7.2 Dispersion in Atmospheric Convective Boundary Layer with Wind Shears: from Laboratory Models to Complex Simulation Studies

Convective boundary layers (CBLs) driven by buoyancy forcings from the bottom or/and from the top and capped by temperature (density) inversions are commonly observed in the lower portion of earth’s atmosphere (Holtslag and Duynkerke 1998). During fairweather daytime conditions, the buoyancy forcing in the boundary layer is primarily represented by convective heat transfer from a warm underlying surface. Such buoyancy forcing generates upand downward motions that effectively mix momentum and scalar fields inside the CBL. Due to active mixing, the wind velocity, potential temperature (buoyancy), and concentrations of atmospheric constituents in the main portion of the CBL (often referred to as convectively mixed layer) do not change considerably with height when averaged over horizontal planes or over time. A typical CBL can be subdivided into three separate layers: the surface layer, in which the meteorological variables change fairly rapidly with height; the mixed layer, where mean vertical gradients of these variables close to zero; and the entrainment zone (also referred to as the inversion layer or interfacial layer), where again the large gradients in meteorological fields are observed. Across the entrainment zone, the freeatmosphere air, which is more buoyant than the CBL air, is entrained into the convectively mixed layer as the CBL grows. Such convective entrainment is maintained by the penetration of the thermals into the stably stratified atmosphere above the CBL and subsequent folding of more buoyant air from aloft into the CBL as these overshooting thermals sink back into the mixed layer. Dispersion patterns in the atmospheric CBL are strongly variable both in time and in space. However, in the meteorological boundary-layer studies, two CBL types different with respect to their spatial-temporal evolution are usually considered. The first CBL type (we will call it the non-steady CBL) is (or assumed to be) statistically quasi-homogeneous over a horizontal plane. In this case, the CBL evolution is regarded as a nonstationary process. Most numerical and laboratory CBL studies reported in the literature have been carried out under the assumption of horizontal homogeneity of the layer. Available field measurement data on dispersion of passive constituents in the CBL usually also refer to this CBL type. Another commonly studied case of the atmospheric CBL is the horizontally evolving CBL, which grows in a neutrally or stably stratified air mass that is advected over a heated underlying surface. This type of the CBL (we will call it the heterogeneous CBL) is a traditional subject of wind-tunnel model studies. Based on the Taylor hypothesis (Willis and Deardorff 1976b), it is generally possible to relate temporal and spatial scales of dispersive turbulent motions in the non-steady and heterogeneous CBLs (Fedorovich et al. 1996). The turbulence structure and characteristics of dispersion in the atmospheric CBL have been rather thoroughly investigated for the case of non-steady CBL without wind shears (hereafter referred to as the case of shear-free CBL). Just a few studies have been devoted to the investigation of effects produced by additional non-convective (or non-buoyant) forcings that contribute to the CBL turbulence regime in conjunction with the dominant buoyant driving mechanism. Wind shear is an example of such forcing. In the developed CBL, the wind (momentum) field inside the layer is well mixed by convective motions and, as explained in Garratt et al. (1982), the flow regions with strong mean wind gradients (shears) are usually located at the surface (surface shear) and at the level of inversion (elevated shear). Erich Plate was one of pioneers of wind tunnel studies of flow and dispersion in CBLs affected by wind shears. Laboratory experiments, conducted by him and his colleagues in the thermally stratified wind tunnel of the University of Karlsruhe (Germany) in the 19801990s, considerably contributed to our present understanding of dispersion processes in the sheared CBL and constituted an essential complementation to famous laboratory studies of dispersion in the shear-free water-tank CBL by Deardorff and Willis (1982, 1984) and Willis and Deardorff (1976a, 1978, 1981, 1983, 1987). In the present paper, the growing complexity of investigated CBL dispersion phenomena and applied model approaches will be discussed in a historical retrospective. Recent results obtained in both nonsteady and heterogeneous CBLs will be presented. The emphasis will be laid on the dispersion of non-buoyant plume of gaseous tracer emitted from a point source located at different elevations within the CBL. ————————————————————————— * Corresponding author address: Evgeni Fedorovich, School of Meteorology, University of Oklahoma, 100 East Boyd, Norman, OK 73019-1013; e-mail: [email protected] 2. LABORATORY MODELS OF CBL FLOWS