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C. W. Fairall

Abstract

A top-down/bottom-up diffusion Model is used to evaluate the relative contributions of humidity temperature, and their cross correlation to the radar refractive index structure function paramer Cn 2 in the cloud-free, convective planetary boundary layer (PBL). Profiles of Cn 2 can be measured continuously in the clear air with Doppler radars. Extraction of information about PBL dynamical parameters is more straightforward if Cn 2 is dominated by either moisture (Cq 2) or temperature (CT 2). In the lower part of the PBL the surface Bowen ratio β0 determines this dominance. In the upper part of the PBL the inversion Bowen ratio β1 is the primary determining factor. The model suggests that humidity accounts for at least 75% of Cn 2 over tropical and midlatitude oceans. Polar oceans, where β0 often exceeds 0.5, may not be dominated by either temperature or moisture. Over land β0 can easily vary from 0.1 to 10. Geographical regions with β0<0.3 will be dominated by Cq 2 in the lower PBL; regions with β0>5 will be dominated by CT 2. Even for large values of β0, the upper part of the PBL will often be dominated by humidity.

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C. W. Fairall

Abstract

A model for inversion layer turbulence properties of a cloud-free entraining mixed-layer with wind shear at the top is developed. Using the approach of Wyngaard and LeMone (1980), expressions for the average values of ε and σw 2 in the entrainment region are developed in terms of a mixed layer scaling velocity Wm, the entrainment velocity We and several mean profile scaling parameters including the flux Richardson numberwhere ΔSk is the inversion wind shear, Γ the lapse rate of θv above the inversion and Δθv the buoyancy jump at the inversion. Deardorff's empirical relation for We/ σw is used to close the set of equations and to obtain a parameterization for We which applies it Rf is greater than a critical value approximately equal to one-half. The wind shear enhancement of entrainment leads to an increase in the refractive index structure parameter, CN 2, in the interfacial region. This increase in CN 2 may be significant under conditions of strong geostrophic forcing combined with a low-level inversion or large baroclinic effects associated with horizontal gradients of mixed-layer temperature or inversion height.

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C. W. Fairall

Abstract

A model of scalar structure function parameters in the entraining, convective boundary layer is developed based on a top-down and bottom-up diffusion approach. The behavior of the structure function parameters is obtained from the large eddy simulations of the scalar variance budget equations given by Moeng and Wyngaard. The conventional convective scaling formalism is augmented with an additional scaling parameter, Rc, which is the ratio of the entrainment flux of the scalar variable, C, to the surface flux. The model is compared to atmospheric measurements of the structure function parameters for temperature (CT 2) and humidity (CQ 2). Two types of comparisons are done: average profiles from several well known measurement campaigns (e.g., Minnesota and AMTEX) and individual profiles from twenty soundings by a light aircraft. The model appears to fit the CQ 2 data better than it fits the CT 2 data, particularly for the average profiles. There is a tendency for the model to underestimate the structure function parameters in the upper mixed-layer, particularly for CT 2.

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J. B. Edson and C. W. Fairall

Abstract

Measurements of the momentum, heat, moisture, energy, and scalar variance fluxes are combined with dissipation estimates to investigate the behavior of marine surface layer turbulence. These measurements span a wide range of atmospheric stability conditions and provide estimates of z/L between −8 and 1. Second- and third-order velocity differences are first used to provide an estimate of the Kolmogorov constant equal to 0.53 ± 0.04. The fluxes and dissipation estimates are then used to provide Monin–Obukhov (MO) similarity relationships of the various terms in the turbulent kinetic energy (TKE) and scalar variance (SV) budgets. These relationships are formulated to have the correct limiting forms in extremely stable and convective conditions. The analyses concludes with a determination of updated dimensionless structure function parameters for use with the inertial–dissipation flux method.

The production of TKE is found to balance its dissipation in convective conditions and to exceed dissipation by up to 17% in near-neutral conditions. This imbalance is investigated using the authors’ measurements of the energy flux and results in parameterizations for the energy flux and energy transport term in the TKE budget. The form of the dimensionless energy transport and dimensionless dissipation functions are very similar to previous parameterizations. From these measurements, it is concluded that the magnitude of energy transport (a loss of energy) is larger than the pressure transport (a gain of energy) in slightly unstable conditions.

The dissipation of SV is found to closely balance production in near-neutral conditions. However, the SV budget can only be balanced in convective conditions by inclusion of the transport term. The SV transport term is derived using our estimates of the flux of SV and the derivative approach. The behavior of the derived function represents a slight loss of SV in near-neutral conditions and a gain in very unstable conditions. This finding is consistent with previous investigations.

The similarity between these functions and recent overland results further suggests that experiments are generally above the region where wave-induced fluctuations influence the flow. The authors conclude that MO similarity theory is valid in the marine surface layer as long as it is applied to turbulence statistics taken above the wave boundary layer.

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A. A. Grachev and C. W. Fairall

Abstract

Recent measurements made onboard the R/P FLIP in the San Clemente Ocean Probing Experiment in September 1993 and onboard the R/V Moana Wave during Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment are used to evaluate the direct dependence between the Monin–Obukhov stability parameter (ratio of height to Obukhov length) ζ and the bulk Richardson number Rib derived from standard meteorological mean observations of water surface temperature, wind speed, air temperature, and humidity. It is found that in the unstable marine surface layer, ζ = C Rib(1 + Rib/Ribc)−1, where the numerical coefficient C and the saturation Richardson number Ribc are analytical functions of the standard bulk exchange coefficients. For measurements at a height of 10–15 m, C is about 10 and Ribc is about −4.5. Their values are insensitive to variations of Rib and ζ over three decades. Thus, a simple dependence between ζ and Rib has a much wider range of applicability than previously believed. The authors show that this behavior is the result of the effects of “gustiness” driven by boundary layer–scale convective eddies. The derived relationship can be used as a first guess for ζ in bulk flux routines and reduces or eliminates the need for lengthy iterative solutions. Using this approximation yields errors in the latent heat flux of a few watts per square meter, compared to the full iterative solution.

