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1.
Wind and stability characteristics in the atmospheric surface boundary layer at a height,Z, less than 20 m above the sea were examined in nine oceanic investigations. The analysis lends further support to the utility of the log-linear wind-profile law in the stability region of –0.4Z/L0.9, whereL is the Monin-Obukhov length. However, it is also shown that, inasmuch as better than 90% of the measurements fall within the range of ¦Z/L¦ 0.25, and inasmuch as this correction to the drag coefficient under neutral conditions amounts to less than 10%, the familiar logarithmic wind law may be used rather than the log-linear form. A wind-stress drag coefficient,C d (=1.2×10–3 between 1.0 m Z 18.3 m), is thus recommended for general deepwater oceanic applications. The situation over shallow water, which is different, is discussed briefly.  相似文献   

2.
Potential temperature, specific humidity and wind profiles measured by radiosondes under unstable but windy conditions during FIFE in northeastern Kansas were analyzed within the framework of Monin-Obukhov similarity. Around 86% of these profiles were found to have a height range over which the similarity, formulated in terms of the Businger-Dyer functions, is valid and for which the resulting surface fluxes are in good agreement with independent measurements at ground stations. When scaled with the surface roughness z 0 = 1.05 m and the displacement height d 0 = 26.9 m, for the potential temperature this height range was 45 (±31) (z – d 0 )/z 0 104 (±54) and the comparison of the profile-derived surface fluxes with the independent measurements gave a correlation coefficient of r = 0.96. For the specific humidity these values are 42 (±29) (z – d 0 )/z 0 96 (±38) and r = 0.94. In terms of the height of the bottom of the inversion H i , in the morning hours the upper limit of (z – d 0 ) in the Monin-Obukhov layer is approximately 0.3H i , whereas for a fully developed ABL it is closer to 0.1H i . Probably, as a result of the short sampling times and perhaps also of the small gradients under the windy conditions, the exact height range of validity was difficult to establish from a mere inspection of these profiles.  相似文献   

3.
Summary Rotational energy spectra are examined in experiments with two energy and enstrophy conserving schemes defined on the semi-staggered grid. One of the schemes conserved enstrophy as defined on an equivalent staggered grid, and the other conserved enstrophy as defined on the semi-staggered grid.As predicted by the theory (Janji, 1984), the rotational energy corresponding to the scheme conserving enstrophy as defined on an equivalent staggered grid decreases faster with increasing intensity of the wave number vector than that of the scheme conserving enstrophy as defined on the semistaggered grid. Moreover, the spectrum obtained with the scheme conserving the equivalent staggered grid enstrophy resembles the observed atmospheric spectra in a large part of the admissible wave number range.With 2 Figures  相似文献   

