Whither the Stable Boundary Layer?

Abstract

C learly no other part of the atmosphere is more important to Earth’s ecosystems than its lowest layer, known as the atmospheric boundary layer (ABL). The land surface exchanges heat, mass, and momentum with the free atmosphere through the ABL, and naturally the ABL is affected by orography, land use, external forcing (e.g., radiation), and Earth’s rotation. Environmental changes, whether due to slowly evolving global warming or rapidly dispersing atmospheric releases, permeate through to living organisms via the ABL. During the daytime, the ABL is driven by surface heating [the convective boundary layer (CBL)], whereas radiative cooling near the ground at night leads to the stable boundary layer (SBL). The nocturnal boundary layer (NBL) is the most common manifestation of SBL, with notable exceptions being areas where the urban heat island eliminates the near-surface stable stratification and polar regions where the SBL can persist continuously for days. The SBL breaks down into a CBL during the “morning transition,” and the CBL collapses to an SBL during the “evening transition.” Over the past half century, the progress in understanding the CBL has far outpaced the SBL; the much stronger forcing in the CBL makes measurement and modeling of turbulence therein much easier. Conversely, the SBL encapsulates a unique mix of processes that are generally much weaker (at least in total) and often difficult to measure at their scales of influence (let alone over multiple scales), 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. A greater emphasis is therefore needed on the interactions of SBL processes and the resulting modification of heat, mass, and momentum fluxes. Modeling of commonly sought meteorological and air quality indicators—surface temperature and wind speed/direction, fog, air pollution, and dispersion of chemical, biological, and radiological contaminants—relies heavily on 