Stable Planetary Boundary Layer Research Articles

Large-Eddy Simulation Of The Stably Stratified Planetary Boundary Layer

In this work, we study the characteristics of a stably stratifiedatmospheric boundary layer using large-eddy simulation (LES).In order to simulate the stable planetary boundary layer, wedeveloped a modified version of the two-part subgrid-scalemodel of Sullivan et al. This improved version of themodel is used to simulate a highly cooled yet fairly windy stableboundary layer with a surface heat flux of(Wθ)o = -0.05 m K s-1and a geostrophic wind speed of Ug = 15 m s-1.Flow visualization and evaluation of the turbulencestatistics from this case reveal the development ofa continuously turbulent boundary layer with small-scalestructures. The stability of the boundary layercoupled with the presence of a strong capping inversionresults in the development of a dominant gravity wave atthe top of the stable boundary layer that appears to be relatedto the most unstable wave predicted by the Taylor–Goldsteinequation. As a result of the decay of turbulence aloft,a strong-low level jet forms above the boundary layer.The time dependent behaviour of the jet is compared with Blackadar'sinertial oscillation analysis.

Turbulence parameterisation for PBL dispersion models in all stability conditions

Accounting for the current knowledge of the planetary boundary layer (PBL) structure and characteristics, a new set of turbulence parameterisations to be used in atmospheric dispersion models has been derived. That is, expressions for the vertical profiles of the Lagrangian length scale l i and time scale T i and diffusion coefficient K i , i=u, v, w , are proposed. The classical statistical diffusion theory, the observed spectral properties and observed characteristics of energy containing eddies are used to estimate these parameters. The results of this new method are shown to agree with previously determined parameterisations. In addition, these parameterisations give continuous values for the PBL at all elevations (z 0⩽z⩽h, z i) and all stability conditions from unstable to stable, where h and z i are the turbulent heights in stable or neutral and convective PBL, respectively, and L is the Monin–Obukhov length. It is the aim of this work to present the general derivations of these expressions and to show how they compare to previous results. Finally, a validation of the present parameterisation applied in a Lagrangian particle model, will be shown. The Copenhagen data set is simulated. This data set is particularly suited for this validation, since most of the Copenhagen tracer experiments were performed in stability conditions that are the result of the relative combination of wind shear and buoyancy forces. As a consequence, a parameterisation scheme, able to deal contemporary with neutral and slightly convective condition, is to be preferred.

Summary of the Lövånger International Workshop on Turbulence and Diffusion in the Stable Planetary Boundary Layer

A workshop on the stable planetary boundary layer (PBL) was held on 21–24 October, 1997 at Lovanger, a small town about 80 km north of Umea, Sweden. Thirty-five scientists representing eight countries participated in the meeting, which was arranged by the U.S. Army Research Office, the Swedish Defence Research Establishment, the U.S. National Oceanic and Atmospheric Administration's Air Resources Laboratory, and the Meteorology Department of Uppsala University. Topics addressed included the very stable boundary layer, gravity wave/turbulence interactions, modeling the stable boundary layer, future observations and new measurement techniques, the role of condensation (fog) and radiative flux divergence, and atmospheric diffusion. Invited papers appear in this special issue. Workshop discussions, informal presentations, and specific recommendations are summarized. Workshop participants and organizers are presented in Appendix A.

Influence of Ocean Surface Conditions on Atmospheric Vertical Thermodynamic Structure and Deep Convection

