On the boundary layer structure over strong topography

The ‘classical' boundary layer structure (SBL=stable boundary layer, NBL=neutral boundary layer, CBL=convective boundary layer) is largely distorted over complex, mountainous terrain. In this sense, already the definition of the PBL, i.e. its height extent, is itself called into question. . We review various definitions and compare them to results from observational studies and (high-resolution) numerical modelling over complex terrain. Most generally, , as expected, the results show that the boundary layer height is not spatially homogeneous and can sometimes be quite ambiguous. During daytime convective conditions, for example, in a steep valley the ‘Mixed Layer portion' of the CBL may stay well within a few hundred meters depth while significant TKE (turbulence kinetic energy) values reach much higher. Experimentally determining turbulence profiles, however, presently still poses considerable problems that are also briefly addressed.

The scaling regimes of the ‘classical' boundary layer, which are based on equilibrium conditions, are not likely to be found over truly complex terrain. Still, some (sparsely available) evidence suggests that with appropriate extensions generalizable results may be possible. On the basis of some examples we will focus the discussion on the basic research requirements and pathways towards scaling in complex terrain.