Ground-based optical astronomical observations must pass through Earth's atmosphere, where the image motion of stars caused by atmospheric turbulence can degrade observational quality.

A new study led by researchers from the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS) identified the atmospheric layer below 200 m above the Muztagh-ata station as the key physical layer determining the site's optical astronomical observing quality.

The study also found a distinct stratified wind structure with significant seasonal and diurnal variations at the site: wind speeds are higher in winter and lower in summer, with a stable prevailing wind direction from the south-west across all altitude layers.

Muztagh-ata has relatively favorable seeing conditions, giving it great potential to become a major optical astronomical observing site in western China. The findings provide an important basis for the site selection of large-aperture telescopes and the development of AO systems at the Muztagh-ata site.

XAO PhD candidate GU Wenbo, under the supervision of his advisor Aili Yishamuding, used coherent Doppler wind lidar to carry out long-term wind-profile observations at the North-1 site of the Muztagh-ata station. By combining these data with simultaneous measurements from a seeing monitor and a 30-meter meteorological tower, GU conducted a joint analysis of optical turbulence and seeing characteristics within the lowest 3 km above the site.

The analysis showed that the median integrated seeing in the 6–1000 m altitude layer is approximately 0.60 arcseconds, and this layer contributes a median of 59% to the total atmospheric seeing. Within this layer, the thin layer from 6 to 200 m above the ground accounts for more than 63% of the turbulence energy in the 6–1000 m range. The study confirms that the atmospheric layer below 200 m is indeed the key physical layer determining the optical observing quality of the Muztagh-ata site.

Researchers also found that atmospheric turbulence at Muztagh-ata is jointly influenced by thermal and dynamic processes. During the daytime, turbulence is dominated by thermal convection caused by surface heating, allowing turbulent motions to develop upward to about 1500–2000 m.

At the same time, mechanical turbulence induced by strong wind shear also serves as a key factor affecting seeing. Across all seasons, turbulence exhibits a pronounced diurnal variation: after sunset, it rapidly weakens, and the atmospheric environment becomes more stable.

In addition, the research team compared and validated ERA5 reanalysis data. The results indicate that ERA5 is highly reliable for estimating nighttime optical turbulence but shows certain biases during the daytime when surface convection is strong. Therefore, such data should be used with caution in daytime turbulence modeling.

This study was jointly supported by the National Natural Science Foundation of China and the "Light of West China" Talent Program of CAS.