Isotropic and anisotropic correlations in turbulent wind speed data     

Katja Henjes 《Boundary-Layer Meteorology》1997,84(1):149-167

The correlations in turbulent wind speed data from the open ocean are studied. They are calculated as Fourier transforms of the spectraldensities, which are computed from Fourier transforms of the measured windspeed components. All correlations are found to decay within a few seconds.It is shown that the spectra are most efficiently calculated using data sections whose length does not exceed the observed correlation decay time.Longer runs include more random noise without containing more information.The ratio of the along-wind to transverse spectral levels is investigatedand found to be in excellent agreement with the assumption of isotropy inthe inertial wave number range.  相似文献   

In situ hydrocarbon concentrations from pressurized cores in surface sediments, Northern Gulf of Mexico     

Katja U. Heeschen  Hans Jürgen Hohnberg  Matthias Haeckel  Friedrich Abegg  Manuela Drews  Gerhard Bohrmann   《Marine Chemistry》2007,107(4):498-515

Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct–Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C₁ through C₅) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg^− 1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH₄. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH₄ concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.  相似文献   

Thermodynamic mixing behavior of synthetic Ca-Tschermak–diopside pyroxene solid solutions: I. Volume and heat capacity of mixing     

Katja Etzel  Artur Benisek  Edgar Dachs  Lado Cemič 《Physics and Chemistry of Minerals》2007,34(10):733-746

The unite cell parameters and heat capacities of a series of synthetic clinopyroxenes on the join Ca-Tschermak (CaTs)−diopside (Di) were measured using X-ray powder diffraction and calorimetric methods, respectively. The volume of mixing at 298 K shows a negative asymmetric deviation from ideality. A two-parameter Margules fit to the data yields W _CaTs−Di ^V = −0.29 ± 0.11 cm³ mol⁻¹ and W _Di−CaTs ^V = −1.14 ± 0.14 cm³ mol⁻¹. Heat capacities were determined between 5 and 923 K by heat-pulse at 5−302 K and differential-scanning calorimetry at 143−923 K. The precision of the low and high temperature C _p data is better than ±1%. Polynomials of the form C _p = a + bT ^−1/2 + cT ⁻² + dT ⁻³ were fitted to the C _p data in the temperature range between 250 and 925 K. Thermal entropy values [S ₂₉₈ − S ₀] and [S ₉₀₀ −S ₀] as well as enthalpies [H ₂₉₈ − H ₀] and [H ₉₀₀ − H ₀] were calculated for all members of the solid solution series. No significant deviation from ideal mixing behavior was observed.  相似文献   

On the Problem of Sample Preservation by Freezing     

Dieter Rinne  Petra Schmitt 《洁净——土壤、空气、水》2003,31(6):501-512

During routine analysis of running water samples differences of the analytical results of water‐soluble anions were observed between analysis carried out immediately after sampling and after storage of the cooled/frozen sample. This phenomenon was investigated for some years with the “model ion” chloride. A high number of samples (ca. 210) were analysed over three years with different analytical methods, and by alteration of the sample containers the discrete physical influence of the wall effect of the sample vessel could be worked out definitely. No significant differences occur with the mean value of the year but the single values have high differences in the positive or negative direction. Assuming a similar behaviour of other water constituents when preserved by freezing the analytical values had to be looked at very critically.  相似文献   

Justification of the Inertial Dissipation Technique in Anisotropic Mean Flow     

Katja Henjes 《Boundary-Layer Meteorology》1998,88(2):161-180

The inertial dissipation technique has been successfully employed for many years to measure the wind stress, especially over the open ocean. This method is based on Kolmogorov's theoretical prediction of universality in the inertial wavenumber range. The theory was developed under the assumption of locally isotropic turbulence, and the dissipation technique has been criticized as lacking justification in a boundary-layer shear flow. In this paper, Kolmogorov's theory is explicitly applied to the anisotropic conditions prevailing in the atmosphere. It is shown that the inertial dissipation method relies on the homogeneity and isotropy of the spectrum _ii(k) for k in the inertial range. This is a weaker condition than Kolmogorov's assumption of isotropy of the correlation function B_ij(r). In high-Reynolds-number shear flows, isotropy of _ii(k) is realized to a good approximation, whereas isotropy of B_ij(r) is not. Some consequences for the experimental implementation are discussed; in particular, sampling times (block lengths) not exceeding the order of the eddy life time are recommended in the calculation of spectra.  相似文献   