4.
The dynamical range of global atmospheric circulations is extended to specialized parameter regions by evaluating the influence of the rotation rate () on axisymmetric, oblique, and diurnally heated moist models. In Part I, we derived the basic range of circulations by altering for moist and dry atmospheres with regular and modified surfaces. Again we find the circulations to be composed of only a few elementary forms. In axisymmetric atmospheres, the circulations consist of a single jet in the rotational midrange (*=1/2–1) and of double jets in the high range (*=2–4), together with one or two pairs of Hadley and Ferrel cells; where (*=/ E ) is the rotation rate normalized by the terrestrial value. These circulations differ from those predicted by firstorder symmetric-Hadley (SH1) theory because the moist inviscid atmosphere allows a greater nonlinearity and prefers a higher-order meridional mode. The axisymmetric circulations do, however, resemble the mean flows of the natural system — but only in low latitudes, where they underlie the quasi-Hadley (QH) element of the MOIST flows. In midlatitudes, the axisymmetric jets are stronger than the natural jets but can be reduced to them by barotropic and baroclinic instablities. Oblique atmospheres with moderate to high tilts ( P =25°–90°) have the equator-straddling Hadley cell and the four basic zonal winds predicted by the geometric theory for the solstitial-symmetric-Hadley (SSH) state: an easterly jet and a westerly tradewind in the summer hemisphere, and a westerly jet and an easterly tradewind in the winter hemisphere. The nonlinear baroclinic instability of the winter westerly produces a Ferrel cell and the same eddy fluxes as the quasi-geostrophic QG element, while the instability of the summer easterly jet produces a QG-Hadley (QGH) element with a unique, vertically bimodal eddy momentum flux. At high P and low *, the oblique atmospheres reach a limiting state having global easterlies, a pole-to-pole Hadley cell, and a warm winter pole. At low tilts P <10°, the oblique circulations have a mix of solstitial and equinoctial features. Diurnal heating variations exert a fundamental influence on the natural-Hadley (NH) circulations of slowly rotating systems, especially in the singular range where the zonal winds approach extinction. The diurnality just modifies the NH element in the upper singular range (1/45*1/16), but completely transforms it into a subsolar-antisolar Halley circulation in the lower singular range (0*<1/45). In the modified NH flows, the diurnality acts through the convection to enhance the generation of the momentum-transferring planetary waves and, thereby, changes the narrow polar jets of the nondiurnal states into broad, super-rotating currents. Circulation theory for these specialized flows remains rudimentary. It does not explain fully how the double jets and the multiple cells arise in the axisymmetric atmospheres, how the QGH element forms in the oblique atmospheres, or how waves propagate in the slowly rotating diurnal atmospheres. But eventually all theories could, in principle, be compared against planetary observation: with Mars testing the QGH elements; Jupiter, the high-range elements; Titan, the equinoctial and solstitital axisymmetric states; and Venus, the diurnally modified NH flows.  相似文献   

5.
Electrochemical modelling may provide fast order-of-magnitude estimates of energy and mass transfer in crops and partly fill the gap between rigid, expensive field experiments and oversimplified mathematical modelling. The technique comprises breaking up a flowing electrolyte in a manner analogous — but not necessarily completely similar — to the way the atmospheric flow is broken up by crops and studying the flow of ions in the electrolyte.The effects of variations in free-flow velocity, plant spacing, row spacing and orientation on velocity field, transfer coefficients and eddy diffusivities have been studied in an electrochemical model under conditions relevant to well-ventilated crops with relatively simple canopy structure. A one-dimensional analytical model is developed for the transfer at solid surfaces, with direct proportionality between transfer coefficient and eddy diffusivity. The proportionality constant has values in the model of (3.5±0.5)×10–3cm–1 at the ground and (5+-2)×10–4cm–1 (for a cylindrical probe) near canopy top. The latter is not too different from the values proposed for foliage surfaces in real canopies, viz., approximately 1×10–3 cm–1 (Philip, 1964) and 7×10–4 cm–1 (Uchijima, 1966). may therefore become a useful parameter in scaling transfer coefficients from systems with different molecular diffusion properties.A tentative extrapolation of measured data to corresponding values in air is given. The agreement between predicted values and presently available field data is encouraging although more precise field data are required for a final judgment on the validity of the model.This paper is published with the permission of the International Institute of Heat and Mass Transfer; an abbreviated version of the paper is given in the Proceedings, Intl. Seminar on Heat and Mass Transfer in the Environment of Vegetation, Dubrovnik, Aug. 26–30, 1974.  相似文献   

