Characteristics of the vertical structure of the atmospheric turbulence in the Tibetan Plateau

The Tibetan Plateau (TP) has unique atmospheric dynamics and thermal structures that originate from its giant terrain and complex climate. High vertical-resolution thermal radiosondes were launched near the central (Lhasa, 91°06′E, 29°36′N, 3670 m above sea level (ASL)) and marginal (Da Qaidam, 95°21′E, 37°51′N, 3180 m ASL) areas of the TP during the summers of 2018 and 2020, respectively. Atmospheric turbulence parameters were calculated, and the characteristics of the atmospheric turbulent vertical structure at sunset in these two areas were analyzed and compared. Affected by TP thermal forcing and stably controlled by the summer monsoon anticyclone, the atmospheric refractive index structure constant (Cn2) tended to increase and then decrease with increasing height, reaching a maximum at the tropopause (∼18 km ASL) at the Lhasa site. Although Cn2 at the Da Qaidam site also tended to increase at the tropopause, the position of the strong turbulent band (STB) (5–7 km ASL) was below the tropopause height corresponding to the potential temperature lapse rate minimum. The vertical distribution of Cn2 at the two sites, particularly regarding the position of the STB, was highly correlated with the atmospheric stability (Ri) and the thermal mixing scale (LT). The significant correlations among the three parameters (STB, Ri, and LT) indicated that the strong fluctuations in temperature caused by thermal mixing were the dominant factor causing the Ri to be less than its critical value of 0.25. Moreover, the suppression strength involving the upward transport of the heat sources was the main reason for the different turbulent vertical structures and STB positions at the two sites. The zonal mean thermodynamic and dynamical fields derived from the reanalysis data also showed a height difference in the heat sources transported to the troposphere at the two sites. In the marginal TP, the material and energy in the lower troposphere were transported by the turbulent atmosphere upward along the slope of the mountain and converged at the central TP (28°N–35°N) with strong thermal forcing up to the tropopause. In the STB of the Lhasa site, the turbulent dissipation rate and eddy diffusion coefficient increased sharply, indicating that the turbulent atmosphere in this central site was highly diffused, and the small-scale turbulence transported the material and energy upward.