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J. C. Kaimal and J. A. Businger

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J. C. Kaimal and J. A. Businger

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The paper describes the characteristics of a convective plume and a dust devil from measurements made at 5.66 and 22.6 m above a flat uniform site in Kansas. The velocity fluctuations were measured with a continuous-wave, three-component sonic anemometer and the temperature fluctuations with a fine platinum wire thermometer. The data show that the plume is basically a non-rotating system; it is more tilted in the downwind direction than the dust devil, travels at a lower velocity than the mean wind speed at 0.5 m, and requires vertical stretching for its maintenance in the presence of wind shear. The dust devil shows a down-draft in the middle, travels at a higher velocity than the mean wind at 32 m, and derives much of its stability from rotation. Both systems tend to transport momentum upward, against the velocity gradient, which probably accounts for the very low and sometimes negative stresses observed during unstable conditions.

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J. W. Deardorff and J. A. Businger

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Not available.

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J. C. Kaimal and J. A. Businger

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A sonic anemometer-thermometer, based on transmission of continuous sound waves and determination of phase angle variations, has been developed. The instrument is designed to measure the fluctuations in the vertical wind component and temperature simultaneously. Measurement of these two quantities enables one to determine the eddy heat flux in the turbulent boundary layer. Field tests indicate that the instrument performs satisfactorily.

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J. A. Businger and J. W. Deardorff

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J. A. Businger and P. M. Kuhn

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Simultaneous night observations with four different sensors for the long-wave atmospheric radiation were obtained with a radiosonde, specially equipped for this purpose. The sensors consisted of (1) a radiometersonde as developed by Sumoi and Kuhn, (2) a disc-type total radiometer, and (3) two spherical total radiometers (i.e., a ‘black ball’ as developed by Gergen and a blackened silver sphere).

The total radiations measured by the disc and the radiometersonde are in fairly good agreement. The total radiations measured with the two spheres showed a difference which may partly be caused by convection and conduction inside the ‘black ball’. The total radiation measured with the spherical sensors tends to be less than that of the disc in the upper part of the sounding which may be caused by the transparency of the atmosphere at those heights. The temperature difference between the air temperature and the radiative equilibrium temperature seems to be more or less related to the divergence of the net radiation.

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J. M. Wilczak and Joost A. Businger

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The energetics of the dry convective boundary layer is studied by partitioning the turbulent heat flux into thermally indirect (w′θ′<0) and thermally direct (w′θ′>0) components as a function of z/Zi. It is found that except for the inversion transition region, the thermally direct and indirect fluxes each have linear profiles. The integrated profiles indicate that a fraction A of the buoyant production of thermal kinetic energy is available to do the work of entrainment, where A is the boundary layer entrainment coefficient. A simple mixing analysis shows that this would require the integrated production of energy due to surface heating to be independent of the entrainment process. Sub-partitioning the thermally indirect flux into two components (w′<, θ′>0 and w′>0, θ′<0) reveals that an upward flux of cold air is the dominant thermally indirect term throughout the bulk of the mixed layer. Further, in the inversion transition layer the measured negative mean entrainment beat flux (w′θ′)i is principally due to a net upward flux of locally colder air, and not to a net downward flux of locally warmer air. These results are interpreted in terms of a highly idealized dome-wisp model of the entrainment mechanism.

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W. G. Large and J. A. Businger

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The DISSTRESS system for remote measurements of the surface wind stress over the ocean from ships and buoys is described. It is fully digital, utilizing the inertial dissipation technique. Parallel processing allows anemometer data to be filtered in natural frequency space; that is, the fitter cutoffs shift linearly with the mean wind speed of the data to be filtered. The construction of the digital Butterworth bandpass filters is presented in detail.

The performance of the system is evaluated by analyzing the results from 28 days of operation during FASINEX. The mean wind speed is checked, the anemometer response function is established, and drag coefficients are compared to previous studies. The capability of the system is demonstrated by continuous time series of the friction velocity computed every 20 min. The conclusion is that the surface wind stress can be measured more reliably and accurately (20%) with this system than from anemometer wind speeds and a bulk formula.

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William J. Shaw and Joost A. Businger

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Aircraft data from the JASIN Experiment have been used to examine the role that intermittency plays in turbulent transfer in the near-neutral marine atmospheric boundary layer. Conditional sampling, using the time-varying dissipation rate as an indicator, was the technique chosen for studying the dimensions of observed bursts of dissipation and their relation to the turbulent transfer. Burst fractional area coverage, γ, showed significant height variability in the surface layer, from a value of 0.45 near the surface decreasing to a constant value of about 0.30 above Z=0.2Zi. It was shown that γ is quite sensitive in the surface layer to the height of measurement and to the surface roughness (scaling with u2*/gZ), while being independent of heat flux.

The plume model of Frisch provided an estimate of the physical dimensions of the bursts. Their area varied little with height and corresponded to an average diameter of 140 m, but the number density decreased with height. The regions of high turbulence activity showed an elongation of 10% in the mean wind direction throughout the ABL.

Bursts of dissipation rate were generally coincident with regions of enhanced flux. Conditional statistics showed that 50–60% of the vertical velocity variance, stress, and water vapor fluxes were concentrated in 30% of the area over most of the ABL. The mean vertical velocity difference, Δw, between the bursts and the ambient state was found to reflect buoyant input of energy into the ABL through a dependence on the convective scaling velocity w *. This observation, the roughness height dependence of γ, and various laboratory findings suggest that plumes may be generated by the shear properties of the flow, rather than by thermal instabilities.

The turbulence kinetic energy balance showed that bursts of dissipation are also regions of enhanced turbulent transfer. In the convective case, buoyant production is concentrated in these regions. The transport of turbulence kinetic energy out of the lower ABL by the bursts actually exceeds the net transport, so that the ambient state transports turbulence kinetic energy to the surface.

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J. M. Wilczak and Joost A. Businger

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A method is developed for retrieving turbulent pressure fluctuations from tower measurements of velocity and temperature, through use of the equations of motion. This method is applied to a series of large-scale eddies which are defined by their characteristic temperature ramp structure. The variance of pressure is found to follow local free-convection.

Large-scale eddy (LSE) pressure fields are used to estimate the pressure transport and pressure-gradient interaction terms in the convective surface-layer budgets of heat flux, stress and turbulent kinetic energy. The LSE pressure terms are found to balance the budgets to within 20–30% of the size of the largest budget terms.

Ensemble fields are formed by averaging individual LSE pressure transport and pressure-gradient interaction fields. The basic characteristics of these ensemble pressure covariance fields are easily related to the cross-products of the ensemble fields of p′, ∂p′/∂x, w′, and so on. This offers a simple way of visualizing the source of the budget pressure covariances in terms of the average LSE structure.

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