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Search Results: 1 - 10 of 95584 matches for " Christopher W. Fairall "
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The Critical Richardson Number and Limits of Applicability of Local Similarity Theory in the Stable Boundary Layer
Andrey A. Grachev,Edgar L Andreas,Christopher W. Fairall,Peter S. Guest,P. Ola G. Persson
Physics , 2012, DOI: 10.1007/s10546-012-9771-0
Abstract: Measurements of atmospheric turbulence made over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are used to determine the limits of applicability of Monin-Obukhov similarity theory (in the local scaling formulation) in the stable atmospheric boundary layer. Based on the spectral analysis of wind velocity and air temperature fluctuations, it is shown that, when both of the gradient Richardson number, Ri, and the flux Richardson number, Rf, exceed a 'critical value' of about 0.20 - 0.25, the inertial subrange associated with the Richardson-Kolmogorov cascade dies out and vertical turbulent fluxes become small. Some small-scale turbulence survives even in this supercritical regime, but this is non-Kolmogorov turbulence, and it decays rapidly with further increasing stability. Similarity theory is based on the turbulent fluxes in the high-frequency part of the spectra that are associated with energy-containing/flux-carrying eddies. Spectral densities in this high-frequency band diminish as the Richardson-Kolmogorov energy cascade weakens; therefore, the applicability of local Monin-Obukhov similarity theory in stable conditions is limited by the inequalities Ri < Ri_cr and Rf < Rf_cr. However, it is found that Rf_cr = 0.20 - 0.25 is a primary threshold for applicability. Applying this prerequisite shows that the data follow classical Monin-Obukhov local z-less predictions after the irrelevant cases (turbulence without the Richardson-Kolmogorov cascade) have been filtered out.
Similarity theory based on the Dougherty-Ozmidov length scale
Andrey A. Grachev,Edgar L Andreas,Christopher W. Fairall,Peter S. Guest,P. Ola G. Persson
Physics , 2014, DOI: 10.1002/qj.2488
Abstract: Local similarity theory is suggested based on the Brunt-Vaisala frequency and the dissipation rate of turbulent kinetic energy instead the turbulent fluxes used in the traditional Monin-Obukhov similarity theory. Based on dimensional analysis (Pi theorem), it is shown that any properly scaled statistics of the small-scale turbulence are universal functions of a stability parameter defined as the ratio of a reference height z and the Dougherty-Ozmidov length scale which in the limit of z-less stratification is linearly proportional to the Obukhov length scale. Measurements of atmospheric turbulence made at five levels on a 20-m tower over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are used to examine the behaviour of different similarity functions in the stable boundary layer. It is found that in the framework of this approach the non-dimensional turbulent viscosity is equal to the gradient Richardson number whereas the non-dimensional turbulent thermal diffusivity is equal to the flux Richardson number. These results are a consequence of the approximate local balance between production of turbulence by the mean flow shear and viscous dissipation. The turbulence framework based on the Brunt-Vaisala frequency and the dissipation rate of turbulent kinetic energy may have practical advantages for estimating turbulence when the fluxes are not directly available.