6.
Eighteen soil samples from central Sudan were fractionated by dry sieving ina size fraction from <45 m to >300 m while aerosols generatedfrom these soils were fractionated in the particle size range from 0.25 mto >16 m. The elemental concentrations of soil samples were determinedby energy-dispersive X-ray fluorescence, while the elemental concentrationsof generated aerosols were analysed by particle-induced X-ray emission. Theelements Al, K and Rb show a slight positive fractionation with decreasingparticle size throughout the particle size range studied. The concentrationsof Ca, Mn, Fe, Sr and Y are maximum in the small soil size fraction (<45m) and decrease for the coarse soil size fractions, while in the mineralaerosol particle sizes (0.25– > 16 m) the concentrations remainmore or less constant. The size distributions for Cr, Ti and Zr show a maximumin the particle size range 45–100 m and the concentrations of theseelements decrease sharply in the aerosol fraction down to 16 m to remainconstant in the smaller aerosol fractions.Enrichment factors for the elements were calculated relative to five referencematerials: average crustal rock, average soil, the investigated Sahara bulksoil, the finest fraction of this soil and the aerosol generated from thissoil, and using four reference elements: Al, Si, Ti and Fe. The enrichmentfactors were found to vary significantly depending on the choice of thereference material or the reference element. The enrichment factors for theSudan mineral aerosol were almost identical to those for Khartoum atmosphericaerosol but different from those for Namib mineral aerosol and Israelatmospheric aerosol following dust storms. Multivariate display methods(cluster analysis, principal component analysis and linear discriminantanalysis) were applied to the element ratios in the mineral aerosol from theSahara and Namib and this showed that these mineral aerosol can bedifferentiated into different groups. An attempt was also made to relate themineral aerosol to its parent soil through the use of these multivariatetechniques and the elemental ratios in both the mineral aerosols and the bulksoils (Namib and Sahara). It was also possible using the elemental ratios andthe multivariate display methods to associate the crustal component to themineral aerosol generated from the Sahara.  相似文献   

7.
A Large Eddy Simulation (LES) model representing the air flow within and above a plant canopy layer has been completed. Using this model, the organized structures of turbulent flow in the early developmental stages of a crop are simulated and discussed in detail.The effect of the drag due to vegetation is expressed by a term added to the three-dimensional Navier-Stokes equation averaged over the grid scale. For the formulation of sub-grid turbulence processes, the equations for the time-dependent SGS (Sub-Grid-Scale) turbulence energy equation is used, which includes the effects of dissipation (both by viscosity and leaf drag), shear production and diffusion.The organized structure of turbulent flow at the air-plant interface, obtained numerically by the model, yields its contribution to momentum transfer. The three-dimensional large eddy structures, which are composed of spanwise vortices (rolls) and streamwise vortices (ribs), are simulated near the air-plant interface. They are induced by the shear instability at inflection points of the velocity profile. The structure clearly has a life cycle. The instantaneous image of the structure is similar to those observed in the field observations of Gaoet al. (1989) and in the laboratory flume experiments of Ikeda and Ota (1992). These organized structures also account for the well known fact that the sweep motion of turbulence dominates momentum transport within and just above a plant canopy, and the motion of ejection prevails in the higher regions.  相似文献   

8.
Summary The integral aerosol optical depths (k ) at the hour of 08:20 Local Standard Time (LST), are compared with those calculated previously at 11:20 and 14:20 LST, for clear days during summer in Athens over the period 1962–1988. The mean values at 08:20 LST were consistently lower than the values at 11:20 and 14:20 LST. The influence of the vertical wind profile on the values ofk was also investigated. A comparison was made of the wind profiles at 02:00 and 14:00 LST, for days in which the 11:20 and 14:20 LST values ofk were 0.200 andk 0.350, respectively. The corresponding bulk wind shear s was also found for the period 1980–1988. The most significant results occurred with the first category of days. The resultant wind velocities from the surface to the 900 hPa level, in each hour were higher by 2–4 m·s–1 with respect to the corresponding values for the second category. At 02:00 LST the bulk wind shear showed a considerable difference (1.8) between the two categories of days in the surface to 700 hPa layer at 02:00 LST. Finally, the associated weather conditions that appear to initiate a period of low values ofk (k 0.200) at 11:20 and 14:20 LST were examined for the period 1980–1988. Fifteen such cases were identified and it was found that they all occurred after the passage of weak cold fronts.With 6 Figures  相似文献   