The authors analyze the influence of Sea Surface Temperature (SST) and surface wind divergence on atmospheric thermodynamic structure and the resulting effects on the occurrence of deep convection using National Meteorological Center radiosonde data and International Satellite Cloud Climatology Program data for July 1983-July 1985. The onset of deep convection requires not only the existence of positive convective available potential energy (CAPE), but also an unstable planetary boundary layer (PBL). A stable PBL is observed to suppress deep convection even when CAPE is positive. Variations of SST have a major effect on CAPE, but surface wind divergence can also affect deep convection by changing the lapse rate in the lower troposphere and humidity in the PBL. Specifically, when SST is greater than or equal to 28 C, CAPE is always positive, and surface wind divergence does not qualitatively change the buoyancy profile above the PBL. Strong surface wind divergence, however, stabilizes the PBL so as to suppress the initiation of deep convection. In warm SST regions, CAPE is greater than 0 regardless of assumptions about condensate loading, although the pseudoadiabatic limit is more consistent with the observed deep convection than the reversible moist-adiabatic limit under these circumstances. When SST is less than 27 C, CAPE is usually negative and inhibits convection, but strong surface wind convergence can destabilize the inversion layer and moisten the PBL enough to make the atmosphere neutrally stable in the mean. As a result, deep convection is generally enhanced either when SST is greater than or equal to 28 C in the absence of strong surface wind divergence or when strong surface wind convergence occurs even if SST is less than 27 C. The anomalous suppression of deep convection in the warm area of the equatorial west Pacific lying between the intertropical convergence zone (ITCZ) and south Pacific convergence zone (SPCZ) is probably caused by dryness in the PBL and an inversion in that area. The seasonal cycles of deep convection and surface wind divergence are in phase with the maximum solar radiation and lead SST for one to three months in the central Pacific. The change of PBL relative humidity plays a critical role in the changeover to convective instability in this case. The seasonal change of deep convection and associated clouds seems not to have important effects on the seasonal change of local SST in the central Pacific.

Wave Exchange between the Ground Surface and a Boundary-Layer Critical Level

Abstract Gravity waves induced by two- and three-dimensional terrain features are examined theoretically in the planetary boundary layer (PBL) using a linear wave model that includes reabsorption at a critical level. The PBL structure is characterized by a constant Brunt-Vaisala frequency and a hyperbolic tangent wind speed profile, which can be adjusted to produce critical levels. It is found that for typical values of wind speed and thermal stratification in the stable PBL and for even mild terrain disturbances, the Reynolds stress and surface drag caused by surface-generated waves can be at least as large as those conventionally associated with surface friction. The wave drag will act on the PBL flow where wave dissipation occurs, for example, at a critical level or in regions of wave breaking. The drag over a given crosswind section of a two-dimensional ridge is about twice as great as that over a three-dimensional of approximately the same horizontal area. An entirely new result is the prediction tha...

Rational approach to the bottom shear stress in tidal planetary boundary layer flow

Abstract The friction velocity associated with the maximum bottom shear stress in neutrally stable tidal planetary boundary layer flow is presented. The directions of the bottom shear stresses for the anticlockwise and clockwise rotating components are also presented. The results are obtained by using similarity theory and are given for flow conditions in the rough, smooth and transitional smooth-to-rough turbulent regime. An approximation for the maximum bottom shear stress by disregarding the rotation of the velocity in the boundary layer as the seabed is approached is also presented.

Velocity profiles, the resistance law and the dissipation rate of mean flow kinetic energy in a neutrally and stably stratified planetary boundary layer

The logarithmic + polynomial approximation is suggested for vertical profiles of velocity components in a planetary boundary layer (PBL) at neutral and stable stratification. The resistance law functions A and B are determined on the basis of this approximation, using integral relations derived from the momentum equations, the Monin-Obukhov asymptotic formula for the wind profile in a stably stratified near-surface layer and the known expressions for the PBL depth. This result gives a realistic and convenient method for calculating the surface friction velocity and direction and the total dissipation rate of mean flow kinetic energy in terms of geostrophic velocity, buoyancy flux at the surface, the roughness parameter and the Coriolis parameter. In the course of these derivations a review is given of current views on the main problems of the neutral and stable PBL.

Switching between First-Order Closure Schemes in a Numerical Mesoscale Model

Abstract Within the numerical mesoscale model described by McNider and Pielke (1981) the eddy exchange coefficients are estimated from either a local or nonlocal scheme. Switching between these is dependent on the sign of the surface heat flux. In inhomogeneous regions this switching is shown to be sensitive to the height of the stable planetary boundary layer. By defining the boundary layer height as the height to which turbulence generated by the wind shear falls to zero, a more realistic estimate can be obtained and the model is able to simulate observed potential temperature fields.

