521 research outputs found
Evaluation of the SeaWinds scatterometer for regional monitoring of vegetation phenology
Phenology, or the seasonality of recurring biological events such as vegetation canopy development and senescence, is a primary constraint on global carbon, water and energy cycles. We analyzed multiseason Ku-band radar backscatter measurements from the SeaWinds-on-QuikSCAT scatterometer to determine canopy phenology and growing season vegetation dynamics from 2000 to 2002 at 27 sites representing major global land cover classes and regionally across most of North America. We compared these results with similar information derived from the MODIS leaf area index (LAI) data product (MOD-15A2). In site-level linear regression analysis, the correspondence between radar backscatter and LAI was significant (p \u3c 0.05) at most but not all sites and was generally higher (R2 \u3e 0.5) for sites with relatively low LAI or where the seasonal range in LAI was large (e.g., \u3e3 m2 m−2). The SeaWinds instrument also detected generally earlier onset of vegetation canopy growth in spring than the optical/near-infrared (NIR) based LAI measurements from MODIS, though the timing of canopy senescence and the end of the growing season were more similar. Over North America, the correlation between the two time series was stratified largely by land cover class, with higher correlations (R ∼ 0.7–0.9) for most cropland, deciduous broadleaf forest, crop/natural vegetation mosaic land cover, and some grassland. Lower correlations were observed for open shrubland and evergreen needleleaf forest. Overall, the results indicate that SeaWinds backscatter is sensitive to growing season canopy dynamics across a range of broadleaf vegetation types and provides a quantitative view that is independent of optical/NIR remote sensing instruments
Model of Double Asteroid Redirection Test Impact Ejecta Plume Observations
The Double Asteroid Redirection Test (DART) spacecraft will impact the moon Dimorphos of the [65803]
Didymos binary in order to demonstrate asteroid deflection by a kinetic impactor. DART will measure the
deflection by using ground-based telescopic observations of the orbital period change of Didymos and will carry
the Light Italian CubeSat for Imaging of Asteroids (LICIACube) cubesat, which will perform a flyby of Didymos
about 167 s after the DART impact, obtaining images of the DART impact ejecta plume. LICIACube images
showing the ejecta plume spatial structure and temporal evolution will help determine the vector momentum
transfer from the DART impact. A model is developed for the impact ejecta plume optical depth, using a pointsource scaling model of the DART impact. The model is applied to expected LICIACube plume images and shows
how plume images enable characterization of the ejecta mass versus velocity distribution. The ejecta plume
structure, as it evolves over time, is determined by the amount of ejecta that has reached a given altitude at a given
time. The evolution of the plume optical depth profiles determined from LICIACube images can distinguish
between strength-controlled and gravity-controlled impacts, by distinguishing the respective mass versus velocity
distributions. LICIACube plume images discriminate the differences in plume structure and evolution that result
from different target physical properties, mainly the strength and porosity, thereby allowing inference of these
properties to improve the determination of DART impact momentum transfer
Constraints on the perturbed mutual motion in Didymos due to impact-induced deformation of its primary after the DART impact
Binary near-Earth asteroid (65803) Didymos is the target of the proposed NASA
Double Asteroid Redirection Test (DART), part of the Asteroid Impact &
Deflection Assessment (AIDA) mission concept. In this mission, the DART
spacecraft is planned to impact the secondary body of Didymos, perturbing
mutual dynamics of the system. The primary body is currently rotating at a spin
period close to the spin barrier of asteroids, and materials ejected from the
secondary due to the DART impact are likely to reach the primary. These
conditions may cause the primary to reshape, due to landslides, or internal
deformation, changing the permanent gravity field. Here, we propose that if
shape deformation of the primary occurs, the mutual orbit of the system would
be perturbed due to a change in the gravity field. We use a numerical
simulation technique based on the full two-body problem to investigate the
shape effect on the mutual dynamics in Didymos after the DART impact. The
results show that under constant volume, shape deformation induces strong
perturbation in the mutual motion. We find that the deformation process always
causes the orbital period of the system to become shorter. If surface layers
with a thickness greater than ~0.4 m on the poles of the primary move down to
the equatorial region due to the DART impact, a change in the orbital period of
the system and in the spin period of the primary will be detected by
ground-based measurement.Comment: 8 pages, 7 figures, 2 tables, accepted for publication in MNRA
The Scientific Measurement System of the Gravity Recovery and Interior Laboratory (GRAIL) Mission
