On the Definition of the Boundary Layer in Complex, Mountainous Terrain

According to Stull's textbook, the atmospheric boundary layer (ABL) is defined ‘as that part of the troposphere that is directly influenced by the presence of the earth's surface, and responds to surface forcing with a timescale of about an hour or less'. This definition tacitly assumes flat and horizontally homogeneous surface conditions, but nevertheless has produced some debate concerning how to determine the boundary layer height. Over complex, mountainous terrain the situation is even more complicated, in as processes of different character (turbulent exchange, meso- and submeso-scale flows) contribute to how the atmosphere responds (‘surface forcing') to the character of the surface. Under daytime conditions and depending on the chosen diagnostic largely different ABL heights result and these are found (from idealized modeling) to depend on terrain geometry. Even more, atmospheric layers diagnosed to be ‘above the ABL' can still exhibit substantial turbulence. Under stable conditions, on the other hand, different elements of terrain (such as a valley basin, the slopes, the ridges) respond quite differently to the surface forcing and therefore give rise to an ‘unconventional' ABL structure when following the traditional definitions. What we call the i-Box project is a combination of multi-site, long-term observations in the Inn Valley with (very) high-resolution numerical modeling in order to understand exchange processes between the boundary layer and the free troposphere in complex mountainous terrain. In this contribution we use i-Box results and observations to work out potential problems with the ABL definition and diagnostics in such terrain and offer a number of suggestions on how to address the open questions.