We present models of the near-infrared (1-5 μm) spectra of Saturn’s F ring obtained by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) at ultra-high phase angles (177.4-178.5°). Modeling this spectrum constrains the size distribution, composition, and structure of F ring particles in the 0.1-100 μm size range. These spectra are very different from those obtained at lower phase angles; they lack the familiar 1.5 and 2 μm absorption bands, and the expected 3 μm water ice primary absorption appears as an unusually narrow dip at 2.87 μm. We have modeled these data using multiple approaches. First, we use a simple Mie scattering model to constrain the size distribution and composition of the particles. The Mie model allows us to understand the overall shapes of the spectra in terms of dominance by diffraction at these ultra-high phase angles, and also to demonstrate that the 2.87 μm dip is associated with the Christiansen frequency of water ice (where the real refractive index passes unity). Second, we use a combination of Mie scattering with Effective Medium Theory to probe the effect of porous (but structureless) particles on the overall shape of the spectrum and depth of the 2.87 μm band. Such simple models are not able to capture the shape of this absorption feature well. Finally, we model each particle as an aggregate of discrete monomers, using the Discrete Dipole Approximation (DDA) model, and find a better fit for the depth of the 2.87 μm feature. The DDA models imply a slightly different overall size distribution. We present a simple heuristic model which explains the differences between the Mie and DDA model results. We conclude that the F ring contains aggregate particles with a size distribution that is distinctly narrower than a typical power law, and that the particles are predominantly crystalline water ice. 相似文献
We evaluate the evidence for the presence of mineral spectral signatures indicative of the past presence of water at two putative paleolakes on Mars using observations by the Mars Reconnaissance Orbiter (MRO) Compact Reconnaissance Image Spectrometer for Mars (CRISM). CRISM spectra of both sites are consistent with laboratory spectra of Mg-rich phyllosilicates. Our analysis represents the first detailed evaluation of these locations. The spatial occurrence and association with topographic features within the craters is distinctly different for the two sites. The occurrence of these minerals supports the conclusion that water was once active in the areas sampled by these craters. The distribution of the phyllosilicates in Luqa does not provide distinctive evidence for the presence of a previous standing body of water and is consistent with either impact emplacement or post-impact alteration. For Cankuzo, the phyllosilicate distribution provides evidence of a layer in the crater wall indicative of aqueous activity, but does not require a paleolake. 相似文献
While restoring hyporheic flowpaths has been cited as a benefit to stream restoration structures, little documentation exists confirming that constructed restoration structures induce comparable hyporheic exchange to natural stream features. This study compares a stream restoration structure (cross‐vane) to a natural feature (riffle) concurrently in the same stream reach using time‐lapsed electrical resistivity (ER) tomography. Using this hydrogeophysical approach, we were able to quantify hyporheic extent and transport beneath the cross‐vane structure and the riffle. We interpret from the geophysical data that the cross‐vane and the natural riffle induced spatially and temporally unique hyporheic extent and transport, and the cross‐vane created both spatially larger and temporally longer hyporheic flowpaths than the natural riffle. Tracer from the 4.67‐h injection was detected along flowpaths for 4.6 h at the cross‐vane and 4.2 h at the riffle. The spatial extent of the hyporheic zone at the cross‐vane was 12% larger than that at the riffle. We compare ER results of this study to vertical fluxes calculated from temperature profiles and conclude significant differences in the interpretation of hyporheic transport from these different field techniques. Results of this study demonstrate a high degree of heterogeneity in transport metrics at both the cross‐vane and the riffle and differences between the hyporheic flowpath networks at the two different features. Our results suggest that restoration structures may be capable of creating sufficient exchange flux and timescales of transport to achieve the same ecological functions as natural features, but engineering of the physical and biogeochemical environment may be necessary to realize these benefits. 相似文献
