WHITHER THE STABLE BOUNDARY LAYER? A Shift in the Research Agenda

Stable boundary layer (SBL) encapsulates a unique mix of processes that are generally much weaker and often difficult to measure at their scales of influence, study in isolation, or parameterize robustly. These processes interact in nonlinear way such that emerging new phenomena overshadow the contributing processes, and direct parameterizations of the former based on an understanding of the latter may not be viable. Viable simulation of SBL processes depends on the type of the boundary layer. When Richardson number exceeds a critical value, the SBL is sufficiently stable to suppress turbulence and confine it to isolated patchy regions of large horizontal extent and small vertical scale that are interspersed in otherwise laminar-like motions. This is called the very stable boundary layer (VSBL). In the SBL, due to damping of turbulence, only weak shear stresses exist between air layers near the ground and those aloft, and the upper layers accelerate to form a low-level jet (LLJ). Near-term challenges include gaining a deeper understanding of processes and their interactions across a wide range of scales.