Computer tomographic analysis of water distribution and flow in porous media     

D. K. Cassel  J. M. Brown  G. A. Johnson 《Theoretical and Applied Climatology》1990,42(4):223-228

Summary Computer tomography (CT) is the reconstruction by computer of a tomographic plane (slice) of an object. The tomograph is developed from multiple X-ray absorption measurements (scans) made around the periphery of the object. Recent research in soil science indicates that CT, which has been used in the medical field for 17 years, may find applications in assessing the degree of uniformity, or lack thereof, of soils and other porous media, in determining the flow paths of water and solutes through soils and porous materials, and in determing the flow paths of water and solutes to roots of plants. This paper discusses the general concept of CT, some of the capabilities associated with software used to format the CT readings into the desired images, and presents some CT scan data for several draining porous media.Paper presented at the Symposium Biophysical Measurements at the American Society of Agronomy Annual Meeting, Anaheim, CA, November, 27–December 1, 1988.—The use of trade names in this publication does not imply endorsement by the North Caroline Agric. Res. Serv. of the product name nor criticism of similar ones not mentioned.With 4 Figures  相似文献   

Balloon Intercomparison Campaigns: Results of remote sensing measurements of HCl     

C. B. Farmer  B. Carli  A. Bonetti  M. Carlotti  B. M. Dinelli  H. Fast  W. F. J. Evans  N. Louisnard  C. Alamichel  W. Mankin  M. Coffey  I. G. Nolt  D. G. Murcray  A. Goldman  G. M. Stokes  D. W. Johnson  W. A. Traub  K. V. Chance  R. Zander  G. Roland  L. Delbouille 《Journal of Atmospheric Chemistry》1990,10(2):237-272

All of the techniques used to measure stratospheric HCl during the two BIC campaigns involved high resolution infrared spectroscopy. The balloon-borne instruments included five different spectrometers, three operating in the solar absorption mode and two in emission (at distinctly different wavelengths). Ground-based and aircraft correlative measurements were made close to the balloon locations, again by near-infrared spectroscopy.Within this set of results, comparisons between different techniques (absorption vs emission) viewing the same airmass (i.e., on the same gondola) were possible, as were comparisons between the same technique used on different gondolas spaced closely in time and location. The final results yield a mean profile of concentration of HC1 between 18 and 40 km altitude; an envelope of ±15% centered on this profile encompasses all of the results within one standard deviation of their individual mean values. The absolute accuracy of the final profile is estimated to be no worse than 10%. It is concluded also that the measurement techniques for HCl have reached a level of performance where a precision of 10% to 15% can be confidently expected.  相似文献   

Decadal water mass variations along 20°W in the Northeastern Atlantic Ocean     

Gregory C. Johnson  Nicolas Gruber   《Progress in Oceanography》2007,73(3-4):277

Water mass variations in the northeastern Atlantic Ocean along 20°W are analyzed with pentadal resolution over the past 15 years using data from four repeat occupations of a meridional hydrographic section running south from Iceland. The section was sampled in 1988, 1993, 1998, and 2003. The results are interpreted in the context of changes in air–sea forcing, ocean circulation, and water properties associated with the North Atlantic Oscillation (NAO). The NAO index oscillated around zero from 1984 to 1988, was strongly positive from 1989 to 1995, after which it shifted to lower positive, and occasionally negative values from 1996 to 2003. Previously published studies suggest that after the 1995–1996 shift of the NAO, the subpolar gyre largely retreated to the northwest in the northeastern Atlantic Ocean, resulting in an increasingly southeastern character of local water masses with time. Water property changes extending from the SubPolar Mode Water (SPMW) just below the seasonal pycnocline through the density range shared by Mediterranean Outflow Water and SubArctic Intermediate Water (SAIW) along 20°W are consistent with changes in wind-driven ocean circulation and air–sea heat flux associated with shifts in the NAO, especially after accounting for ocean memory. After periods of lower NAO index the SPMW is warmer, saltier, and lighter. At these same times, large increases of apparent oxygen utilization (AOU) and potential vorticity are found at the SPMW base, consistent with SPMW ventilation to lighter densities during lower NAO index periods. Deeper and denser in the water column, the cold, fresh, and dense SAIW signature within the permanent pycnocline that was most strongly present in 1993, near the culmination of a period of high NAO index, is much reduced in 1988 and 1998. In 2003, after a prolonged period of lower NAO index, increasing influence of warmer, saltier subtropical waters is clear within the permanent pycnocline. The deep penetration of the changes implies that they are caused primarily by circulation changes resulting from NAO-associated wind shifts, but changes in air–sea heat flux could also have played a role.  相似文献   

