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Review: the Atmospheric Boundary Layer REVIEWS El..SEVIER Earth-Science Reviews 37 (1994) 89-134 Review: the atmospheric boundary layer J.R. Garratt CSIRO Division of Atmospheric Research, Private Bag No. 1, Mordialloc, Victoria 3195, Australia Received 28 December, 1993; revised and accepted 27 April, 1994 Abstract An overview is given of the atmospheric boundary layer (ABL) over both continental and ocean surfaces, mainly from observational and modelling perspectives. Much is known about ABL structure over homogeneous land surfaces, but relatively little so far as the following are concerned, (i) the cloud-topped ABL (over the sea predominantly); (ii) the strongly nonhomogeneous and nonstationary ABL; (iii) the ABL over complex terrain. These three categories present exciting challenges so far as improved understanding of ABL behaviour and improved representation of the ABL in numerical models of the atmosphere are concerned. 1. lntroduction factors controlling the erosion and ultimate breakdown of this inversion. 1.1. Role of the atmospheric boundary layer (b) Control and Management of Air Quality is closely associated with the transport and disper­ The atmospheric (or planetary) boundary layer sal of atmospheric pollutants, including industrial plays an important role in many fields, including plumes. Processes of concern include turbulent air pollution, agricultural meteorology, hydrology, mixing in the ABL, particularly the role of con­ aeronautical meteorology, mesoscale meteorol­ vection, photochemistry and dry and wet deposi­ ogy, weather forecasting and climate. We can tion to the surface. In this general area, research summarise just a few of the problems for which on atmospheric turbulence has a very important boundary-layer knowledge is important, as fol­ practical application, and local meteorology, in­ lows cluding the role of mesoscale circulations (sea (a) Urban Meteorology is associated with the breezes, slope winds, valley flows) and the phe­ low-level urban environment and air pollution, nomenon of decoupling of the low-level flow and including air pollution episodes involving photo­ the large scale upper flow, is also of major rele­ chemical smog and accidental releases of danger­ vance. ous gases. The dispersal of smog and low-level (c) Aeronautical Meteorology is concerned with pollutants depends strongly on meteorological boundary-layer phenomena such as low cloud, conditions. Of particular importance is informa­ low-level jets and intense wind shear leading to tion on the likely growth of the shallow mixed high intensity turbulence, particularly for aircraft layer resulting from surface heating, and on the landing and take-off. In the case of low clouds 0012-8252/94$26.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-8252(94)00032-R 90 J.R. Garratt / Earth-Science Reviews 37 (1994) 89-134 and low-level jets, factors affecting their forma­ turbulence is strongly influenced by the diurnal tion, maintenance and dissipation are of great cycle of surface heating and cooling, by the pres­ importance. ence of clouds and by horizontal variability in (d) Agricultural Meteorology and Hydrology surface properties. Neutra! flow, in which buoy­ are concerned with processes such as the dry ancy effects are absent, is readily produced in the deposition of natural gases and pollutants to wind tunnel, and may be closely approximated in crops, evaporation, dewfall and frost formation. the atmosphere in windy conditions with a com­ The last three are intimately associated with the plete cloud cover. The unstably-stratified ABL, state of the ABL, with the intensity of turbulence or convective boundary layer (CBL), occurs when and with the energy balance at the surface. strong surface heating (due to the sun) produces (e) Numerical Weather Prediction (NWP) and thermal instability or convection in the form of Climate Simulation based on dynamical models thermals and plumes, and when upside-down of the atmosphere depend on the realistic repre­ convection is generated by cloud-top radiative sentation of the Earth's surface and the major cooling. In strongly unstable conditions driven by physical processes occurring in the atmosphere. lt surface heating, the outer region of the boundary has been said (Stewart, 1979) that no general layer in particular is dominated by convective circulation model is conceptually complete with­ motions and is often referred to as the mixed out the inclusion of boundary-layer effects, and layer. In contrast, the stably-stratified ABL oc­ that no prediction model can succeed without a curs mostly (though not exclusively) at night, in sufficiently accurate inclusion of the influence of response to surface cooling by lang wave emission the boundary. The boundary layer affects both to space. The unstable ABL is characterized by a the dynamics and thermodynamics of the atmo­ near-surface superadiabatic layer, and the stable sphere. There are a variety of dynamic effects: ABL by the presence of a