The Gravity Recovery and Interior Laboratory (GRAIL) mission to the Moon utilized an integrated scientific measurement system comprised of flight, ground, mission, and data system elements in order to meet the end-to-end performance required to achieve its scientific objectives. Modeling and simulation efforts were carried out early in the mission that influenced and optimized the design, implementation, and testing of these elements. Because the two prime scientific observables, range between the two spacecraft and range rates between each spacecraft and ground stations, can be affected by the performance of any element of the mission, we treated every element as part of an extended science instrument, a science system. All simulations and modeling took into account the design and configuration of each element to compute the expected performance and error budgets. In the process, scientific requirements were converted to engineering specifications that became the primary drivers for development and testing. Extensive simulations demonstrated that the scientific objectives could in most cases be met with significant margin. Errors are grouped into dynamic or kinematic sources and the largest source of non-gravitational error comes from spacecraft thermal radiation. With all error models included, the baseline solution shows that estimation of the lunar gravity field is robust against both dynamic and kinematic errors and a nominal field of degree 300 or better could be achieved according to the scaled Kaula rule for the Moon. The core signature is more sensitive to modeling errors and can be recovered with a small margin
Non-linear glacier response to calving events, Jakobshavn Isbræ, Greenland
Jakobshavn Isbræ, a tidewater glacier that produces some of Greenland’s largest icebergs
and highest speeds, reached record-high flow rates in 2012 (Joughin and others, 2014). We use terrestrial
radar interferometric observations from August 2012 to characterize the events that led to record-high
flow.Jakobshavn Isbræ, a tidewater glacier that produces some of Greenland’s largest icebergs
and highest speeds, reached record-high flow rates in 2012 (Joughin and others, 2014). We use terrestrial
radar interferometric observations from August 2012 to characterize the events that led to record-high
flow. We find that the highest speeds occurred in response to a small calving retreat, while several larger
calving events produced negligible changes in glacier speed. This non-linear response to calving events
suggests the terminus was close to flotation and therefore highly sensitive to terminus position. Our
observations indicate that a glacier’s response to calving is a consequence of two competing feedbacks:
(1) an increase in strain rates that leads to dynamic thinning and faster flow, thereby promoting desta-
bilization, and (2) an increase in flow rates that advects thick ice toward the terminus and promotes
restabilization. The competition between these feedbacks depends on temporal and spatial variations
in the glacier’s proximity to flotation. This study highlights the importance of dynamic thinning and
advective processes on tidewater glacier stability, and further suggests the latter may be limiting the
current retreat due to the thick ice that occupies Jakobshavn Isbræ’s retrograde bed.We are grateful to many people and organizations that sup-
ported this project. TRIs were purchased with funds from
the Gordon and Betty Moore Foundation (GBMF2627).
Field work was completed through NASA (NNX08AN74G).
Cassotto was supported by the New Hampshire
Space Grant Consortium (NNX10AL97H) and later by a
NASA Earth and Space Science Fellowship Program
(NNX14AL29H). We thank CH2 M HILL Polar Services and
Air Greenland for logistics support, and Judy McIlrath and
Denis Voytenko for assistance in the field. Landsat imagery
courtesy of the US Geological Survey. Joe Licciardi, Tim
Bartholomaus and an anonymous reviewer provided valu-
able insight that improved this manuscript. Data are available
upon request by contacting the primary author.Ye
Numerical simulation of bar and island morphodynamics in anabranching mega-rivers
Onlineopen article ©2013 American Geophysical Union.Bar and island morphodynamics in the world's largest anabranching rivers are investigated using a new numerical model of hydrodynamics, sediment transport, bank erosion, and floodplain development, operating over periods of several hundred years. Simulated channel morphology is compared to that of natural rivers and shown to be realistic, both in terms of the statistical characteristics of channel width, depth, and bar shape distributions, and mechanisms of unit bar, compound bar, and island evolution. Results demonstrate that bar and island stability may be sensitive to hydrologic regime, because greater variability in flood magnitude encourages the formation of emergent bars that can be stabilized by vegetation colonization. Simulations illustrate a range of mechanisms of unit bar generation that are linked to local bed or bank instabilities. This link may explain the reduced frequency of unit bars observed in some large anabranching rivers that are characterized by stable vegetated islands and slow rates of channel change. Model results suggest that the degree to which sand-sized bed material is carried in suspension likely represents an important control on bar morphodynamics and channel network evolution, because of its influence on sand transport direction. Consequently, differences in the partitioning of the total sand load between bed load and suspension may provide a partial explanation for contrasting styles of anabranching in the world's largest sand-bed rivers. These results highlight a need for spatially-distributed flow and sediment transport data sets from large rivers, in order to support improved parameterizations of sand transport mechanics in morphodynamic models.Natural Environment Research Council (NERC). Grant Numbers: NE/I023228/1, NE/E016022/
Minimum Energy Configurations in the -Body Problem and the Celestial Mechanics of Granular Systems
Minimum energy configurations in celestial mechanics are investigated. It is
shown that this is not a well defined problem for point-mass celestial
mechanics but well-posed for finite density distributions. This naturally leads
to a granular mechanics extension of usual celestial mechanics questions such
as relative equilibria and stability. This paper specifically studies and finds
all relative equilibria and minimum energy configurations for and
develops hypotheses on the relative equilibria and minimum energy
configurations for bodies.Comment: Accepted for publication in Celestial Mechanics and Dynamical
Astronom
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