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851.
Observations of the Hudson River plume were taken in the spring of 2006 in conjunction with the Lagrangian Transport and Transformation
Experiment using mooring arrays, shipboard observations, and satellite data. During this time period, the plume was subjected
to a variety of wind, buoyant, and shelf forcings, which yield vastly different responses in plume structure including a downstream
recirculating eddy. During weak and downwelling winds, the plume formed a narrow buoyant coastal current that propagated downstream
near the internal wave speed. Freshwater transport during periods when the downwelling wind was closely aligned with the coast
was near the river discharge values. During periods with a cross-shore component to the wind, freshwater transport in the
coastal current estimated by the mooring array is less than the river discharge due to a widening of the plume that leads
to the internal Rossby radius scaling for the plume width to be invalid. The offshore detachment of plume and formation of
a downstream eddy that is observed surprisingly persisted for 2 weeks under a variety of wind forcing conditions. Comparison
between mooring, shipboard, and satellite data reveal the downstream eddy is steady in time. Shipboard transects yield a freshwater
content equal to the previous 3 days of river discharge. The feature itself was formed due to a large discharge following
a strong onshore wind. The plume was then further modified by a brief upwelling wind and currents influenced by the Hudson
Shelf Valley. The duration of the detachment and downstream eddy can be explained using a Wedderburn number which is largely
consistent with the wind strength index described by Whitney and Garvine (J Geophys Res 110:C03014 1997). 相似文献
852.
Estimates of surface drifter trajectories in the equatorial Atlantic: a multi-model ensemble approach 总被引:3,自引:3,他引:0
Robert Bruce Scott Nicolas Ferry Marie Drévillon Charlie N. Barron Nicolas C. Jourdain Jean-Michel Lellouche Edward Joseph Metzger Marie-Hélène Rio Ole Martin Smedstad 《Ocean Dynamics》2012,62(7):1091-1109
We compared the estimates of surface drifter trajectories from 1 to 7?days in the equatorial Atlantic over an 18-month period with five eddying ocean general circulation model (OGCM) reanalyses and one observational product. The cumulative distribution of trajectory error was estimated using over 7,000?days of drifter trajectories. The observational product had smaller errors than any of the individual OGCM reanalyses. Three strategies for improving trajectory estimates using the ensemble of five operational ocean analysis and forecasting products were explored: two methods using a multi-model ensemble estimate and also spatial low-pass filtering. The results were insensitive to the method used to create the ensemble estimates, and by most measures, the results were better than the observational product. Comparison of relative skill of the various OGCM reanalyses suggested promising avenues for exploration for further improvements: forcing with higher frequency wind stress and quality control of input data. One of the lowest horizontal resolution OGCMs, with 1/4° longitude horizontal resolution, made the best trajectory estimates. The individual OGCMs were dominated by errors at spatial scales smaller than about 100 to 200?km, i.e., less than the local deformation radius. But buried in those errors were valuable signals that could be retrieved by combining all the OGCM velocity fields to produce a multi-model ensemble-based estimate. This estimate had skill down to spatial scales about 75?km. Results from this study are consistent with previous work showing that ensemble-mean forecast skill is superior to individual forecasts. 相似文献
853.
Limited change in dune mobility in response to a large decrease in wind power in semi-arid northern China since the 1970s 总被引:2,自引:0,他引:2
Joseph A. Mason James B. Swinehart Huayu Lu Xiaodong Miao Phueng Cha Yali Zhou 《Geomorphology》2008,102(3-4):351-363
The climatic controls on dune mobility, especially the relative importance of wind strength, remain incompletely understood. This is a key research problem in semi-arid northern China, both for interpreting past dune activity as evidence of paleoclimate and for predicting future environmental change. Potential eolian sand transport, which is approximately proportional to wind power above the threshold for sand entrainment, has decreased across much of northern China since the 1970s. Over the same period, effective moisture (ratio of precipitation to potential evapotranspiration) has not changed significantly. This “natural experiment” provides insight on the relative importance of wind power as a control on dune mobility in three dunefields of northern China (Mu Us, Otindag, and Horqin), although poorly understood and potentially large effects of human land use complicate interpretation. Dune forms in these three regions are consistent with sand transport vectors inferred from weather station data, suggesting that wind directions have remained stable and the stations adequately represent winds that shaped the dunes. The predicted effect of weaker winds since the 1970s would be dune stabilization, with lower sand transport rates allowing vegetation cover to expand. Large portions of all three dunefields remained stabilized by vegetation in the 1970s despite high wind power. Since the 1970s, trends in remotely sensed vegetation greenness and change in mobile dune area inferred from sequential Landsat images do indicate widespread dune stabilization in the eastern Mu Us region. On the other hand, expansion of active dunes took place farther west in the Mu Us dunefield and especially in the central Otindag dunefield, with little overall change in two parts of the Horqin dunes. Better ground truth is needed to validate the remote sensing analyses, but results presented here place limits on the relative importance of wind strength as a control on dune mobility in the study areas. High wind power alone does not completely destabilize these dunes. A large decrease in wind power either has little short-term effect on the dunes, or more likely its effect is sufficiently small that it is obscured by human impacts on dune stability in many parts of the study areas. 相似文献
854.
