The Mountain Boundary Layer (MoBL) – do we need yet another variety?

The Atmospheric Boundary Layer (ABL) is the lowest layer of the atmosphere and as such responsible for the Earth-atmosphere interaction in the climate system. It determines – from local to global scales – the exchange of energy, mass and momentum between ‘the surface’ and ‘the atmosphere’, and hence the water cycle, the energy balance, the carbon budget (to name only a few). Due to friction and radiative processes  at the surface, the flow in the ABL is turbulent – and turbulence is often considered the defining characteristic of ABL flows. In numerical models for weather and climate, turbulence is quite generally sub-grid scale (and will remain so at least for global scale models for the coming decades), so that the ‘lower boundary condition’, i.e., ABL turbulence, needs to be parameterized.

With almost 70 percent of the Earth’s land surface being covered by hills or mountains, the question arises whether current understanding of ABL turbulence is sufficient to adequately describe the surface-atmosphere exchange over non-flat or even mountainous terrain. In this contribution, a case is therefore made for the introduction of a Mountain Boundary Layer (MoBL). The MoBL is not just another variety of the boundary layer – such as the CBL or SBL  describing convective vs. stable boundary layer states, respectively: the lowest layer of the troposphere over complex mountainous terrain cannot be assumed to be horizontally homogeneous (what poses challenges for numerical modeling)  and is forced by gravitational processes as well as interactions with meso-scale flows. With this, ABL parameterizations in numerical models – which are based on the assumption of horizontal homogeneity, i.e., the ‘Boundary-Layer Approximation’, and the dominance of turbulence, e.g., in the diagnosis of the ABL height – are not likely to perform well. Some of the salient characteristics of the probably youngest in the ‘BL family’ will be presented, which emerge from recent research on boundary layer dynamics over complex terrain. Also, the largest challenges we still face in the description, physical characterization and modeling of the MoBL will be addressed.