used to carry out a long-term and systematic evaluation of optical turbulence parameters at different locations.(2)Based on the ERA5 data from 1999 to 2018,we studied the vertical wind speed distribution at Gaomeigu Station and Lhasa Station and the seasonal variation trend of the average wind speed at 200 hPa.At the same time,we use different optical turbulence parameters to compare the stability of the free atmosphere over Gaomeigu Station and Lhasa Station.For the Gaomeigu Station and Lhasa Station during autumn to spring(except September),the strongest vertical wind speed occurred at the jet stream level(200 hPa),being a typical phonomenon in the mid-latitude.In summer and September,the wind speed at the two stations is obviously lower than that in other months.Especially at the Gaomeigu station in September,the median wind speed below 12 km does not exceed 5 m/s.Generally speaking,from January to April and from July to August,the wind speed intensity at Gaomeigu Station is basically stronger than or equal to that at Lhasa Station.The wind speed at the jet stream layer above the two stations also shows obvious seasonal variation,the wind speed is relatively small in summer,but being strong in winter.Moreover,the wind speed over Gaomeigu Station and Lhasa Station has not changed much in the past 20 years.Through the 1/Ri profile map,we can conclude that the atmosphere at both stations is in a stable state at an altitude above 20 km.For these two sites,The monthly median 1/Ri profile and the monthly average value of V200 show that the atmosphere in the two sites is stable from June to September,and optical turbulence is not easy to trigger;In other months,there are two regions near the altitudes of 14-18 km and 5-9 km which are easy to trigger turbulence,and the triggered turbulence intensity in Lhasa is stronger than that in Gaomeigu.Generally speaking,the atmospheric condition at Gaomeigu Station is stable than at Lhasa Station.In addition,the atmosphere at Gaomeigu Station is relatively stable in June and September,which is beneficial to astronomical observation.(3)Based on the reanalysis data of ERA5 data from 1999 to 2018,the characteristics of parameters such as wind speed,((?)?/(?)h)?((?)V/(?)h)2?1/Ri at Ali station and Mt Graham station near the same latitude are studied.Looking at the vertical wind speed profiles over Ali Station and Mt Graham Station,the maximum wind were always appeared at the jet stream layer except in July and August.And in July and August,the wind speed at the two stations is obviously lower than that in other months.On the whole,the wind speed at Ali station is stronger than that at Mt Graham station.The monthly average has obvious seasonal changes.For Ali station and Mt Graham station,the monthly median profile and the monthly average value of V200 show that atmospheric turbulence is not easy to occur in July and August,and the atmospheric stability is high;in other months,there are 2-3 regions where strong turbulence is easy to trigger,and the intensity of triggered turbulence is stronger than that at Mt Graham station.On the whole,the atmospheric stability of Mt Graham station is stronger than that of Ali station,and it is less likely to generate turbulence.(4)Based on the radiosonde data obtained at Lhasa Station for the first time,the characteristics of the turbulent profile in Lhasa in August are analyzed,and it is found that there are strong turbulent layers near 5 km and 12.5 km in the measured profile,which proves the validity of the 1/Ri profile estimation results.Based on the tradiosonde data obtained at Lhasa station for the first time,the HV 5/7 model and HMN model in Lhasa were fitted respectively.The differences between the measured balloon values and the estimated values from different external scales(including Coulman,HMN and Dewan models)are compared.The analysis results show that the two fitted models can well reflect the turbulence characteristics of Lhasa station,and HMN model performs best. |
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