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A. A. Grachev and C. W. Fairall

Abstract

This paper focuses on the study of momentum flux between ocean and atmosphere in light winds and is based on the data collected during several field campaigns, the Atlantic Stratocumulus Transition Experiment, the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment, and the San Clemente Ocean Probing Experiment. Weak wind at sea is frequently accompanied by the presence of fast-traveling ocean swell, which dramatically affects momentum transfer. It is found that the mean momentum flux (uw covariance) decreases monotonically with decreasing wind speed, and reaches zero around a wind speed U ≈ 1.5–2 m s−1, which corresponds to wave age c p/U ≈ 10 for wave/swell conditions of the experiments in this study. Further decrease of the wind speed (i.e., increase of the wave age) leads to a sign reversal of the momentum flux, implying negative drag coefficient. Upward momentum transfer is associated with fast-traveling swell running in the same direction as the wind, and this regime can be treated as swell regime or mature sea state. In the swell regime the surface stress vector is nearly opposite to wind and swell directions, and the wind is roughly aligned in the swell direction. Thus, a weak wind over ocean swell can be frequently associated with upward momentum transfer (i.e., from ocean to atmosphere).

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S. S. Zilitinkevich, A. A. Grachev, and C. W. Fairall

Abstract

The heat and mass transfer over the sea is considered in terms of the sea surface roughness lengths for scalars, z 0T for potential temperature θ, and z 0q for specific humidity q, or alternatively, in terms of the roughness-layer scalar increments, δθ and δq. A new scaling reasoning is proposed in support of the familiar square root dependence of the above increments on the roughness Reynolds number, Re0u = z 0u u∗/ν, where z 0u is the sea surface aerodynamic roughness length, u∗ is the friction velocity, and ν is the molecular viscosity of the air. Scaling predictions are validated using data from measurements made by the National Oceanic and Atmospheric Administration’s Environmental Technology Laboratory aboard the R/V Moana Wave in the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment in 1992–93 and the R/P FLIP in the San Clemente Ocean Probing Experiment in September 1993. Data presented as the dimensionless scalar increments (or the ratios z 0u/z 0T and z 0u/z 0q) versus Re0u show a good agreement with theoretical predictions, especially at Re0u > 2 (over stormy sea). The resulting roughness-length formulations are recommended for practical use in climate and mesoscale air–sea interaction models.

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C. W. Fairall, K. L. Davidson, and G. E. Schacher

Abstract

Microthermal sensors contaminated by salt aerosol droplets are subject to erroneous temperature fluctuations caused by water vapor exchange in response to fluctuations in humidity. The effect was studied by comparing the values of mean-square temperature fluctuations indicated by contaminated and clean sensors. The effect was negligible for ambient relative humidities above 85%, primarily due to the lack of humidity fluctuations. The errors were significantly diminished by frequent washing of the sensors.

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C. W. Fairall, J. E. Hare, and J. B. Snider

Abstract

As part of the First International Satellite Cloud Climatology Regional Experiment (FIRE), a surface meteorology and shortwave/longwave irradiance station was operated in a marine stratocumulus regime on the northwest tip of San Nicolas island off the coast of Southern California. Measurements were taken from March through October 1987, including a FIRE Intensive Field Operation (IFO) held in July. Algorithms were developed to use the longwave irradiance data to estimate fractional cloudiness and to use the shortwave irradiance to estimate cloud albedo and integrated cloud liquid water content. Cloud base height is estimated from computations of the lifting condensation level. The algorithms are tested against direct measurements made during the IFO; a 30% adjustment was made to the liquid water parameterization. The algorithms are then applied to the entire database. The stratocumulus clouds over the island are found to have a cloud base height of about 400 m, an integrated liquid water content of 75 gm−2, a fractional cloudiness of 0.95, and an albedo of 0.55. Integrated liquid water content rarely exceeds 350 g m−2 and albedo rarely exceeds 0.90 for stratocumulus clouds. Over the summer months, the average cloud fraction shows a maximum at sunrise of 0.74 and a minimum at sunset of 0.41. Over the same period, the average cloud albedo shows a maximum of 0.61 at sunrise and a minimum of 0.31 a few hours after local noon (although the estimate is more uncertain because of the extreme solar zenith angle). The use of joint frequency distributions of fractional cloudiness with solar transmittance or cloud base height to classify cloud types appears to be useful.

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S. E. Larsen, J. Hojstrup, and C. W. Fairall

Abstract

Hot wires respond to temperature as well as to velocity, whereas cold wires respond to velocity as well as to temperature. The static and dynamic response characteristics are summarized and it is shown that the frequency transfer functions for the four different responses in general are different. The influence of the transfer characteristics on measurements of turbulence statistics is discussed; it is shown that the nonideal response behavior influences, most strongly, statistics involving the correlation between velocity and temperature and here, most seriously, parameters involving small-scale turbulence.

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