Characteristics of atmospheric optical turbulence in the free atmosphere over China derived from radiosonde data using the Thorpe method

The distribution characteristics of optical turbulence are an essential factor to consider for the performance design of astronomical telescopes and adaptive optics instruments. This study used the Thorpe method to estimate the vertical and horizontal distribution characteristics of optical turbulence in the free atmosphere over China, based on the sounding data. Vertically, the optical turbulence is stronger in the troposphere than in the stratosphere, with median values of the atmospheric refractive index structure constant (<i>C</i><i>n</i>2) of -16.66 and -17.14 under logarithmic (the unit of <i>C</i><i>n</i>2 is m^-2/3), respectively. In addition, the vertical profile of <i>log</i>₁₀<i>C</i><i>n</i>2 exhibits a distinct peak near the tropopause, and the vertical and seasonal variations of <i>log</i>₁₀<i>C</i><i>n</i>2 are the result of the synergistic effects of the square of Brunt-Vaisala frequency (<i>N</i>²), wind shear (<i>S</i>), and Thorpe length (<i>L</i>_<i>T</i>). Horizontally, there are obvious seasonal differences in the seeing, with higher values in spring and summer; the stratification analysis reveals that the seeing are highest in the BT (below the tropopause), followed by NT (near the tropopause), and lowest in AT (above the tropopause), and the BT region shows a distribution pattern of "high in the south and low in the north". In addition, the analysis of the relationship between optical turbulence (<i>log</i>₁₀<i>C</i><i>n</i>2) and <i>N</i>² and <i>S</i> reveals that the troposphere exhibits stronger optical turbulence due to smaller <i>N</i>² and larger <i>S</i>, and the enhancement of static instability significantly increases the incidence of optical turbulence when <i>N</i>² approaches zero, while the stratosphere has weaker optical turbulence and a relatively stable distribution. The optical turbulence characteristics can be used directly for astronomical observations and free-space optical communications. Although this study focused on the characteristics of optical turbulence in the free atmosphere over China, the results provided valuable insights into the distribution of optical turbulence worldwide, and can help to offer a scientific basis for future studies in the field of optical astronomy.