Structure of Turbulence in Katabatic Flows below and above the Wind-Speed Maximum
Andrey A. Grachev,Laura S. Leo,Silvana Di Sabatino,Harindra J. S. Fernando,Eric R. Pardyjak,Christopher W. Fairall
Physics , 2015, DOI: 10.1007/s10546-015-0034-8
Abstract: Measurements of small-scale turbulence made over the complex-terrain atmospheric boundary layer during the MATERHORN Program are used to describe the structure of turbulence in katabatic flows. Turbulent and mean meteorological data were continuously measured at multiple levels at four towers deployed along the East lower slope (2-4 deg) of Granite Mountain. The multi-level observations made during a 30-day long MATERHORN-Fall field campaign in September-October 2012 allowed studying of temporal and spatial structure of katabatic flows in detail, and herein we report turbulence and their variations in katabatic winds. Observed vertical profiles show steep gradients near the surface, but in the layer above the slope jet the vertical variability is smaller. It is found that the vertical (normal to the slope) momentum flux and horizontal (along the slope) heat flux in a slope-following coordinate system change their sign below and above the wind maximum of a katabatic flow. The vertical momentum flux is directed downward (upward) whereas the horizontal heat flux is downslope (upslope) below (above) the wind maximum. Our study therefore suggests that the position of the jet-speed maximum can be obtained by linear interpolation between positive and negative values of the momentum flux (or the horizontal heat flux) to derive the height where flux becomes zero. It is shown that the standard deviations of all wind speed components (therefore the turbulent kinetic energy) and the dissipation rate of turbulent kinetic energy have a local minimum, whereas the standard deviation of air temperature has an absolute maximum at the height of wind-speed maximum. We report several cases where the vertical and horizontal heat fluxes are compensated. Turbulence above the wind-speed maximum is decoupled from the surface, and follows the classical local z-less predictions for stably stratified boundary layer.
Determining the sea-air flux of dimethylsulfide by eddy correlation using mass spectrometry
B. W. Blomquist,B. J. Huebert,C. W. Fairall
Atmospheric Measurement Techniques Discussions , 2009,
Abstract: Mass spectrometric measurement of DMS by atmospheric pressure ionization with an isotopically labeled standard (APIMS-ILS) is a sensitive method with sufficient bandpass for direct flux measurements by eddy correlation. Use of an isotopically labeled internal standard greatly reduces instrumental drift, improving accuracy and precision. APIMS-ILS has been used in several recent campaigns to study ocean-atmosphere gas transfer and the chemical budget of DMS in the marine boundary layer. This paper provides a comprehensive description of the method and errors associated with DMS flux measurement from ship platforms. The APIMS-ILS instrument used by most groups today has a sensitivity of 100–200 counts s 1 pptv 1, which is shown to be more than sufficient for flux measurement by eddy covariance. Mass spectral backgrounds (blanks) are determined by stripping DMS from ambient air with gold. The instrument is found to exhibit some signal loss, with a half-power frequency of ≈1 Hz, but a correction based on an empirically determined instrument response function is presented. Standard micrometeorological assumptions of steady state and horizontal uniformity are found to be appropriate for DMS flux measurement, but rapid changes in mean DMS mixing ratio serve as a warning that measured flux may not represent the true surface flux. In addition, bias in surface flux estimates arising from the flux gradient are not generally significant, but conditions of lowered inversion and high surface flux may lead to a significant difference between measured flux and true surface flux. The effects of error in motion corrections and of vertical motion within the surface layer concentration gradient are discussed and the estimated maximum error from these effects is ≤18%.
Chemical sensor resolution requirements for near-surface measurements of turbulent fluxes
M. D. Rowe, C. W. Fairall,J. A. Perlinger
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2011,
Abstract: Businger and Delany (1990) presented an approach to estimate the sensor resolution required to limit the contribution of the uncertainty in the chemical concentration measurement to uncertainty in the flux measurement to 10 % for eddy covariance, gradient, and relaxed eddy accumulation flux measurement methods. We describe an improvement to their approach to estimate required sensor resolution for the covariance method, and include disjunct eddy covariance. In addition, we provide data to support selection of a form for the dimensionless scalar standard deviation similarity function based on observations of the variance of water vapor fluctuations from recent field experiments. We also redefine the atmospheric parameter of Businger and Delany in a more convenient, dimensionless form. We introduce a "chemical parameter" based on transfer velocity parameterizations. Finally, we provide examples in which the approach is applied to measurement of carbon dioxide, dimethylsulfide, and hexachlorobenzene fluxes over water. The information provided here will be useful to plan field measurements of atmosphere-surface exchange fluxes of trace gases.
