Impact of surface and entrainment heat fluxes on the thermodynamic structure of the convective boundary layer over the Tibetan Plateau: observations and modelling analysis

Detailed convective boundary layer (CBL) structure and the impact factors over the Tibetan Plateau has not been clearly understood, particularly for the level of neutral stability (z_n), at which statically unstable lower CBL begins to transit into slightly stable upper CBL. Substantial uncertainties still exist in numerical models with different planetary boundary layer (PBL) schemes to reproduce such detailed structure. In this study, detailed CBL structure and processes over the Tibetan Plateau are examined using multi-year radiosonde data and large-eddy simulation (LES), particularly focusing on the impact of surface heating and entrainment on z_n. The results indicated that the values of z_n spatially ranged within 0.16–0.38z_i on the plateau, with z_i representing the CBL depth, and z_n was higher in the southwestern region and lower in the southeastern region. Surface-/entrainment-induced large-scale thermals (corresponding to nonlocal fluxes) tended to suppress/elevate z_n, due to warm turbulence penetrating into the upper/lower CBL, whereas small-scale eddies (corresponding to local fluxes) played an opposite role on modifying z_n. The LES results suggested that z_n increased before 08:00 Local Time (about 80 minutes after sunrise) because surface-induced small eddies dominated during the early stage of CBL growth and z_n decreased afterwards as large-scale surface-induced thermals became more active. These improved understanding provides guidance for further improvement of PBL schemes.