Temporal variations in bedload transport rates associated with the migration of bedforms     

Basil Gomez  Richard L. Naff  David W. Hubbell 《地球表面变化过程与地形》1989,14(2):135-156

Temporal variations in bedload transport rates that occur at a variety of timescales, even under steady flow conditions, are accepted as an inherent component of the bedload transport process. Rarely, however, has the cause of such variations been explained clearly. We consider three data sets, obtained from laboratory experiments, that refer to measurements of bedload transport made with continuously recording bedload traps. Each data set is characterized by a predominant low-frequency oscillation, on which additional higher-frequency oscillations generally are superimposed. The period of these oscillations, as isolated through the use of spectral analysts, ranged between 0·47 and 168 minutes, and was associated unequivocally with the migration of bedforms such as ripples, dunes, and bars. The extent to which such oscillatory behaviour may be recognized in a data set depends on the duration of sampling and the length of the sampling time, with respect to the period of a given bedform. Several theoretical probability distribution functions have been developed to describe the frequency distributions of (relative) bedload transport rates that are associated with the migration of bedforms (Einstein, 1937b; Hamamori, 1962; Carey and Hubbell, 1986). These distribution functions were derived without reference to a sampling interval. We present a modification of Hamamori's (1962) probability distribution function, generated by Monte Carlo simulation, which permits one to specify the sampling interval, in relation to the length of a bedform. Comparisons between the simulated and observed frequency distributions, that were undertaken on the basis of the data described herein, are good (significant at the 90 per cent confidence level). Finally, the implications that temporal variability, which is associated with the migration of bedforms, have for the accurate determination of bedload transport rates are considered.  相似文献   

Performance of the electrochemical ozone sonde OSR     

U. Feister  K. H. Grasnick  G. Peters 《Pure and Applied Geophysics》1985,123(3):422-440

Summary The accuracy of the electrochemical ozone sonde, type OSR, has been estimated by analysing tandem ozone soundings of the balloon-borne electrochemical ozone sonde OSR at the Lindenberg Observatory from May to November 1982. A negative bias, though not significant, has been observed above about 28 km for soundings having high single correction factors. Random errors are at their minimum just above the level of the maximum of ozone partial pressure, and reach their maximum in the troposphere. Except at heights above about 28 km the random error of ozone sondes is a factor 2 to 3 times less than the error of the short Umkehr method. Provided that soundings with too high correction factors are neglected, the ozone sonde OSR has an accuracy comparable to that of other Brewer type sondes.The maximum amount of information on the vertical ozone distribution can be drawn from sonde measurements in the lower stratosphere. A study is underway to improve the sensitivity of the sonde OSR and thus to further enhance its reliability.  相似文献   

Measurements of tropospheric OH concentrations: A comparison of field data with model predictions     

D. Perner  U. Platt  M. Trainer  G. Hübler  J. Drummond  W. Junkermann  J. Rudolph  B. Schubert  A. Volz  D. H. Ehhalt  K. J. Rumpel  G. Helas 《Journal of Atmospheric Chemistry》1987,5(2):185-216

Using long path UV absorption spectroscopy we have measured OH concentrations close to the earth's surface. The OH values observed at two locations in Germany during 1980 through 1983 range from 0.7×10⁶ to 3.2×10⁶ cm^-3. Simultaneously we measured the concentrations of O₃, H₂O, NO, NO₂, CH₄, CO, and the light non methane hydrocarbons. We also determined the photolysis rates of O₃ and NO₂. This allows calculations of OH using a zero dimensional time depdendent model. The modelled OH concentrations significantly exceed the measured values for low NO _x concentrations. It is argued that additional, so far unidentified. HO _x loss reactions must be responsible for that discrepancy.  相似文献   

Aerosol concentrations and size distribution in the troposphere and lower stratosphere over Thumba     

B. H. Subbaraya  A. Jayaraman 《Pure and Applied Geophysics》1982,120(3):407-421

Rocket-borne ultraviolet photometers operating at =0.31 m have been used at Thumba (8°33N, 76°52E) to make measurements of atmospheric scattering at tropospheric and lower stratospheric altitudes. Measurements could be made of the amplitude of the scattered fluxes and the angular distribution up to an altitude of about 24 km on three rocket flights conducted as part of the 16 February 1980 solar eclipse campaign. These measurements have been used to study the size distribution as well as the number densities of aerosols in the troposphere and lower stratosphere over Thumba.  相似文献   