The SeaSat-A satellite scatterometer   总被引：1，自引：0，他引：1  

Grantham  W. Bracalente  E. Jones  W. Johnson  J. 《Oceanic Engineering, IEEE Journal of》1977,2(2):200-206

This paper describes the methods used to develop performance requirements and design characteristics for the microwave scatterometer (SASS) ocean-surface wind sensor on the NASA SeaSat-A satellite. Wind vector measurement requirements from the SeaSat user community such as wind speed and direction accuracy, resolution cell size, grid spacing, and swath width formed the basis for defining instrument characteristics. The resulting scatterometer is designed for 14.6 GHz using four fan beam antennas to measure wind speed and direction over a 1000-km swath width with a resolution cell size50 times 50km. Results presented show scatterometer accuracy satisfies user requirements for wind speed from 4 m/s to greater than 24 m/s for the nominal SeaSat-A orbit of 790 km altitude,108deginclination, and 0.001 eccentricity.  相似文献   

Can traditional harvesting methods for cockles be accommodated in a Special Area of Conservation?     

McLaughlin  Emma; Portig  Alex; Johnson  Mark P. 《ICES Journal of Marine Science》2007,64(2):309-317

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Temporal and spatial variability of biogenic particles fluxes during the JGOFS northeast Atlantic process studies at 47°N, 20°W     

Philip P. Newton  Richard S. Lampitt  Timothy D. Jickells  Penny King  Clare Boutle   《Deep Sea Research Part I: Oceanographic Research Papers》1994,41(11-12)

Particle fluxes to 3100 m depth at 45°50′N, 19°30′W were measured using time-series sediment traps during a 17 month period encompassing 1989 and 1990 JGOFS spring bloom process studies in the northeast Atlantic. There was a marked intra-annual variability in fluxes of mass, particulate inorganic carbon (PIC), particulate organic carbon (POC) and opal, appearing as two major flux events in each year. In 1989, the first flux event represented the settlement of spring bloom-type material, whereas the second, in autumn, was heavily enriched in mucopolysaccharides. In 1990, in contrast, the two flux events comprised spring bloom-type material and arrived at depth at different times relative to the 1989 events. The intra- and interannual variability evident for all three biogenic components was most notable for POC: (i) the autumn 1989 event supplied twice as much POC to 3100 m as the earlier spring bloom settlement—a quite unexpected observation—and (ii) the annual average POC flux in 1989 was 3–4 times more than in 1990. A synthesis of process study datasets with sediment trap data enables an evaluation of the coupling of deep fluxes with surface-water events. Spatial variability of the 1989 deep flux events is assessed by comparing the sediment trap data reported here with those from a second site 100 km away (Honjo and Manganini,Deep-Sea Research II,40, 587–607, 1993). The timing and magnitude of the 1989 spring bloom settlement was indistinguishable in the two datasets, indicating no spatial variability in flux between these sites. In contrast, the autumn 1989 flux event was barely recorded at the second site. Given the biogeochemical importance of this latter event to deep waters, most notable in terms of its contribution to POC flux, this observation of deep-water mesoscale flux variability indicates a significant problem in determining regional carbon budgets. Construction of basin-scale budgets is a central goal of JGOFS and for this to be achieved further studies of mesoscale variability of particle flux are essential.  相似文献   