9.
The structure of supercritical western boundary currents is investigated using a quasi-geostrophic numerical model. The basic flow is of meridional Munk balance, and the input boundary is perturbed by the most unstable wave solution obtained from linear spatial instability calculations. Self-preserving (or equilibrium) solutions are obtained for the model runs at Re=30, 60, 90, and 120, and their energy and vorticity budgets are analyzed. In an analogy with the laboratory turbulence of wall boundary layers, the western boundary layer is divided into inner and outer layers. In the inner layer, the mean energy is dissipated via direct viscous dissipation, while in the outer layer it is converted to the eddy energy via turbulence production. The main scenario is that the mean energy is produced in the inner layer via ageostrophic pressure work divergence, and it is partly removed due to viscous action within a narrow region near the wall, defined here as viscous sub-layer. The remaining portion is converted to the eddy energy via turbulence production in the outer layer, which is in turn transported to the inner layer, then again to the viscous sub-layer where it is ultimately dissipated. In the near-wall side, the vorticity balance of the mean flow is maintained by viscous effect and Reynolds flux divergence, while in the offshore side it is maintained by beta effect and Reynolds flux divergence. The length scale of the supercritical boundary current is roughly , where LM is the Munk length, as observed from a dimensional analysis.  相似文献   

10.
Impacts of different closure schemes in the Mixed Spectral Finite-Difference model (Beljaarset al., 1987) for neutrally stratified atmospheric surface-layer flow over complex terrain are studied. Six different closure schemes, (Z+z 0), Mixing Length,E–(Z+z 0),E–,E–– andq 2 l are compared. Model results for flow over an infinite series of sinusoidal ridges are examined in the context of the inner and outer layers defined by Jackson and Hunt (1975). Results are compared with rapid distortion estimates of the changes in normal stresses. The effects of streamline curvature are also examined in a qualitative sense.  相似文献   

11.
Summary Evaporation rates determined by energy balance and bulk transfer equations and confirmed with soil moisture sampling was regressed against average daytime temperature, vapour pressure deficit and wind speed over several chinook events between 1986 and 1988. The equationE = 0.45 exp (0.35 + 0.025 — 0.133 [e * —e]) yielded reasonable estimates of evaporation with surface soil moisture between saturation and near wilting point.E is evaporation in mmd –1,T, u ande * ande are temperature (°C), wind speed (ms–1), saturation vapour pressure and vapour pressure (mb) respectively. The overbar denotes daytime average.With 8 Figures  相似文献   

12.
The vertical turbulent fluxes have been determined during the Atlantic Trade Wind Experiment (ATEX) both by direct and profile methods. The drag coefficient obtained from direct measurements was c D = 1.39 × 10–3. A distortion of the wind profile due to wave action could be demonstrated, this produced an increased drag coefficient estimated by the profile method. The dissipation technique using the downwind spectrum gave a lower drag coefficient of 1.26 × 10–3, probably due to non-isotropic conditions (the ratio of vertical to downwind spectrum at high frequencies scattered considerably with an average of 1 instead of 4/3).From direct measurements, the sensible heat flux showed a poor correlation with the bulk parameter product U, contrary to the heat flux obtained from profiles. It is shown that this is due to the higher frequency part of the cospectrum, say above 0.25 Hz, which contributes more than 50 % of the total flux. Determination of the heat flux from temperature fluctuations by the dissipation method would be in agreement with the direct determination only if the corresponding Kolmogoroff constant were 2.1 instead of 0.8.For the vertical flux of water vapor obtained from profiles, the bulk transfer coefficient was 1.28 × 10–3.This work was supported by the Deutsche Forschungsgemeinschaft, Schwerpunktprogramm Meeresforschung and later the Sonderforschungsbereich Meeresforschung Hamburg.  相似文献   

13.
Neutrally buoyant atmospheric surface-layer flow over a thin vertical wall has been studied using a turbulence closure scheme designed specifically to address flow problems containing high shears. The turbulent flow model consists of a general solution of the time averaged, steady state, twodimensional Navier-Stokes equations, where theE- turbulence model has been used to close the system of equations. Model output compares favorably with measurements made in both a full-scale field study and in an atmospheric wind tunnel. In the simulation of flow over a solid wall, two recirculation eddies are produced. The smallest eddy is located windward of the wall with a separation point located atx/h=–0.8, and the largest is located in the lee of the wall atx/h=5.8. Immediately downwind of the wall top, the turbulent kinetic energy, the energy dissipation rate, and the momentum flux all reach a local maximum. These peak values generally maintain their height positionz/h=1.0, but decrease progressively downwind. The turbulent viscosity is strongly modified under the influence of the wall, with a local maximum forming in the lee of the wall top, and a local minimum forming at a heightz/h=2.0 above the lee recirculation eddy. The surface momentum flux reduction due to the presence of the wall begins atx/h=–10.0. Minimum zero fluxes occur at the surface separation points, and a local peak in momentum flux is produced at the centers of each recirculation eddy. Downwind of the wall, the modeled surface flux reaches an equilibrium at roughlyx/h=30.0.  相似文献   

