Boundary Layer Complexity; or Non-Canonical Boundary Layers

The planetary boundary layer is commonly in states more complex than those presented in textbooks. Furthermore, it is often desired to assign a “PBL height” which is often ill-defined. The canonical cases are the well-mixed PBL driven by strong surface heat flux, the neutral PBL driven by wind shear, and the (weakly) stable PBL with moderate downward heat flux, all without clouds. These cases don't come close to exhausting the variety found in reality. Strongly stable BLs with intermittent turbulence have received some attention. Spatial and temporal transitions and clouds are also well-known if somewhat under-studied. BLs with weak forcings may not be well-mixed even if the stratification is stable. There may be dissimilarity between profiles of temperature, humidity, wind speed and direction, and other scalars. We will present estimates of the prevalence of various PBL states on a global scale as well as locally based on observations in the U.K. We will discuss when the PBL height is well-defined and when it should be avoided, with remarks on model evaluation.