Wave prediction in the Southern North Sea     

Sasithorn Aranuvachapun  John A. Johnson 《Ocean Engineering》1980,7(2):347-356

Predictions of maximum wave height H_max are made at Cromer, Happisburgh and Lowestoft on the East Anglian Coast using the formula H_max = CK_RK_S U²/g where C is a constant, K_R, K_S are refraction and shoaling cofficients, U is wind speed and g is the acceleration due to gravity. Comparisonsare made with the models of Darbyshire Draper (1963) and Bretschneider (1958) Using this wave prediction formula, an estimate of the wave climate in the southern North Sea is deduced for the gales of 2–3 January 1976.  相似文献   

Polycyclic aromatic hydrocarbons in gulf of maine sediments: Distributions and mode of transport     

Peter F. Larsen  Donald F. Gadbois  Anne C. Johnson 《Marine environmental research》1986,18(4)

Sediments from 19 stations in the Gulf of Maine were analyzed for 16 priority polycyclic aromatic hydrocarbon (PAH) compounds in 1983. Thirteen of the compounds were distributed widely with total concentrations ranging from 10–512 ppb (dry wt). These values are an order of magnitude lower than those observed in the coastal zone but higher than those on Georges Bank. PAH concentration was directly related to grain size, total organic carbon (TOC) and distance from source. PAHs appear to be accumulating in Wilkinson and Jordan Basins but not in Georges Basin. Observed PAH distributions support the conclusion that the principal transport mechanism is through the atmosphere with localized augmentation by sediment resuspension and transport from coastal embayments.  相似文献   

Equatorially trapped Rossby waves in the presence of meridionally sheared baroclinic flow in the Pacific Ocean   总被引：1，自引：0，他引：1  

D.B. Chelton  M.G. SchlaxJ.M. Lyman  G.C. Johnson 《Progress in Oceanography》2003,56(2):323-380

TOPEX/POSEIDON altimeter data are analyzed for the 8.5-year period November 1992 to May 2001 to investigate the sea surface height (SSH) and geostrophic velocity signatures of quasi-annual equatorially trapped Rossby waves in the Pacific. The latitudinal structures of SSH and both components of geostrophic velocity are found to be asymmetric about the equator across the entire Pacific with larger amplitude north of the equator. The westward phase speeds are estimated by several different methods to be in the range 0.5-0.6 m s⁻¹. These observed characteristics are inconsistent with the classical theory for first vertical, first meridional mode equatorially trapped Rossby waves, which predicts a phase speed of about 0.9 m s⁻¹ with latitudinally symmetric structures of SSH and zonal velocity and antisymmetric structure of meridional velocity. The observations are even less consistent with the latitudinal structures of SSH and geostrophic velocity components for other modes of the classical theory.The latitudinal asymmetries deduced here have also been consistently observed in past analyses of subsurface thermal data and altimeter data and have been variously attributed to sampling errors in the observational data, a superposition of multiple meridional Rossby wave modes, asymmetric forcing by the wind, and forcing by cross-equatorial southerly winds in the eastern Pacific. We propose a different mechanism to account for the observed asymmetric latitudinal structure of low-frequency equatorial Rossby waves. From the free-wave solutions of a simple 1.5-layer model, it is shown that meridional shears in the mean equatorial current system significantly alter the potential vorticity gradient in the central and eastern tropical Pacific. The observed asymmetric structures of sea surface height and geostrophic velocity components are found to be a natural consequence of the shear modification of the potential vorticity gradient. The mean currents also reduce the predicted westward phase speed of first meridional mode Rossby waves, improving consistency with the observations.  相似文献   

Regional setting of a complex backarc: New Hebrides Arc,northern Vanatu-eastern Solomon Islands     

D. P. Johnson  P. C. Maillet  R. Price 《Geo-Marine Letters》1993,13(2):82-89

New GLORIA imagery over the complex backarc area of the northern New Hebrides Arc clearly outlines the deeply faulted Jean Charcot Troughs. The troughs are 2400–3000 m deep and are not magmatically active. They appear to be fragmented older crust and are not backarc basins in the usual sense.Horst and graben structures on the Hazel Holme Fracture Zone (HHFZ) cut obliquely across the trend of the Zone, which therefore is not a simple transform. The enechelon ridges may form a relay, but Y-shaped grabens could be remnant spreading zones, formed earlier in the history of the western part of the North Fiji Basin.  相似文献