14.
The analytical model of finite-amplitude, quasi-geostrophic ‘free mode’ baroclinic eddies and mean zonal flows in a Cartesian channel, presented recently by Read, is extended to take account of vertical variations in the buoyancy frequency N. A series of exact solutions is presented to illustrate the effect of monotonically varying static stability on the structure and properties of the flow. The analytical solutions are then compared with a corresponding series of numerical simulations of steady wave flows in a rotating fluid annulus subject to internal heating and sidewall cooling. By suitable choices of internal heating distributions and boundary conditions, several different forms of N2 profile could be obtained in the simulated flows, in which N2 was concentrated to a greater or lesser extent towards the upper boundary. The resulting steady flows exhibited strong qualitative similarities in their structure and dependence upon the form of N2(z) to that of the analytical solutions when realistic profiles of N2 were included in the latter, especially when an equivalent-barotropic component was included, although the latter component is unable to satisfy the simplest (internal jet) form of horizontal boundary condition as usually applied to Rossby waves.The relatively weak, though crucially important, forcing and dissipation processes in the annulus are examined using approximate quasi-geostrophic diagnostics of the major terms in the budget of potential enstrophy for the numerical simulations. Internal heating is found to be the major source of potential enstrophy for the mean zonal flow, solely by virtue of the variation of N2 with height, but has only a minor direct effect upon the eddy flow component. Because of the presence of critical layers in the flow, all non-linear terms (including the third-order potential enstrophy flux divergence) are found to be significant in certain regions. Some implications for the value and applicability of EP flux diagnostics are discussed. Potential enstrophy budgets for horizontal regions enclosed by geostrophic streamlines are used to shed further insight into the maintenance of the flow against ‘friction’, and on the form of the potential vorticity-streamfunction relationship. Some implications of the results for other systems of geophysical interest are also discussed.  相似文献   

15.
This article reviews the available data, measurement techniques, and present understanding of the millimeter thick aqueous thermal boundary layer. A temperature difference between the surface and lower strata, T, of the order of a few tenths to –1 °C have been observed. Techniques ranging from miniature mercury thermometers and electrical point sensors to optical interferometry and infrared radiometry have been employed. Many processes influence the temperature structure in this thin boundary layer. Among them are: the net upward heat flux due to evaporation and sensible heat transfer; infrared and solar radiation; and the turbulence near the interface due to wind mixing, wave breaking and current shear. Presence of solute and surface-active materials stimulate or dampen these mixing processes thereby influencing boundary-layer thickness and temperature structure.Department of Atmospheric Sciences Contribution Number 354.  相似文献   

16.
A numerical model of convective heat transfer due to isolated thermals in the atmospheric boundary layer is used to describe the temperature profile transformation in undisturbed conditions as a result of intensive dry free convection. Based on some assumptions, the heat transfer Equation (2) is transformed to the form (14) in which the coefficients and the function F are expressed by (d/dz)(ln ) and by parameters of thermals. Equation (14) has been solved numerically with the help of Equation (15) obtained from the statics equation because of Equation (8). The size distribution function f(z, r, t) of the thermals is discrete (Table I), according to Vulf'son (1961). On Figures 1 and 2 are plotted successive temperature profiles for a ground inversion, transformed due to free convection and turbulence (Figures 1a and 2a), and due to turbulence only (Figures 1b and 2b). The profiles are computed from Equation 14 (Figures 1a and 2a) and Equation 16 (Figures 1b and 2b) for k z= 1 m2 s–1 (Figure 1) and k z= 10 m2 s–1 (Figure 2). On Figure 3 the real temperature profiles in Sofia for June 22nd 1976 are compared with the profiles computed using the real initial profile for 4.30 h local time. Good qualitative agreement can be seen.  相似文献   