Deterministic chaos at the ocean surface: applications and interpretations
A. J. Palmer,C. W. Fairall,R. A. Kropfli
Nonlinear Processes in Geophysics (NPG) , 1998,
Abstract: Ocean surface, grazing-angle radar backscatter data from two separate experiments, one of which provided coincident time series of measured surface winds, were found to exhibit signatures of deterministic chaos. Evidence is presented that the lowest dimensional underlying dynamical system responsible for the radar backscatter chaos is that which governs the surface wind turbulence. Block-averaging time was found to be an important parameter for determining the degree of determinism in the data as measured by the correlation dimension, and by the performance of an artificial neural network in retrieving wind and stress from the radar returns, and in radar detection of an ocean internal wave. The correlation dimensions are lowered and the performance of the deterministic retrieval and detection algorithms are improved by averaging out the higher dimensional surface wave variability in the radar returns.
Water-side turbulence enhancement of ozone deposition to the ocean
C. W. Fairall,D. Helmig,L. Ganzeveld,J. Hare
Atmospheric Chemistry and Physics Discussions , 2006,
Abstract: A parameterization for the deposition velocity of an ocean-reactive atmospheric gas (such as ozone) is developed. The parameterization is based on integration of the turbulent-molecular transport equation (with a chemical source term) in the ocean. It extends previous work that only considered reactions within the oceanic molecular sublayer. The sensitivity of the ocean-side transport to reaction rate and wind forcing is examined. A more complicated case with a much more reactive thin surfactant layer is also considered. The full atmosphere-ocean deposition velocity is obtained by matching boundary conditions at the interface. For an assumed ocean reaction rate of 103 s-1, the enhancement for ozone deposition by oceanic turbulence is found to be up to a factor of three for meteorological data obtained in a recent cruise off the East Coast of the U.S.
Determination of oceanic ozone deposition by ship-borne eddy covariance flux measurements
L. Bariteau,D. Helmig,C. W. Fairall,J. E. Hare
Atmospheric Measurement Techniques Discussions , 2009,
Abstract: A fast response ozone analyzer based on the ozone-nitric oxide chemiluminescence method was integrated into the NOAA-ESRL flux system to achieve the first ship-borne, direct ozone flux measurements over the open ocean. Air was collected from an inlet at 18 m height over the ocean surface mounted to the bow-jackstaff and via a 30 m-long sampling line to the ozone instrument on the ship deck. A "puff" system was used for accurate and regular determination of the sample transport time (lag) between the inlet and the chemical analyzer. A Nafion-membrane dryer facilitated removal of fast water vapor fluctuations, which eliminated the need for quenching and density correction of the ozone signal. The sampling-analyzer system was found to have a ~0.25–0.40 s response time at a sensitivity of ~2800 counts s 1 per ppbv of ozone. Quality control and data filtering procedures for eliminating data that did not meet measurement requirements were critically evaluated. The new ozone flux system was deployed during several cruises aboard the NOAA Ship Ronald H. Brown, and evaluated using results obtained during several research cruises off the coasts of the North and South America continents.
Water-side turbulence enhancement of ozone deposition to the ocean
C. W. Fairall,D. Helmig,L. Ganzeveld,J. Hare
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2007,
Abstract: A parameterization for the deposition velocity of an ocean-reactive atmospheric gas (such as ozone) is developed. The parameterization is based on integration of the turbulent-molecular transport equation (with a chemical source term) in the ocean. It extends previous work that only considered reactions within the oceanic molecular sublayer. The sensitivity of the ocean-side transport to reaction rate and wind forcing is examined. A more complicated case with a much more reactive thin surfactant layer is also considered. The full atmosphere-ocean deposition velocity is obtained by matching boundary conditions at the interface. For an assumed ocean reaction rate of 103 s 1, the enhancement for ozone deposition by oceanic turbulence is found to be up to a factor of three for meteorological data obtained in a recent cruise off the East Coast of the U.S.
The use of the eye lens technique in deriving the age structure and life table of an Impala (Aepyceros melampus)
N. Fairall
Koedoe : African Protected Area Conservation and Science , 1969, DOI: 10.4102/koedoe.v12i1.748
Abstract: The use of the eye lens technique in deriving the age structure and life table of an Impala (Aepyceros melampus)
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