To reduce the hazards from debris flows in drainage basins burned by wildfire, erosion control measures such as construction of check dams, installation of log erosion barriers (LEBs), and spreading of straw mulch and seed are common practice. After the 2002 Missionary Ridge Fire in southwest Colorado, these measures were implemented at Knight Canyon above Lemon Dam to protect the intake structures of the dam from being filled with sediment. Hillslope erosion protection measures included LEBs at concentrations of 220–620/ha (200–600% of typical densities), straw mulch was hand spread at concentrations up to 5.6 metric tons/hectare (125% of typical densities), and seeds were hand spread at 67–84 kg/ha (150% of typical values). The mulch was carefully crimped into the soil to keep it in place. In addition, 13 check dams and 3 debris racks were installed in the main drainage channel of the basin.The technical literature shows that each mitigation method working alone, or improperly constructed or applied, was inconsistent in its ability to reduce erosion and sedimentation. At Lemon Dam, however, these methods were effective in virtually eliminating sedimentation into the reservoir, which can be attributed to a number of factors: the density of application of each mitigation method, the enhancement of methods working in concert, the quality of installation, and rehabilitation of mitigation features to extend their useful life. The check dams effectively trapped the sediment mobilized during rainstorms, and only a few cubic meters of debris traveled downchannel, where it was intercepted by debris racks.Using a debris volume-prediction model developed for use in burned basins in the Western U.S., recorded rainfall events following the Missionary Ridge Fire should have produced a debris flow of approximately 10,000 m3 at Knight Canyon. The mitigation measures, therefore, reduced the debris volume by several orders of magnitude. For comparison, rainstorm-induced debris flows occurred in two adjacent canyons at volumes within the range predicted by the model. 相似文献
855.
Joseph A. Nuth III John L. Lowrance George R. Carruthers 《Earth, Moon, and Planets》2008,102(1-4):495-504
The prime measurement objective of the Near Earth Object Chemical Analysis Mission (NEOCAM) is to obtain the ultraviolet spectra
of meteors entering the terrestrial atmosphere from ∼125 to 300 nm in meteor showers. All of the spectra will be collected
using a slitless ultraviolet spectrometer in Earth orbit. Analysis of these spectra will reveal the degree of chemical diversity
in the meteors, as observed in a single meteor shower. Such meteors are traceable to a specific parent body and we know exactly
when the meteoroids in a particular shower were released from that parent body (Asher, in: Arlt (ed.) Proc. International
Meteor Conference, 2000; Lyytinen and van Flandern, Earth Moon Planets 82–83:149–166, 2000). By observing multiple apparitions
of meteor showers we can therefore obtain quasi-stratigraphic information on an individual comet or asteroid. We might also
be able to measure systematic effects of chemical weathering in meteoroids from specific parent bodies by looking for correlations
in the depletions of the more volatile elements as a function of space exposure (Borovička et al., Icarus 174:15–30, 2005).
By observing the relation between meteor entry characteristics (such as the rate of deceleration or breakup) and chemistry
we can determine if our meteorite collection is deficient in the most volatile-rich samples. Finally, we can obtain a direct
measurement of metal deposition into the terrestrial stratosphere that may act to catalyze atmospheric chemical reactions. 相似文献
856.