17.
The interpretation of ultra-high resolution radar observations of thin clear-air echo strata is made with the aid of fine-scale aircraft measurements. The echo layer, generally comprising two sub-strata each 5 m thick and spaced 7–10 m apart, is found within a 10–20 m deep section of a strong inversion where the thermal stability and shear are maximized, and the Richardson number is close to 0.25. Mechanical turbulence is restricted entirely to this layer; the variance of the N-S velocity component, 3, is the strongest, consistent with the orientation of the shear vector in this stratum. Spectra and cospectra of a 9-s slant run through the echo stratum show remarkably ordered motions. A strong negative peak of <w> covariance at 80-m scale, accompanied by a zero of <uw> covariance and bulges in the longitudinal () and vertical (w) velocity spectra, is identified with breaking Kelvin-Helmholtz waves oriented in the N-S direction along the shear vector. A synthesis of the temperature and velocity structures from measurements along the flight path confirms the ordered motion deduced from the spectra and reveals a group of K-H waves of 80-m length and 10-m height at the height of the radar echo. Microscale K-H ripples of 3–4 m length are also deduced to be present in the 0.5 m thick interfacial region where the thermal gradient and shear are strongly enhanced by the larger shearing K-H wave.Two possible sources of the echoes are proposed: (1) scatter from fully developed turbulence within the interfacial zone in an inertial subrange falling entirely in sub-meter scales; and (2) the incoherent summation of specular reflections from properly oriented portions of the microscale K-H ripples. While the authors favor the first of these mechanisms, both require stringent conditions of the physical microstructure which are beyond the available observations. Fossil turbulence is precluded as an echo mechanism.This paper is based in part on the doctoral dissertation by the senior author.Present affiliation: Air Force Cambridge Research Laboratories, Bedford, Mass., U.S.A.  相似文献   

18.
This paper is written to report observations of the structure of the atmospheric surface layer over a coastal industrialized equatorial area. The observations were recorded at Prai Industrial Park, Penang (5° 22′ N, 100° 23′ E) a relatively simple terrain area during the south-west monsoon season in the period of three months using slow response systems. The limitations of the instruments used and its effects on the results are discussed. Wind turbulence and temperature were measured on a 10 m tower and analyzed using eddy correlation method and Monin–Obukhov similarity relations to obtain the normalized standard deviation of longitudinal (σu/u), lateral (σv/u) and vertical wind velocity fluctuations (σw/u) with respect to stability parameter z/L. From the results of the analysis, we found that most of turbulence is generated by shear or mechanical force. It was found that the average neutral value of σu/u is 2.35, 1.98 for σv/u and 1.47 for σw/u with a significantly lower than the proportionality to the power of 1/3 during unstable atmospheric conditions, and thus do not obey Monin–Obukhov similarity theory. It was observed that σu/u and σv/u values increase linearly in the range of 0 < z/L < 2 and fairly well correlated while σw/u does not.  相似文献   

19.
Stability correction functions which combine the exponent of z/L, and a linear term in z/L, are proposed for the unstable case. The functions provide a reasonably close fit to the m and h results of Dyer and Hicks (1970) for 0 < –z/L 1, but they cannot be extended to cases of strong instability. Attractive features are the ability to integrate the expressions directly in terms of z/L, and a particularly close fit of the integrated result to experimentally derived m values.  相似文献   

20.
The growth of a two-dimensional internal boundary layer (IBL), which develops when a neutral or unstably stratified flow over a uniform terrain encounters a step-change in surface roughness, is numerically investigated by a higher-order turbulence closure theory. It is found that the thickness of the IBL increases as ~ x n, where x is the downstream distance from the roughness-transition line. For a given set of upstream conditions, the value of the exponent n depends only on the Monin-Obukhov length L, and it is approximately independent of the roughness-change parameter M = In (z01/z02). At large fetches, increases markedly with increasing instability.NRC-NAS Resident Research Associate at AFCRL, 1973–74  相似文献   

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