The qanat water supply technology, which gravity drains mountain aquifers into valleys, is considered as a culturally appropriate
and ecological sustainable design to meet northern Cyprus’ drinking water development needs. This research estimates the boundary
and water budget for the proposed qanat recharge area of 370 km2, which is in the upper elevations of the limestone dominated Five Finger Mountain Range. The mountain drainage was analyzed
using global elevation data from the Shuttle Ranging Topography Mission (SRTM). Efforts to use Tropical Rainfall Measuring
Mission (TRMM) annual precipitation for water budget inputs failed due to extreme error when tested against 10–30 years of
meteorological station data; TRMM under-estimated depths on the narrow mountain peaks. Gage records, while few in number,
were area averaged to set average annual precipitation inputs at 530 mm year−1. Evaporation was estimated using a complementary relationship areal evapotranspiration (CRAE) model, setting average atmospheric
outputs at 221 mm year−1. Recharge to the qanat aquifer was set by subtracting evaporation from precipitation, and then allocating 50% of the remaining
water to environmental services. At 25% development, the qanat system supplies 14 mm3 year−1 of water, meeting the drinking water deficit of 13 mm3. 相似文献
In part 1 of this two-paper series, a brief summary of the basic concepts and theories used in developing the Generalized Stream Tube model for Alluvial River Simulation （GSTARS） computer models was presented. Part 2 provides examples that illustrate some of the capabilities of the GSTARS models and how they can be applied to solve a wide range of river and reservoir sedimentation problems. Laboratory and field case studies are used and the examples show representative applications of the earlier and of the more recent versions of GSTARS. Some of the more recent capabilities implemented in GSTARS3, one of the latest versions of the series, are also discussed here with more detail. 相似文献
Earth, Moon, and Planets - Observations of lunar radio emission were made at 3.09 mm wavelength (97.1 GHz) from April 18 to May 20, 1971. Absolute brightness temperatures were measured for five... 相似文献
The seasonal variability of sea surface height (SSH) and currents are defined by analysis of altimeter data in the NE Pacific Ocean over the region from Central America to the Alaska Gyre. The results help to clarify questions about the timing of seasonal maxima in the boundary currents. As explained below, the long-term temporal mean of the SSH values must be removed at each spatial point to remove the temporally invariant (and large) signal caused by the marine geoid. We refer to the resulting SSH values, which contain all of the temporal variations, as the ‘residual’ SSH. Our main findings are:
1. The maximum surface velocities around the boundaries of the cyclonic Alaska Gyre (the Alaska Current and the Alaska Stream) occur in winter, at the same time that the equatorward California Current is weakest or reversed (forming the poleward Davidson Current); the maximum surface velocities in the California Current occur in summer. These seasonal maxima are coincident with the large-scale atmospheric wind forcing over each region.
2. Most of the seasonal variability occurs as strong residuals in alongshore surface currents around the boundaries of the NE Pacific basin, directly connecting the boundaries of the subpolar gyre, the subtropical gyre and the Equatorial Current System.
3. Seasonal variability in the surface velocities of the eastward North Pacific Current (West Wind Drift) is weak in comparison to seasonal changes in the surface currents along the boundaries.
4. There is an initial appearance next to the coast and offshore migration of seasonal highs and lows in SSH, alongshore velocity and eddy kinetic energy (EKE) in the Alaska Gyre, similar to the previously-described seasonal offshore migration in the California Current.
5. The seasonal development of high SSH and poleward current residuals next to the coast appear first off Central America and mainland Mexico in May–June, prior to their appearance in the southern part of the California Current in July–August and their eventual spread around the entire basin in November–December. Similarly, low SSH and equatorward transport residuals appear first off Central America and Mexico in January–February before spreading farther north in spring and summer.