Numerical simulation of a stable planetary boundary layer over the Argentine continental shelf

A two-dimensional planetary boundary-layer model is employed to simulate numerically observed temperature and humidity profiles of an airflow over the Argentine continental shelf. Predicted profiles satisfactorily agree with observed ones which are characterized by a stable boundary layer.

A proposed method for estimating diffusion in a freely convective boundary layer by acoustic sounding

An acoustic sounder system can be configured to provide estimates of the vertical profile of wind velocity and the dissipation rates of kinetic and potential energy. A method based on the concepts of boundary-layer similarity theory is outlined by which the characteristic velocity, w ∗ , in a freely convective boundary layer is deduced from the inversion height and a combination of any two of the mean wind speeds in the layer, the dissipation rate of turbulent kinetic energy and that of turbulent potential energy. These variables, in conjunction with the mean wind velocity and source discharge rate are used to establish the normalized heights, downwind and crosswind distances and the crosswind integrated concentrations of passive contaminants, as well as the distance to and magnitude of the maximum ground concentration of initially buoyant or momentum-driven contaminants. The normalized lateral and vertical spreading is deduced from empirically established laboratory relationships for non-buoyant participates. A preliminary analysis is presented for the feasibility of a similar application to the near-neutral and stable planetary boundary layer.

Simulation of the planetary boundary layer in a multiple-jet wind tunnel

A new method has been devised for generating turbulent shear flows with nearly independently controlled mean velocity and turbulence intensity characteristics and in particular, for simulating neutrally stable planetary boundary layer flows. The method involves the use of a small, open-circuit wind tunnel which is driven on the ejector principle by an array of 64 jets whose velocities may be individually controlled. By appropriate adjustment of these velocities, two-dimensional turbulent flows possessing virtually any desired mean velocity profile at any plane in the test section of the tunnel have been produced. With the use of simple “tripping” barriers and varying degrees of surface roughness, differing turbulence levels have been obtained for flows with virtually identical mean velocity profiles. Reasonable simulations of the flow in the planetary boundary layer over differing types of terrain have been achieved with respect to both mean velocity and turbulence characteristics.

A Comparison of the Low-Level Radiosonde and the Acoustic Echo Sounder for Monitoring Atmospheric Stability

Temperature profiles measured with a radiosonde are compared with returns from an acoustic echo sounder throughout the height range 50–700 m. In general, the sounder records indicate the temperature inversion to be somewhat lower than do the radiosonde records. Reasons for this apparent discrepancy and the advantage of the sounder, in being able to monitor continuously the inversion structure and undulations produced by wind shear, are discussed. It was proved feasible to operate the acoustic sounder in a noisy commercial district with little detrimental effect on the ability to detect atmospheric temperature structure. It is concluded from this preliminary investigation that the combination of the radiosonde and the acoustic echo sounder provides a much more valuable tool for monitoring structure in the stable planetary boundary layer than either device used alone.

Acoustic echo-sounding techniques and their application to gravity-wave, turbulence, and stability studies

This paper is a brief summary of recent experimental studies conducted by the WPL staff in order to: (a) compare acoustic echo strengths with those predicted from measured turbulence intensities and scatter theory; (b) develop optimum experimental geometries for Doppler work, and (c) use the acoustic echo-sounder as a quantitative tool in studies of gravity wave dynamics in, and immediately above, the stable planetary boundary layer. We find that the observed acoustic echo strengths are roughly an order of magnitude greater than those predicted theoretically. This discrepancy might be in part due to partial reflection although the comparison is somewhat clouded by uncertainties in our knowledge of the equipment characteristics, propagation losses, etc. Comparisons between Doppler and in situ wind measurements give confidence in the Doppler results, but further experimentation and comparisons are needed. Preliminary use of acoustic Doppler data in a case study of gravity-wave dynamics in the planetary boundary layer has yielded boundary-layer wind speed and direction profiles which give insight into the mechanisms responsible for the wave generation. The Doppler data yield estimates of the wave associated momentum fluxes (∼a few dyn cm−2) as well. The results derived from the acoustic techniques are quite encouraging, but thus far remain unsubstantiated by independent wind and flux measurements.