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 four experiments over an elevated plateau (4.2 km above mean sea level (a.m.s.l.)): FLAT (no local orography), REAL (realistic terrain), and FLATu10/REALu10 (including a 10 m s−1 upper-level wind). ABL height is diagnosed using a passive tracer method and cross-validated against turbulence and thermodynamic fields. All simulations produce a very deep CBL, reaching ∼9 km a.m.s.l. by late afternoon, consistent with the record-high values observed over the TiP. Small-scale orography accelerates early CBL growth and anchors persistent thermals. REAL shows only a modest (∼1 %–2 %) increase in domain-mean ABL height relative to FLAT, yet locally the ABL is up to 20 % higher over the ridge, revealing strong spatial heterogeneity. Imposed upper-level shear (FLATu10, REALu10) produces deeper and more laterally uniform CBLs, with shear-driven rolls increasing the mean ABL height by roughly 10 %–15 % and homogenizing the tracer field. Orography primarily affects the upper tail of the distribution, with ridge-top 99th-percentile ABL heights exceeding flat-terrain values by 10 %–20 %, while shear is the dominant control on domain-mean deepening. These results show that unresolved fine-scale orography and shear strongly modulate both the depth and spatial variability of the TiP CBL, and can substantially influence entrainment under weak stability. Their omission may lead weather and climate models to underestimate CBL growth and vertical exchange over high-altitude regions.