155 research outputs found
Nearshore wave forecasting and hindcasting by dynamical and statistical downscaling
A high-resolution nested WAM/SWAN wave model suite aimed at rapidly
establishing nearshore wave forecasts as well as a climatology and return
values of the local wave conditions with Rapid Enviromental Assessment (REA) in
mind is described. The system is targeted at regions where local wave growth
and partial exposure to complex open-ocean wave conditions makes diagnostic
wave modelling difficult.
SWAN is set up on 500 m resolution and is nested in a 10 km version of WAM. A
model integration of more than one year is carried out to map the spatial
distribution of the wave field. The model correlates well with wave buoy
observations (0.96) but overestimates the wave height somewhat (18%, bias 0.29
m).
To estimate wave height return values a much longer time series is required
and running SWAN for such a period is unrealistic in a REA setting. Instead we
establish a direction-dependent transfer function between an already existing
coarse open-ocean hindcast dataset and the high-resolution nested SWAN model.
Return values are estimated using ensemble estimates of two different
extreme-value distributions based on the full 52 years of statistically
downscaled hindcast data. We find good agreement between downscaled wave height
and wave buoy observations. The cost of generating the statistically downscaled
hindcast time series is negligible and can be redone for arbitrary locations
within the SWAN domain, although the sectors must be carefully chosen for each
new location.
The method is found to be well suited to rapidly providing detailed wave
forecasts as well as hindcasts and return values estimates of partly sheltered
coastal regions.Comment: 20 pages, 7 figures and 2 tables, MREA07 special issue on Marine
rapid environmental assessmen
How the IMF Induces a Local Component During Northward IMF and Characteristic Timescales
We use the Lyon-Fedder-Mobarry global magnetohydrodynamics model to study the
effects of the interplanetary magnetic field (IMF) component
on the coupling between the solar wind and magnetosphere-ionosphere system when
IMF 0. We describe the evolution of how a magnetospheric
component is induced on closed field lines during these
conditions. Starting from dayside lobe reconnection, the magnetic tension on
newly reconnected field lines redistribute the open flux asymmetrically between
the two hemispheres. This results in asymmetric magnetic energy density in the
lobes. Shear flows are induced to restore equilibrium, and these flows are what
effectively induces a local component. We show the radial
dependence of the induced and compare the results to the
induced during southward IMF conditions. We also show the
response and reconfiguration time of the inner magnetosphere to IMF
reversals during northward IMF . A superposed
epoch analysis of magnetic field measurements from seven Geostationary
Operational Environmental Satellite spacecraft at different local times both
for negative-to-positive and positive-to-negative IMF
reversals is presented. We find that the induced responds
within 16 min of the arrival of IMF at the bow shock, and it
completely reconfigures within 47 min
A high-resolution hindcast of wind and waves for The North Sea, The Norwegian Sea and The Barents Sea
A combined high-resolution atmospheric downscaling and wave hindcast based on
the ERA-40 reanalysis covering the Norwegian Sea, the North Sea and the Barents
Sea is presented. The period covered is from September 1957 to August 2002. The
dynamic atmospheric downscaling is performed as a series of short prognostic
runs initialized from a blend of ERA-40 and the previous prognostic run to
preserve the fine-scale surface features from the high-resolution model while
maintaining the large-scale synoptic field from ERA-40. The nested WAM wave
model hindcast consists of a coarse 50 km model covering the North Atlantic
forced with ERA-40 winds and a nested 10-11 km resolution model forced with
downscaled winds.
