The Characteristics at the Ali Observatory Based on Radiosonde Observations

摘要

As part of the conceptual and preliminary design process of the Chinese Large Telescope, we have spent the last several years measuring and statistically analyzing the atmospheric characteristics above the high Ali Observatory region, in the most western part of the Tibetan Plateau. This is the first article in a series discussing the Ali Observatory site-testing campaign, and we analyze the radiosonde data sets for the first time in that area. The profiles of precipitable water vapor (PWV), upper wind, and optical turbulence are depicted, in addition the statistical characteristics are obtained. PWV profiles in the Ali area indicate that PWV is mainly concentrated below the altitude of 12 km above mean sea level(msl), decreasing with altitude, and has seasonal variations affected by monsoons. In 2016, the median PWV at the Ali Observatory was about 2.0 mm, and less than 1.0 mm during the winter. The wind profiles show strong seasonal variations of wind speed in the troposphere, where there is a prevailing stable west wind. The median wind speed at the height of the 200hP alpha pressure level was 27 m/s in 2016, with a mean speed over 50 m/s in February and 10 m/s in August. The optical turbulence profiles also show seasonal variations, with C-n(2) values between 10(-17) m(-2/3) and 10(-18) m(-2/3) in the free atmosphere. Three turbulent layers are found at the heights around 6-8 km, 12-13 km, and 18-20 km above msl, respectively. In 2016, the median seeing was estimated to be 0.'' 35 at the Ali Observatory, and 0.'' 24 in the summer. The results suggest that very low water vapor in winter, weak upper wind, and good seeing conditions in summer are present at the Ali Observatory, yielding high infrared transparency and high sensitivity for future ground-based optical and infrared telescopes. Given the unique longitude and high altitude, the Ali Observatory might be a promising site in the northern hemisphere for astronomical observations.