surface inversion. more than a half of the atmosphere's kinetic energy loss occurs in the ABL (Palmen and New­ 1.3. Boundary-layer depth ton, 1969). Boundary-layer friction produces cross-isobar flow in the lower atmosphere, whilst The top of the boundary layer in convective boundary-layer interaction permits air masses to conditions is often weil defined by the existence modify their vorticity. From the thermodynamic of a stable layer (capping inversion) into which perspective, all water vapour entering the atmo­ turbulent motions from beneath are generally sphere by evaporation from the surface must en­ unable to penetrate very far, though they may ter through the ABL. Even the oceans are strongly continually erode it, particularly where latent heat influenced by the ABL, since it is through the is released in rising elements of air. The height of boundary layer that they gain most of their mo­ this elevated stable layer is quite variable, but is mentum so influencing the oceanic circulation. generally below 2 to 3 km. The top of a convec­ tive boundary layer is weil defined in Fig. 1 by the 1. 2. Turbulence sharp decrease in aerosol concentration at a height of about 1200 m, and coincides with the From both a climate and local weather per­ base of a deep and intense subsidence invcrsion. spective, the most important ABL processes that Over deserts in mid-summer under strong surface need to be parametrised in numerical models of heating the ABL may be as much as 5 km deep, the atmosphere are vertical mixing and the for­ and even deeper in conditions of vigorous cumu­ mation, maintenance and dissipation of clouds. Ionimbus convection. In stable conditions, in con­ Those Iand-surface properties that are potentially trast to the above, the boundary layer is not so crucial to accurate climate simulation include readily identified, turbulence is much weaker than albedo, roughness, moisture content and vegeta­ in the unstable case, and consequently the depth tion cover. no more than a few hundred metres at most. At Over land in particular, the structure of ABL night over land, under clear skies and light winds, J.R. Garratt / Earth-Science Reviews 37 (1994) 89-134 91 a capping stratocumulus layer and a depth on the order of 0.5 km. Such a shallow boundary layer is also found in coastal regions when warm air flows from land over a relatively cool sea. In the trop­ ics, the mean structure is very much dependent on the season, and whether conditions are dis­ turbed (in the vicinity of the inter-tropical conver­ (a) 1000 gence zone) or undisturbed. In the former, devel­ oping cumulus clouds result in poor definition of the ABL top, whilst in undisturbed conditions, the ABL top is well defined by the trade-wind 5!Xl inversion. Under special circumstances, the ABL depth over the ocean can be comparable to that over land in the middle of the day. This can occur during intense cold-air outbreaks over the ocean, 100 200 Aerosol concentration when !arge "jumps" in temperature and humidity that identify the ABL top are particularly notice­ able, and are the result of cold, dry air flowing out from the continent over relatively warm sea. The purpose of this review paper is to provide 1500 the reader with an overview of boundary-layer structure and behaviour over land and sea (in the (b) latter case, often cloud-topped), for homoge­ 1000 neous and heterogeneous surface conditions. In addition, some aspects of boundary-layer parametrisations for use in regional and global climate models are discussed. For more intensive 500 reading, the classical texts should be consulted, e.g. Sutton (1953), Priestley (1959), Lumley and Panofsky (1964), Zilitinkevich (1970), Tennekes 0 L___ __j__:::==~-'-~---'----__J and Lumley (1972), Haugen (1973) and Brown 285 290 295 300 305 Potential temperature (K) (1974). Advanced texts dealing with specialised aspects of the ABL include Wyngaard (1980), Fig. 1. Vertical profiles of (a) aerosol concentration in arbi­ trary units and (b) potential temperature observed overland in Nieuwstadt and Van Dop (1982), Stull (1988), convective conditions (see Garratt, 1992; fig. 1.2). Sorbjan (1989) and Garratt (1992). For a general overview up to the late seventies consult McBean et al. (1979), and for a comprehensive introduc­ it may be even smaller, perhaps no more than tion to instruments and techniques for making 50-100 m and strongly influenced by internal measurements in the ABL see Lenschow (1986). wave motions. In recent times, Hess (1992) reviewed aspects of Over the open oceans, where low-level layer observations and scaling of the ABL, with atten­ cloud (stratus and stratocumulus) is prevalent, tion given to the analysis of observations based the ABL depth may be no more than a few on a framework of similarity scaling. The treat­ hundred metres and, in extratropical latitudes, ment for inhomogeneous surfaces was particu­ may have a structure quite similar to that over larly emphasised. Most recently, an up-to-date land.
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