Transport and deposition of suspended particles in saturated porous media: hydrodynamic effect   总被引：1，自引：1，他引：1  

Nasre-Dine Ahfir  Hua Qing Wang  Ahmed Benamar  Adbellah Alem  Nicolas Massei  Jean-Paul Dupont 《Hydrogeology Journal》2007,15(4):659-668

A laboratory study was undertaken to determine the transport and deposition rate of suspended particles in columns of saturated porous media (gravel and glass beads), where the porous media were subjected to steady-state flow. Silt particles with a mode of 14 μm diameter (used as the suspended particles) and fluorescein (as the conservative tracer) were injected into the columns in short pulses. The breakthrough curves were competently described with the analytical solution of a convection–dispersion equation with a first-order deposition rate. The experiments were performed using different flow rates. The suspended particle size distribution, the porous media, and the flow rates themselves were the main factors retained in this study to investigate the mechanisms governing the transport and deposition kinetics in detail. The results showed the existence of a flow rate, beyond which suspended particles travel faster than the conservative tracer. A decrease of the deposition rate of suspended particles beyond a critical flow velocity was also observed. Such behaviour led to consideration of the couple hydrodynamic-gravity forces at high flow rates. As the hydrodynamic force increases, particle deposition rates are reduced due to the effect of hydrodynamic forces inhibiting the deposition.  相似文献   

Application of wavelet transformation to determine wavelengths and phase velocities of gravity waves observed by lidar measurements     

R. Werner  K. Stebel  G.H. Hansen  U. Blum  U.-P. Hoppe  M. Gausa  K.-H. Fricke 《Journal of Atmospheric and Solar》2007,69(17-18):2249

During the last two decades, important advances have been made in the investigation of gravity waves. However, more efforts are needed to study certain aspects of gravity waves. In the real atmosphere, gravity waves occur with different properties at different altitudes and, most often, simultaneously. In this case, when there is more than one dominant wave, the determination of gravity wave characteristics, such as the vertical wavelength and the phase velocity, is difficult. The interpretation of temperature perturbation plots versus the altitude and time as well as the application of the Fourier spectral analysis can produce errors.Exact knowledge of the wave characteristics is important both for determination of other characteristics, for example, the horizontal wave components, and for study of wave climatology. The wavelet analysis of vertical temperature profiles allows one to examine the wave's location in space. Up to now, gravity waves have been studied mainly by continuous wavelet transformation to determine dominant waves. We apply wavelet analysis to a time series of temperature profiles, observed by the ALOMAR ozone lidar at Andoya, Norway, and by the U. Bonn lidar system at ESRANGE, Sweden, both for determination of the dominant waves and for specifying the vertical wavelengths and the vertical component of the phase velocities. For this purpose, the wavelet amplitude spectra and the wavelet phase spectra are filtered and Hovmöller diagrams for dominant wavelengths are constructed. The advantage of this type of diagrams is that they give clear evidence for the localization of the dominant waves in space and time and for the development of their phase fronts.  相似文献   

Simulation of Atmospheric Boundary Layer Characteristics during Indian Summer Monsoon using Observations from Monsoon Trough Boundary Layer Experiment at Jodhpur, India     

T. V. B. P. S. Rama Krishna  Maithili Sharan 《Pure and Applied Geophysics》2007,164(8-9):1839-1859

The diurnal structure of the boundary layer during Indian summer monsoon period is studied using a one-dimensional meteorological boundary layer model and the observations collected from the Monsoon Trough Boundary Layer Experiment conducted in 1990 at Jodhpur, India. The model was initialized with the observed temperature profiles at 0530 LST on 17 July, 1990 at Jodhpur and was run for 26 hours. The study is carried out with a geostrophic wind speed of 9.5 m s⁻¹ corresponding to the strong wind simulation. The mean thermodynamic and wind structure simulated by the model are in good agreement with those observed from 30 m tower. The computed surface layer characteristics such as the surface fluxes, TKE and standard deviations of velocity components are found to be reasonably in good agreement with those based on turbulence measurements. The shear and buoyancy budget computed from the model are also compared with the turbulence measurements. The integrated cooling budget in the nocturnal boundary layer is examined.  相似文献   

