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

Abstract

Substantial uncertainties remain in numerical models incorporating different planetary boundary layer (PBL) schemes when it comes to reproducing the detailed thermodynamic structure of the convective boundary layer (CBL), particularly for the level of neutral stability (z_n), at which statically unstable lower CBL begins to transit into slightly stable upper CBL. Using multi-year radiosonde data from 12 stations and large-eddy simulation (LES), we examined the detailed CBL thermodynamic structure and processes over the Tibetan Plateau, 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 air penetrating into the upper/lower CBL, whereas small-scale eddies (corresponding to local fluxes) exert the opposite effect on z_n. The LES results indicated that z_n increased before 08:00 Local Time (about 80 min after sunrise), as surface-induced small eddies dominated during the early stage of CBL development. After this time, z_n decreased as large-scale surface-induced thermals became more active. This improved understanding provides guidance for further improvement of PBL schemes.

Plain language summary

Convective boundary layer (CBL) over the Tibetan Plateau plays an important role in the climate system in East Asia. Nevertheless, detailed CBL structure and the impact factors over the plateau have not been fully examined yet. Combining the multi-year radiosonde fine-resolution profiles of potential temperature (θ) with the large eddy simulation (LES), we investigate the CBL structure, particularly the position of neutral point (z_n), a transition level separating the statically unstable lower CBL from the slightly statically stable upper CBL. We also examine the impacts of surface heating and entrainment process on detailed CBL structure. θ profiles exhibit a three-layer structure, with z_n spatially varying over the plateau. Different eddies with different sizes initiated from the surface and from the entrainment zone at the CBL top synergistically affect the evolution of the CBL and the altitude of z_n. This improved understanding of the detail CBL structure and the relevant physical processes provides a new angle to evaluate and calibrate numerical weather prediction (NWP) models with PBL schemes.