Analysis of local-scale transport in a deep dry boundary layer over the Tibetan Plateau in boreal winter.

Abstract

<p>The Tibetan Plateau (TiP) is the highest and largest plateau in the world. During the boreal winter and spring, a deep atmospheric boundary layer (up to 5 km above the plateau level) is frequently observed in the area. The ultimate goal of the present work is to assess the impact of small-scale topography and thermal circulations on the structure of the dry-season deep boundary layer over the TiP and on tracer exchange with the upper troposphere (UT). The deep boundary layer is responsible for the tracer transport from atmospheric boundary layer (ABL) to upper troposphere and lower stratosphere (UTLS). To this end, we performed a set of idealized large-eddy simulations (LES) resolving the turbulent transport from the surface to the UT using the CM1 model. The initial conditions for the first simulated dry case are based on a very deep boundary layer over TiP (at Gerze station) on 4 March 2008 documented in Chen et al. (2016). The high resolution LES performed for the winter case during boreal winter using the CM1 model. The simulations use a grid spacing of 50 m to ensure that the turbulence and thermal circulations are adequately resolved. Tracer transport and exchange with the UT are investigated using passive tracers released at different height levels. To assess the impact of small-scale topography, the plateau site simulated with flat terrain, idealized small-scale topography, and real topography. The results are generalized by a systematic investigation of the impact of turbulence and thermal circulations on boundary layer depth simulated in an idealized setup using the CM1 model.</p>