The determination of turbulent structures in the atmospheric surface layer

J.L.J. Schols

    Research output: Thesisinternal PhD, WU

    Abstract

    The turbulent flow in the atmospheric surface layer (ASL) contains turbulent structures, which are defined as spatially coherent, organized flow motions. 'Organized' means that characteristic patterns, observed at a point in space, occur almost simultaneously in more than one turbulence signal and are repeated periodically.<p/>These turbulent structures play an important role in the processes of production and transport of turbulence, and they are central to understanding the mechanism of turbulence. A real life turbulent field is a superposition of effects from numerous structures of varying sizes in different stages of development. Van Atta (1977) suggested that a turbulent flow contains a hierarchy of organized motions in which smaller scale motions are superimposed on larges ones.<p/>The detection of turbulent structures forms a problem with two aspects: First, which flow properties must be used to define a turbulent structure. Second, how can be decided that on the basis of these properties, a turbulent structure is present. The VITA (variable interval time averaging) method concentrates exclusively on the intense, high frequency components of a single turbulent signal at one point in space.<p/>The application of the VITA detection method to ASL turbulence data is described in Chapter II. It works well, since the signalto-noise ratio is favorable, especially for the temperature fluctuations in the unstable ASL. The qualitative behavior of the conditional averages, using the VITA method, is not sensitive to the selection of the detection parameters, i.e. threshold level and short averaging time. The conditional averages represent ensemble-averaged values of the flow quantities inside the turbulent structures.<p/>The VITA method reveals the presence of vertically coherent turbulent structures in the ASL, which look similar to those in laboratory shear flows. At the moment when a sharp temperature interface appears, the horizontal alongwind velocity shows a sharp increase, along with a sudden decrease of vertical velocity, independent of the thermal stability conditions in the ASL. This determines a velocity interface, which is important in the turbulent transport processes as can be inferred from its relationship to the temperature interface.<p/>The turbulent structures in the ASL produce between 30 and 50 percent of the mean turbulent vertical transport of horizontal alongwind momentum and they contribute to between 40 and 50 percent of the mean turbulent vertical heat transport, both during 15 to 20 percent of the total observation time. Chapter III contains a quantitative description of those turbulent structures, which are most significant for the turbulent transport processes in the ASL. The turbulent structures are the result of the formation of longitudinal convection cells in the unstable ASL. The diameter of these cells scales on the ASL parameters u <sub>*</sub> and T <sub>*</sub> , while the translation speed of the turbulent structures scales on u <sub>*</sub> . Kline et al. (1967) found that the lateral dimensions of the turbulent structures in a laboratory shear layer scaled on the wall region parameters, viz. the wall friction velocity and the molecular kinematic viscosity. Beljaars (1979) presented a deterministic mathematical model for turbulent exchange in a laboratory boundary layer. He introduced counterrotating streamwise vortices to explain the existence of periodic instability zones in the wall region. These instabilities refer to the burst events, which are part of the turbulent structures. The burst events contain both the ejection and sweep phases of a turbulent structure.<p/>The turbulent structures in the ASL are determined by a large scale temperature field. Convective motions. which encompass the whole depth of the planetary boundary layer (PBL), penetrate into the ASL. The inclination angle of the temperature interface at the upstream edge of the turbulent structures to the surface is about 40°; that of the velocity interface, also called internal shear layer, is approximately 60°. The internal shear layer at the back of laboratory shear structures has been found to be inclined at an angle of about 18° to the wall (Thomas and Bull, 1983).<p/>Chapter IV describes the dynamical properties of the turbulent structures in the near neutral ASL. The vertical gradient of turbulent static pressure provides the link between the small scale and large scale organized motions inside the turbulent structures in the ASL: The internal shear layer, as determined by the velocity interface, carries a positive static pressure pulse. On the downstream side of the internal shear layer, at the upstream back of a turbulent structure, there exists a negative spatial turbulent static pressure gradient which causes the ejection of low speed fluid in the turbulent structure.<p/>The turbulent static pressure field contains two dominant time scales: An intense small scale, pulse shaped, fluctuation is imbedded in a large scale variation around the upstream interface of a turbulent structure in the ASL. The large scale pressure field travels at a speed, which is equal to about twice the value of the speed for the small scales. Emmerling's (1973) wall pressure data obtained in a laboratory turbulent shear layer, indicate that intense, individual, small scale pressure fluctuations travel downstream at speeds as low as about half the speed at which the large scale pressure fluctuations move as measured by e.g. Willmarth and Wooldridge (1962). The small scale wall pressure fluctuations are very intense relative to the intensity of the large scale wall pressure fluctuations.<p/>In chapter V the present results are discussed and suggestions for future research are presented.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    Supervisors/Advisors
    • Wartena, L., Promotor, External person
    • Beljaars, A.C.M., Co-promotor, External person
    Award date21 Dec 1984
    Place of PublicationWageningen
    Publisher
    Publication statusPublished - 1984

    Keywords

    • turbulence
    • boundary layer
    • land surface

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