Chaos and “Bursting” in the Planetary Boundary Layer

Abstract

A theoretical study of the intermittent breakdowns of the planetary boundary layer (PBL) has been carried out using a one-dimensional, ?flat? terrain, PBL model of the type originally developed by A.K. Blackadar. It was found that the turbulent burst effects predominate in the lowest tens of meters of the PBL where an alternation between successive regimes of laminar and turbulent flow drive either rapid cooling or rapid heating, respectively, of the near-surface air temperature under certain conditions. This rather discontinuous change of the air temperature occurs with variable amplitudes at irregular intervals depending critically on the specified surface aerodynamic roughness length, the prevailing geostrophic wind, the magnitude of the radiation cooling with respect to the soil and its thermal properties, etc. For an aerodynamic roughness length of 1 m, collapse was found to occur within only a very narrow range of geostrophic winds (between 1.5 and 3 m s?1, with a maximum response in the air temperature changes occurring at about 2 m s?1). This sensitivity can help explain why some reports of the phenomenon are common at some locations while not nearly as common at others, depending on the strength of the prevailing geostrophic wind speed and on its direction at a given location during the year. The increased levels of ozone that are commonly found following the breakdown of the nocturnal PBL make the solution to this problem not only a fundamental one for fluid mechanics, but also one of great interest to air pollution research. The aperiodicity of the turbulent bursts and their extreme sensitivity to the initial conditions of the forcing is suggestive of the fact that the PBL can at times behave as a deterministic chaotic system.