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R. A. Antonia
and
A. J. Chambers

Abstract

Measurements of velocity and scalar fluctuations were made using a stable platform in Bass Strait at a height of ∼5 m above the mean surface of the ocean. These measurements were obtained over a period of time where the wind velocity increased steadily before reaching an approximately constant value that was maintained for a duration of ∼10 h. During the initial period of the experiment, induced wave fluctuations, centered about the dominant wave frequency, are observed an spectra of the longitudinal u and vertical w velocity fluctuations, and on the uw cross spectrum. The cross spectrum indicates a relatively important transfer of momentum in the direction sea to air, at the wave frequency. No measurable wave influence is detected on either temperature θ or humidity q spectra, but wθ and wq cross spectra are either negligible or show a change of sign at the wave frequency. Although the effect on w persists, wave-induced disturbances on the cross spectra are small enough to be neglected when the ratio of wave phase speed to friction velocity becomes smaller than ∼40.

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A. J. Chambers
and
R. A. Antonia

Abstract

The influence of surface waves on the Reynolds shear stress and the heat flux, measured in the atmospheric surface layer ∼5 m above the ocean, is discussed using the method of Lu and Willmarth (1973). During the observational period, the phase velocity C associated with the dominant wave frequency is 33–80 times larger than the friction velocity u *. When contributions to the momentum flux − uw are sorted out into the four quadrants of the (u,w) plane, the contribution from the interaction quadrants (u>0, w>0; u<0, w<0) increases as C/u * increases. The contributions to the heat flux are not appreciably affected by C/u *. While the probability that sweep and ejection quadrants associated with instantaneous shear-stress and heat-flux fluctuations occur at the same time is large and approximately independent of C/u *, the probability of simultaneous occurrence of interaction events increases as C/u * increases. The period between ejection events also increases with C/u *.

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R. A. Antonia
,
S. Rajagopalan
, and
A. J. Chambers

Abstract

Conditional sampling and averaging techniques are used to obtain statistics of convectively-driven quasi-ordered structures at a height of 4 m within the atmospheric surface layer. The fraction of time 'y occupiedby these structures, and their frequency of occurrence I can depend on detection criteria parameters, suchas the threshold and hold time. The effect of these parameters on 'y and f is investigated for two conditionalsampling techniques. Both techniques indicate that y decreases continuously with increasing threshold,whereas there is a region in which I is independent of both parameters. When the parameters are suitablyselected, reasonable agreement for both 'y and f can be obtained between the techniques. This agreementdoes not depend on whether the velocity or the temperature fluctuation is used as the basis of detection forone of the techniques.

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R. A. Antonia
,
A. J. Chambers
,
S. Rajagopalan
,
K. R. Sreenivasan
, and
C. A. Friehe

Abstract

Measurements of turbulent momentum, heat and moisture fluxes have been made in Bass Strait from a stable platform, at a height of approximately 5 m above water. Direct measurements of these fluxes are compared with estimates obtained from spectra of velocity, temperature and humidity fluctuations with the use of the inertial dissipation technique. Directly measured momentum and moisture flux values are in reasonable agreement with inertial dissipation values. The sensible heal flux obtained by the inertial dissipation technique is about twice as large as the directly measured heat flux. The dependence on wind speed of bulk transfer coefficients of momentum, heat and moisture and of variances of velocity and scalar fluctuations is discussed and compared with available data.

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R. A. Antonia
,
A. J. Chambers
,
C. A. Friehe
, and
C. W. Van Atta

Abstract

A review of the evidence for the organized temperature structure observed in both the atmospheric surface layer and the laboratory boundary layer reveals similar features between the two turbulent flows. This similarity suggests that the atmospheric temperature ramp may be interpreted as the signature of an organized large-scale motion rather than a necessary consequence of the presence of buoyant plumes. An experiment was conducted in which the translation velocity Ut of the sharp edge of the temperature ramp is determined from the transit time of the ramp between two thermistors placed at the same height in the marine surface layer but separated in a direction parallel to the wind. Ut was found to be in more nearly constant ratio to the local velocity than to the friction velocity. Velocities determined from the phase angle of the temperature cross spectrum and from the optimum temperature cross correlation obtained from the two thermistors are in reasonable agreement with Ut . Cross correlations of temperature signals from thermistors separated in either vertical or lateral directions are briefly discussed in the context of the spatial geometry of the organized temperature structure.

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