A comparison against in situ and satellite observations of wind and sea state
reveals significant improvement in mean values and upper percentiles of wind
vectors and the significant wave height over ERA-40. Improvement is also found
in the mean wave period. ERA-40 is biased low in wind speed and significant
wave height, a bias which is not reproduced by the downscaling. The atmospheric
downscaling also reproduces polar lows, which can not be resolved by ERA-40,
but the lows are too weak and short-lived as the downscaling is not capable of
capturing their full life cycle.Comment: 34 pages, 12 figures, 6 table
Efficient Bit-Decomposition and Modulus-Conversion Protocols with an Honest Majority
We propose secret-sharing-based bit-decomposition and modulus conversion protocols for a prime order ring with an honest majority: an adversary can corrupt parties of parties and . Our protocols are secure against passive and active adversaries depending on the components of our protocols. We assume a secret is an -bit element and , where in the passive security and in the active security. The outputs of our bit-decomposition and modulus-conversion protocols are tuple of shares in and a share in , respectively, where is the modulus to be converted. If and are small, the communication complexity of our passively secure bit-decomposition and modulus-conversion protocols are bits and bits, respectively. Our key observation is that a quotient of additive shares can be computed from the \emph{least} significant bits. If a secret is ``shifted\u27\u27 and additively shared by in as , the least significant bits of determines since is an odd prime and the least significant bits of are s
The Acceleration and Storage of Radioactive Ions for a Beta-Beam Facility
The term beta-beam has been coined for the production of a pure beam of
electron neutrinos or their antiparticles through the decay of radioactive ions
circulating in a storage ring. This concept requires radioactive ions to be
accelerated to as high Lorentz gamma as 150. The neutrino source itself
consists of a storage ring for this energy range, with long straight sections
in line with the experiment(s). Such a decay ring does not exist at CERN today,
nor does a high-intensity proton source for the production of the radioactive
ions. Nevertheless, the existing CERN accelerator infrastructure could be used
as this would still represent an important saving for a beta-beam facility.Comment: beta-beam working group website at http://cern.ch/beta-bea
Testing nowcasts of the ionospheric convection from the expanding and contracting polar cap model
The expanding/contracting polar cap (ECPC) model, or the time-dependent Dungey cycle, provides a theoretical framework for understanding solar wind-magnetosphere-ionosphere coupling. The ECPC describes the relationship between magnetopause reconnection and substorm growth phase, magnetotail reconnection and substorm expansion phase, associated changes in auroral morphology, and ionospheric convective motions. Despite the many successes of the model, there has yet to be a rigorous test of the predictions or nowcasts made regarding ionospheric convection, which remains a final hurdle for the validation of the ECPC. In this study we undertake a comparison of ionospheric convection, as measured in situ by ion drift meters on board DMSP (Defense Meteorological Satellite Program) satellites and from the ground by SuperDARN (Super Dual Auroral Radar Network), with motions nowcasted by a theoretical model. The model is coupled to measurements of changes in the size of the polar cap made using global auroral imagery from the IMAGE FUV (Imager for Magnetopause to Aurora Global Exploration Far Ultraviolet) instrument, as well as the dayside reconnection rate, estimated using the OMNI data set. The results show that we can largely nowcast the magnitudes of ionospheric convection flows using the context of our understanding of magnetic reconnection at the magnetopause and in the magnetotail
Understanding China’s past and future energy demand: an exergy efficiency and decomposition analysis
There are very few useful work and exergy analysis studies for China, and fewer still that consider how the results inform drivers of past and future energy consumption. This is surprising: China is the world’s largest energy consumer, whilst exergy analysis provides a robust thermodynamic framework for analysing the technical efficiency of energy use. In response, we develop three novel sub-analyses. First we perform a long-term whole economy time-series exergy analysis for China (1971–2010). We find a 10-fold growth in China’s useful work since 1971, which is supplied by a 4-fold increase in primary energy coupled to a 2.5-fold gain in aggregate exergy conversion efficiency to useful work: from 5% to 12.5%. Second, using index decomposition we expose the key driver of efficiency growth as not ‘technological leapfrogging’ but structural change: i.e. increasing reliance on thermodynamically efficient (but very energy intensive) heavy industrial activities. Third, we extend our useful work analysis to estimate China’s future primary energy demand, and find values for 2030 that are significantly above mainstream projections
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