Alluvial floodplain classification by multivariate clustering and discriminant analysis for low‐relief glacially conditioned river catchments 下载免费PDF全文
River classifications provide useful frameworks to understand complex fluvial landscapes and to manage freshwater ecosystems. Alluvial floodplains for rivers in low‐relief glacially conditioned catchments of southern Ontario (Canada) are classified and tested using a sequence of multivariate statistical analyses. An original dataset of 109 floodplain sites is investigated using k‐means clustering, principal component analysis, and discriminant analysis statistical approaches. Four primary floodplain types are proposed representing basic morphological, stratigraphical, and sedimentological characteristics. Classifications are successfully discriminated by two principal dimensions: (1) stream power‐resistance; and (2) floodplain sedimentology. The latter is most efficiently represented by the availability of alluvial sand, and specifically a new variable defined as floodplain sand equivalent (FSE). Floodplain types are generally consistent with previous river classifications, however the glacial legacy requires refined classifications which account for inherited cobble bed materials and patterns of sand supply. Representing the residual variability of stream power‐resistance correlations, a third explanatory dimension of sediment transport is suggested, and may explain some within‐class variability in channel morphology. Balancing the opposing concepts of fluvial process domains and landform continuums, the potential for transitional floodplain types is also explored. The proposed first‐order alluvial floodplain classifications provide a basis from which to further investigate geomorphological diversity within the context of complex glacial legacy effects in low‐relief settings. Future research to reveal the spatial arrangement and linkages of distinct morphological groups within a regional landscape mosaic is expected to provide insights into patterns of post‐glacial fluvial adjustment. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
857.
Nicolas Reul Severine Fournier Jaqueline Boutin Olga Hernandez Christophe Maes Bertrand Chapron Gaël Alory Yves Quilfen Joseph Tenerelli Simmon Morisset Yann Kerr Susanne Mecklenburg Steven Delwart 《Surveys in Geophysics》2014,35(3):681-722
While it is well known that the ocean is one of the most important component of the climate system, with a heat capacity 1,100 times greater than the atmosphere, the ocean is also the primary reservoir for freshwater transport to the atmosphere and largest component of the global water cycle. Two new satellite sensors, the ESA Soil Moisture and Ocean Salinity (SMOS) and the NASA Aquarius SAC-D missions, are now providing the first space-borne measurements of the sea surface salinity (SSS). In this paper, we present examples demonstrating how SMOS-derived SSS data are being used to better characterize key land–ocean and atmosphere–ocean interaction processes that occur within the marine hydrological cycle. In particular, SMOS with its ocean mapping capability provides observations across the world’s largest tropical ocean fresh pool regions, and we discuss from intraseasonal to interannual precipitation impacts as well as large-scale river runoff from the Amazon–Orinoco and Congo rivers and its offshore advection. Synergistic multi-satellite analyses of these new surface salinity data sets combined with sea surface temperature, dynamical height and currents from altimetry, surface wind, ocean color, rainfall estimates, and in situ observations are shown to yield new freshwater budget insight. Finally, SSS observations from the SMOS and Aquarius/SAC-D sensors are combined to examine the response of the upper ocean to tropical cyclone passage including the potential role that a freshwater-induced upper ocean barrier layer may play in modulating surface cooling and enthalpy flux in tropical cyclone track regions. 相似文献
858.
Antonio Pipino Joseph Silk Francesca Matteucci 《Monthly notices of the Royal Astronomical Society》2009,392(1):475-482
The overabundance of Mg relative to Fe, observed in the nuclei of bright ellipticals, and its increase with galactic mass, poses a serious problem for all current models of galaxy formation. Here, we improve on the one-zone chemical evolution models for elliptical galaxies by taking into account positive feedback produced in the early stages of supermassive central black hole growth. We can account for both the observed correlation and the scatter if the observed anti-hierarchical behaviour of the AGN population couples to galaxy assembly and results in an enhancement of the star formation efficiency which is proportional to galactic mass. At low and intermediate galactic masses, however, a slower mode for star formation suffices to account for the observational properties. 相似文献
859.
860.
Lidia Pieri Massimiliano Lattanzi Joseph Silk 《Monthly notices of the Royal Astronomical Society》2009,399(4):2033-2040
The presence of dark matter in the halo of our Galaxy could be revealed through indirect detection of its annihilation products. Dark matter annihilation is one possible interpretation of the recently measured excesses in positron and electron fluxes, provided that boost factors of the order of 103 or more are taken into account. Such boost factors are actually achievable through the velocity-dependent Sommerfeld enhancement of the annihilation cross-section. Here, we study the expected γ-ray flux from two local dwarf galaxies for which Cherenkov telescope measurements are available, namely Draco and Sagittarius. We use recent stellar kinematical measurements to model the dark matter haloes of the dwarfs and the results of numerical simulations to model the presence of an associated population of subhaloes. We incorporate the Sommerfeld enhancement of the annihilation cross-section. We compare our predictions with the observations of Draco and Sagittarius performed by MAGIC and HESS, respectively, and derive exclusion limits on the effective annihilation cross-section. We also study the sensitivities of Fermi and of the future Cherenkov telescope array to cross-section enhancements. We find that the boost factor due to the Sommerfeld enhancement is already constrained by the MAGIC and HESS data, with enhancements greater than ∼104 being excluded. 相似文献