The Extremely High-ranging Planetary Boundary Layer over the Western Tibetan Plateau in Winter

With a depth of some 5000 m above ground the planetary boundary layer (PBL) over the Tibetan Plateau can reach an unmatched height of 9515 m above sea level in late winter / early spring. The proximity of this height to the tropopause facilitates exchange between the stratosphere and the boundary layer. However, the underlying mechanisms responsible for this uniquely deep PBL have remained unclear. In this contribution, we explore different possible mechanisms and their relative importance using measurements (soundings) of the PBL, the associated surface fluxes, single-column and regional numerical simulations as well as global reanalysis data. Our results indicate that the dry conditions of both ground soil and atmosphere in late winter cannot explain the special PBL. Rather, results from a single-column model demonstrate the key influence of weak stability of the free atmosphere upon the growth of extremely deep PBLs over the Tibetan Plateau. Simulations with the numerical weather prediction model COSMO exhibit good correspondence with the observed mean PBL structure and realistic turbulent kinetic energy distributions throughout the PBL. Using ERA-Interim reanalysis we furthermore find that weak atmospheric stability and the resulting deep PBLs are associated with higher upper-level potential vorticity (PV) values, which in turn correspond to a more southerly jet position and higher wind speeds. Upper-level PV structures and jet position thus influence the PBL development over the Tibetan Plateau.