When and where horizontal advection is critical to alter atmospheric boundary layer dynamics over land: The need for a conceptual framework

The thermodynamic properties of the atmospheric boundary layer (ABL) play an important role in several atmospheric processes such as convection initiation, turbulence mixing, the exchange of heat and momentum, and cloud-microphysics. Since the ABL depth (henceforth, BLD) defines the volume of the ABL, many studies consider BLD to be a key scaling parameter to understand and quantify ABL mixing processes. However, most of these studies attributed both BLD temporal and horizontal variability on various scales solely to the impact of underlying surface forcing via locally generated buoyancy fluxes and static stability. We argue that the impact of horizontal advection is often neglected yet important for a more thorough understanding of ABL thermodynamics and kinematics. Here we identified four potentially advection-dominated ABL regimes across (1) the urban-rural interface, (2) complex terrain and adjacent plains, (3) the land-sea interface where horizontal transport of marine boundary layer airmasses influences the regional ABL over coastal areas, and (4) frontal environments where mid-latitude cyclones affect ABL processes via passages of cold and warm frontal boundaries. We then introduced a conceptual framework based on observations so that ABL processes are explained not only by surface forcing but also by horizontal advection of mass, momentum, and energy. This work will help advance our understanding of ABL processes and single out potential sources that trigger drastic changes in ABL thermodynamic features including the BLDs under diverse horizontal advection environments.