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Satellite altimetry has been widely used to determine surface elevation changes in polar ice sheets. The original height measurements are irregularly distributed in space and time. Gridded surface elevation changes are commonly derived by repeat altimetry analysis (RAA) and subsequent spatial interpolation of height change estimates. This article assesses how methodological choices related to those two steps affect the accuracy of surface elevation changes, and how well this accuracy is represented by formal uncertainties. In a simulation environment resembling CryoSat-2 measurements acquired over a region in northeast Greenland between December 2010 and January 2014, different local topography modeling approaches and different cell sizes for RAA, and four interpolation approaches are tested. Among the simulated cases, the choice of either favorable or unfavorable RAA affects the accuracy of results by about a factor of 6, and the different accuracy levels are propagated into the results of interpolation. For RAA, correcting local topography by an external digital elevation model (DEM) is best, if a very precise DEM is available, which is not always the case. Yet the best DEM-independent local topography correction (nine-parameter model within a 3,000 m diameter cell) is comparable to the use of a perfect DEM, which exactly represents the ice sheet topography, on the same cell size. Interpolation by heterogeneous measurement-error-filtered kriging is significantly more accurate (on the order of 50% error reduction) than interpolation methods, which do not account for heterogeneous errors.
相似文献2.
Natural Hazards - Vegetation and tropical forests in particular have a central role in mitigating the effects of increasing levels of atmospheric CO2. Photosynthesis is the fundamental process... 相似文献
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Improved GRACE science results after adjustment of geometric biases in the Level-1B K-band ranging data 总被引：3，自引：1，他引：2
Martin Horwath Jean-Michel Lemoine Richard Biancale Stéphane Bourgogne 《Journal of Geodesy》2011,85(1):23-38
The GRACE (Gravity Recovery and Climate Experiment) satellite mission relies on the inter-satellite K-band microwave ranging
(KBR) observations. We investigate systematic errors that are present in the Level-1B KBR data, namely in the geometric correction.
This correction converts the original ranging observation (between the two KBR antennas phase centers) into an observation
between the two satellites’ centers of mass. It is computed from data on the precise alignment between both satellites, that
is, between the lines joining the center of mass and the antenna phase center of either satellite. The Level-1B data used
to determine this alignment exhibit constant biases as large as 1–2 mrad in terms of pitch and yaw alignment angles. These
biases induce non-constant errors in the Level-1B geometric correction. While the precise origin of the biases remains to
be identified, we are able to estimate and reduce them in a re-calibration approach. This significantly improves time-variable
gravity field solutions based on the CNES/GRGS processing strategy. Empirical assessments indicate that the systematic KBR
data errors have previously induced gravity field errors on the level of 6–11 times the so-called GRACE baseline error level.
The zonal coefficients (from degree 14) are particularly affected. The re-calibration reduces their rms errors by about 50%.
As examples for geophysical inferences, the improvement enhances agreement between mass variations observed by GRACE and in-situ
ocean bottom pressure observations. The improvement also importantly affects estimates of inter-annual mass variations of
the Antarctic ice sheet. 相似文献
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GOCE, Satellite Gravimetry and Antarctic Mass Transports 总被引：1，自引：0，他引：1
Reiner Rummel Martin Horwath Weiyong Yi Alberta Albertella Wolfgang Bosch Roger Haagmans 《Surveys in Geophysics》2011,32(4-5):643-657
In 2009 the European Space Agency satellite mission GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) was launched. Its objectives are the precise and detailed determination of the Earth’s gravity field and geoid. Its core instrument, a three axis gravitational gradiometer, measures the gravity gradient components V xx , V yy , V zz and V xz (second-order derivatives of the gravity potential V) with high precision and V xy , V yz with low precision, all in the instrument reference frame. The long wavelength gravity field is recovered from the orbit, measured by GPS (Global Positioning System). Characteristic elements of the mission are precise star tracking, a Sun-synchronous and very low (260 km) orbit, angular control by magnetic torquing and an extremely stiff and thermally stable instrument environment. GOCE is complementary to GRACE (Gravity Recovery and Climate Experiment), another satellite gravity mission, launched in 2002. While GRACE is designed to measure temporal gravity variations, albeit with limited spatial resolution, GOCE is aiming at maximum spatial resolution, at the expense of accuracy at large spatial scales. Thus, GOCE will not provide temporal variations but is tailored to the recovery of the fine scales of the stationary field. GRACE is very successful in delivering time series of large-scale mass changes of the Antarctic ice sheet, among other things. Currently, emphasis of respective GRACE analyses is on regional refinement and on changes of temporal trends. One of the challenges is the separation of ice mass changes from glacial isostatic adjustment. Already from a few months of GOCE data, detailed gravity gradients can be recovered. They are presented here for the area of Antarctica. As one application, GOCE gravity gradients are an important addition to the sparse gravity data of Antarctica. They will help studies of the crustal and lithospheric field. A second area of application is ocean circulation. The geoid surface from the gravity field model GOCO01S allows us now to generate rather detailed maps of the mean dynamic ocean topography and of geostrophic flow velocities in the region of the Antarctic Circumpolar Current. 相似文献