An Estimate of Global Mean Sea-level Rise Inferred from Tide-gauge Measurements Using Glacial-isostatic Models Consistent with the Relative Sea-level Record   总被引：1，自引：0，他引：1  

J. M. Hagedoorn  D. Wolf  Z. Martinec 《Pure and Applied Geophysics》2007,164(4):791-818

This study is concerned with the influence of the glacial-isostatic adjustment caused by the last Pleistocene deglaciation on the present-day sea level. The viscoelastic deformation caused by the time-variable ice and ocean loads is simulated by computing the resulting perturbations for a spherical, self-gravitating, incompressible, Maxwell-viscoelastic earth model. The associated variation of the earth rotation is described in terms of the Liouville equation, which is solved by means of the MacCullagh formulae. This allows the determination of the vertical displacement and geoid height and, thus, the solution of the sea-level equation. We test several viscosity and ice models and evaluate them by comparison of the computed response with the Holocene relative sea-level record. Using the optimum combination of viscosity and ice models, we then estimate the influence of the last Pleistocene deglaciation on the tide-gauge measurements. A comparison between the observational and residual linear trends for the tide-gauge measurements shows a significant reduction of the variance and geographical variability for the latter, in particular for the formerly ice-covered regions of North America and Scandinavia. The favoured value determined for the global mean sea-level rise is (1.46±0.2) mm a⁻¹.  相似文献   

Comparison of resection–intersection algorithms and projection geometries in radiostereometry     

Niclas Brlin 《ISPRS Journal of Photogrammetry and Remote Sensing》2002,56(5-6)

Three resection–intersection algorithms were applied to simulated projections and clinical data from radiostereometric patients. On simulated data, the more advanced bundle-adjustment-based algorithms outperformed the classical Selvik algorithm, even if the error reductions were small for some parameters. On clinical data, the results were inconclusive.The two different projection geometries had a much larger influence on the error size and distribution. For the biplanar configuration, the position and motion errors were small and almost isotropic. For the uniplanar configuration, the position errors were comparably high and anisotropic, but still resulted in a high accuracy for some motion parameters at the expense of others.The simplified resection–intersection algorithm by Selvik may still be considered a good and robust algorithm for radiostereometry. More studies will have to be performed to find out how the theoretical advantages of the bundle methods can be utilized in clinical radiostereometry.  相似文献   

Energy Balance Of A Sparse Coniferous High-Latitude Forest Under Winter Conditions   总被引：1，自引：0，他引：1  

Sven-Erik Gryning  Ekaterina Batchvarova  H. A. R. De Bruin 《Boundary-Layer Meteorology》2001,99(3):465-488

Measurements carried out in Northern Finland on radiation and turbulent fluxes over a sparse, sub-arctic boreal forest with snow covered ground were analysed. The measurements represent late winter conditions characterised by low solar elevation angles. During the experiment (12–24 March 1997) day and night were about equally long. At low solar elevation angles the forest shades most of the snow surface. Therefore an important part of the radiation never reaches the snow surface but is absorbed by the forest. The sensible heat flux above the forest was fairly large, reaching more than 100 W m^-2. The measurements of sensible heat flux within and above the forest revealed that the sensible heat flux from the snow surface is negligible and the sensible heat flux above the forest stems from warming of the trees. A simple model for the surface energy balance of a sparse forest is presented. The model treats the diffuse and direct shortwave (solar) radiation separately. It introduces a factor that accounts for the shading of the ground at low solar elevation angles, and a parameter that deals with the partial transparency of the forest.Input to the model are the direct and diffuse incoming shortwave radiation.Measurements of the global radiation (direct plus diffuse incoming shortwaveradiation) above the forest revealed a considerable attenuation of the globalradiation at low solar elevation. A relation for the atmospheric turbidity asfunction of the solar elevation angle is suggested. The global radiation wassimulated for a three month period. For conditions with a cloud cover of lessthan 7 oktas good agreement between model predictions and measurementswere found. For cloud cover 7 and 8 oktas a considerable spread can beobserved. To apply the proposed energy balance model, the global radiationmust be separated into its diffuse and direct components. We propose a simpleempirical relationship between diffuse shortwave and global radiation asfunction of cloud cover.  相似文献