Impact of small-scale orography on deep boundary layer evolution and structure over the Tibetan Plateau

Abstract. We investigate how small-scale orography influences the evolution and structure of the exceptionally deep convective boundary layer (CBL) over the Tibetan Plateau (TiP). Using large-eddy simulations (LES) at 50 m resolution under semi-idealized dry conditions, we compare three experiments over an elevated plateau (4.2 km above sea level, ASL): FLAT (no orography), REAL (realistic terrain), and REALu10 (REAL plus an upper-level wind of 10 m s⁻¹). All simulations produce very deep CBLs, reaching ~9 km ASL by late afternoon, consistent with record-high values observed over the TiP. Small-scale orography substantially accelerates early growth: by midday the CBL in REAL is ~80 m higher than in FLAT, and locally above the mountain it is ~500 m deeper. This terrain-induced advantage narrows later in the day as all cases attain extreme depths. In REALu10, the CBL grows even higher, reaching 9.4 km ASL by 19 LT. Added shear organizes convection into longitudinal roll vortices, contrasting with the isolated terrain-anchored plumes in the no-shear case, and enhances vertical mixing.

These results demonstrate that unresolved small-scale orography can increase daytime CBL height by up to 15 % and expedite entrainment of free-tropospheric air. Under clear-sky conditions, the plateau’s CBL can exceed 9 km within a single day given strong surface heating and weak stability aloft. Our findings highlight the importance of including fine-scale terrain and shear effects in models, as their omission may underestimate CBL growth and vertical exchange over high-altitude regions.