A bio-engineered soft-bottom environment: The impact of Lanice conchilega on the benthic species-specific densities and community structure   总被引：2，自引：0，他引：2  

Marijn Rabaut  Katja Guilini  Gert Van Hoey  Magda Vincx  Steven Degraer 《Estuarine, Coastal and Shelf Science》2007,75(4):525-536

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Overcoming Challenges Associated with the Analysis of Nacre by Atom Probe Tomography     

Katja Eder  Laura M. Otter  Limei Yang  Dorrit E. Jacob  Julie M. Cairney 《Geostandards and Geoanalytical Research》2019,43(3):385-395

In this study atom probe tomography was used to study nacre, an important biocomposite material that is challenging to prepare and analyse by atom probe and, when successful, yields data that is challenging to interpret. It was found that these challenges mainly arise from the insulating and heterogeneous nano‐scale properties of nacre. We outline our current best practice for preparing and running atom probe tips, such as using a low acceleration voltage (< 3 kV) and current (≤ 50 pA) to avoid damage to the microstructure, and using transmission electron microscopy to confirm that the region of interest is located close to the apex of the atom probe tip. Optimisation of the preparation parameters led to several successful atom probe experiments, with one of the data sets containing part of an organic membrane and others showing organic inclusions within the reconstruction.  相似文献   

Increased record-breaking precipitation events under global warming   总被引：1，自引：0，他引：1  

Jascha Lehmann  Dim Coumou  Katja Frieler 《Climatic change》2015,132(4):501-515

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Climate simulation over CORDEX Africa domain using the fifth-generation Canadian Regional Climate Model (CRCM5)     

Leticia Hernández-Díaz  René Laprise  Laxmi Sushama  Andrey Martynov  Katja Winger  Bernard Dugas 《Climate Dynamics》2013,40(5-6):1415-1433

The new fifth-generation Regional Climate Model (CRCM5) was driven by ERA reanalyses for the period 1984–2008 over the African continent following the CORDEX experimental protocol. Overall the model succeeds in reproducing the main features of the geographical distribution and seasonal cycle of temperature and precipitation, the diurnal cycle of precipitation, and the West African Monsoon (WAM). Biases in surface temperature and precipitation are discussed in relation with some circulation defects noted in the simulation. In the African regions near the equator, the model successfully reproduces the double peak of rainfall due to the double passage of the tropical rainbelt, although it better simulates the magnitude and timing of the second peak of precipitation. CRCM5 captures the timing of the monsoon onset for the Sahel region but underestimates the magnitude of precipitation. The simulated diurnal cycle is quite well simulated for all of the regions, but is always somewhat in advance for the timing of rainfall peak. In boreal summer the CRCM5 simulation exhibits a weak cold bias over the Sahara and the maximum temperature is located too far south, resulting in a southward bias in the position of the Saharan Heat Low. The region of maximum ascent in the deep meridional circulation of the Hadley cell is well located in the CRCM5 simulation, but it is somewhat too narrow. The core of the African Easterly Jet is of the right strength and almost at the right height, but it is displayed slightly southward, as a consequence of the southward bias in the position of the Saharan Heat Low and the thermal wind relationship. These biases appear to be germane to the WAM rainfall band being narrower and not moving far enough northward, resulting in a dry bias in the Sahel.  相似文献   

On Probing the Inertial Subrange     

Katja Henjes 《Boundary-Layer Meteorology》1999,91(3):367-384

For an extensive turbulent wind speed data set collected on the open ocean, the optimum sampling time is determined to calculate inertial-range spectra. Such spectra are widely used to estimate surface fluxes. The optimum sampling time corresponds to the turbulent memory time, the minimum period after which a given time series can be interpreted as a new independent measurement. In our case, this interval is found to be only about six seconds.Separate spectral levels are calculated from the three measured vector components of the wind. Their ratios are compatible with the assumption of an isotropic inertial subrange, but scatter considerably. Alignment factors are developed based on Kolmogorov's theory, and the aligned spectral levels coincide within a few percent. It is proposed that alignment might be a more reliable test than the ratio of the spectral levels to determine whether observations have taken place in a universal inertial subrange. It is recommended that all three wind components are used to estimate the dissipation rate.  相似文献