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Mima-type mounds in southwest Missouri: Expressions of point-centered and locally thickened biomantles 总被引：1，自引：0，他引：1
Mima-type mounds, formed in gravelly soils of the Diamond Grove Prairie Natural Area near Joplin, Missouri, are the focus of this study. Emphasis is on the spatial and morphological aspects of the mounds, and more particularly on the analysis of mound soils and gravel distributions as a means for shedding light on mound origins in this region. The results strongly suggest that hierarchically dominant point-centered bioturbation by small vertebrates is the mode of mound genesis. Pocket gophers (Geomys bursarius), aided by other biota, create mounds as they burrow in residual gravelly soils that have evolved dense, relatively impermeable claypans that perch water during wet periods. Although pocket gophers do not presently inhabit the Diamond Grove area, evidence of past occupation, along with laboratory and field data, support them as the dominant role in forming the mounds. We conclude that these mounds are expressions of point-centered and locally thickened biomantles. Various subsidiary processes such as aeolian inputs, water erosion, and physical and chemical weathering also have genetically impacted Diamond Grove mounds. 相似文献
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Bellie Sivakumar Wesley W. Wallender William R. Horwath Jeffrey P. Mitchell Samuel E. Prentice Brian A. Joyce 《水文研究》2006,20(8):1723-1736
This study investigates the dynamic nature of rainfall observed at the Sustainable Agriculture Farming Systems (SAFS) site in California's Sacramento Valley, which was established to study the benefits of winter cover cropping in Mediterranean irrigated‐arid systems. Rainfall data of four different temporal scales (i.e. daily, weekly, biweekly, and monthly) are analysed to determine the dynamic nature of precipitation in time. In an arid climate with seasonal precipitation this has large implications for land and water management, both in the short term and in the long term. A nonlinear dynamic technique (correlation dimension method) that uses the phase‐space reconstruction and dimension concepts is employed. Bearing in mind the possible effects of the presence of zeros (i.e. no rain) on the outcomes of this analysis, an attempt is also made to compare the dynamic nature of all‐year rainfall and winter rainfall. Analysis of 15 years of data suggests that rainfall dynamics at this site are dominated by a large number of variables, regardless of the scales and seasons studied. The dimension results also suggest that: (1) rainfall dynamics at coarser resolutions are more irregular than that at finer resolutions; (2) winter rainfall has a higher variability than all‐year rainfall. These results are indeed useful to gain information about the complexity of the rainfall process at this site with respect to (temporal) scales and seasons and, hence, the appropriate model (high‐dimensional) type. However, in view of the potential effects of certain rainfall data characteristics (e.g. zeros, measurement errors, scale effects) on the correlation dimension analysis, the discussion also emphasizes the need for further verification, and possibly confirmation, of these results. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
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M. Hosse R. Pail M. Horwath N. Holzrichter B. D. Gutknecht 《Surveys in Geophysics》2014,35(6):1393-1415
The Central Andean subduction system is one of the most active geological structures on Earth. Although there have been a few previous studies, the structure and dynamics of the system are still not well understood. In the present study, we determine a combined regional gravity model of the Andean convergent subduction region for constraining lithospheric models. After a thorough validation and cleaning of the terrestrial gravity and height databases, the method of Least Squares Collocation was applied to consistently combine terrestrial and satellite gravity data, putting much emphasis on the stochastic modelling of the individual data components. As a result, we computed the first high-resolution regional gravity model of the study region that includes GOCE satellite gravity information. The inclusion of GOCE is an essential distinction from the independent global gravity model EGM2008. Validation against EGM2008 reveals that our regional solution is very consistent in regions where terrestrial gravity data are available, but shows systematic differences in areas with terrestrial data gaps. Artefacts in the EGM2008 of up to 150 mGal could be identified. The new combined regional model benefits from the very homogeneous error characteristics and accuracy of GOCE gravity data in the long-to-medium wavelengths down to 80–100 km. Reliable density modelling became possible also in the region of Central Andes, which lacks terrestrial gravity data. Finally, density models were adapted to fit the new regional gravity field solution. The results clearly demonstrate the capabilities of GOCE to better constrain lithospheric models. 相似文献
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