6. The maximum values of EKE occur when each of the boundary currents are maximum.
Article Outline
1. Introduction and background
2. Data and methods
2.1. Altimeter and tide gauge data
2.2. Atmospheric forcing—sea level pressure
2.3. Statistical gridding
3. Results
4. Summary and discussion
4.1. Alaska Gyre
4.2. Connections around the boundaries of the subarctic and subtropical gyres
4.3. Connections to the North Pacific Current
4.4. Offshore ‘propagation’ of the seasonal height and transport signals
4.5. Connections to the equatorial current systems along the boundaries
Acknowledgements
References
1. Introduction and background
This is the first of a two-part analysis of temporal variability of the NE Pacific Ocean’s surface circulation, as measured by satellite altimeters. Here we examine the seasonal variability. In Part 2 (Strub & James, 2002) we analyze the non-seasonal anomalies of the surface circulation over the 1993–1998 period, during which the 1997–1998 El Niño creates the largest signal. Formation of the seasonal cycles discussed here is the first step in creating the non-seasonal anomalies. The seasonal cycles themselves, however, provide new information on the response of the NE Pacific to strong seasonal forcing, on scales not previously addressed. This analysis quantifies the degree of connection, on seasonal time scales, between the boundary currents in the eastern subarctic and subtropical gyres, as well as the connection between the boundaries and the interior NE Pacific. It further shows a connection to the equatorial current system.Numerous papers describe aspects of the seasonal cycles for certain parameters in subregions of our larger domain. Chapters in Robinson and Brink (1998) review some of the past results from the coastal ocean in the regions between the Equator and the Alaska Gyre ( Badan; Hickey and Royer). Fig. 1 presents the climatological surface dynamic height field (relative to 500 m) in the NE Pacific, calculated from the long-term mean climatological temperature and salinity data of Levitus and Gelfeld (1992). The 500 m reference level is used to concentrate on the surface flow seen by altimeters. Although this climatology is overly smooth, it shows the major currents in the area. The broad, eastward North Pacific Current (also called the West Wind Drift) splits into the counterclockwise Alaska Gyre and the equatorward California Current. South of 20°N in summer, the California Current turns westward and flows into the North Equatorial Current, while in winter–spring, part of it continues along the Mexican mainland before turning westward ( Badan; Fiedler and Fiedler). The long-term climatology shows both paths. The North Equatorial Countercurrent (NECC) flows eastward between 5° –10°N to approximately 120°W, but is only weakly seen in the annual climatology from there to the cyclonic flow around the Costa Rica Dome near 8°N, 92°W. The NECC is a shallow current (found in the upper 200 m) and might appear more strongly if a shallower reference were used, but it is also seasonally intermittent. When the Intertropical Convergence Zone (ITCZ) is in its northern location near 10°N (summer), surface divergences and upwelling create a zonal trough in surface height, driving the NECC along the southern side of the trough. When the ITCZ moves south in winter, the NECC weakens or reverses. 相似文献
This article considers Thailand's problems with the 1982 LOS Convention, especially the exclusive economic zones. Despite the disadvantages, it recommends that Thailand should ratify the Convention. 相似文献
The Newport Hydrographic (NH) Line along 44.65°N off central Oregon was sampled seasonally during two epochs: 1961-1971 through the TENOC program and 1997-2003 through the GLOBEC Northeast Pacific Long Term Observations Program (LTOP); some observations are available for 2004 and 2005. During TENOC, the line extended 305 km offshore to 128°W, with stations 18 km apart over the continental shelf and 36 km offshore. During LTOP, the line was shorter (to 126°W) with closer station spacing over the continental shelf (9 km apart) and slope (18 km apart). LTOP cruises included biochemical sampling and underway current measurements. During both TENOC and LTOP, the seasonal cycle is very strong (accounting for >50% of the variance in surface layer properties), with rapid transitions in spring and fall. The summer regime is subject to coastal upwelling driven by southward winds, equatorward surface currents, and advection of low-salinity waters from the Columbia River. The winter regime off Newport is subject to coastal downwelling and poleward surface currents driven by northeastward winds. Comparison between TENOC and LTOP summer regimes shows the near-surface layer (0-100 m) at most locations is significantly warmer and fresher during LTOP than TENOC, and steric heights over the continental margin are significantly higher. Comparison of LTOP and TENOC winters shows that average differences at most locations were not statistically significant, but that the variance of steric height and shelf-break temperatures was significantly higher during LTOP than TENOC. Interannual variability of climate indices is also stronger during LTOP, which included a rare Subarctic invasion in 2002 as well as the strong 1997-1998 El Niño. During both TENOC and LTOP, interannual variability of steric height is closely related to the El Niño/La Niña cycle. Nutrient concentrations and nitrate-to-phosphate ratios of upwelling-source waters vary inversely with halocline temperature. Both reflect alongshore advection by coastal currents: southward currents bring cool, nitrate-rich waters in summer (especially during the Subarctic invasion), and northward currents bring relatively warm, nitrate-poor waters to the NH line in winter (especially during El Niño). Seasonal and interannual variations in the nutrient level of upwelling-source water are reflected in time series of vertically-integrated chlorophyll over the LTOP survey region (about 150 km by 300 km). Seasonal variations in chlorophyll and currents are congruent with seasonal variations in copepod biomass and diversity. We were not successful in establishing a clear connection between chlorophyll levels and interannual variations in copepod biomass or diversity, nor in explaining the large decrease in the survival rate of coho salmon between TENOC (6%) and LTOP (3%). 相似文献
