137 research outputs found
Global linear stability analysis of kinetic Trapped Ion Mode (TIM) turbulence in tokamak plasma using spectral method
Trapped ion modes (TIM) which belong to the family of ion temperature
gradient (ITG) modes, is one of the important ingredients in heat turbulent
transport at the ion scale in tokamak plasmas. It is essential to properly
estimate their linear growth rate to understand their influence on ion-scale
turbulent transport. A global linear analysis of a reduced gyro-bounce kinetic
model for trapped particle modes is performed, and a spectral method is
proposed to solve the dispersion relation. Importantly, the radial profile of
the particle drift velocity is taken into account in the linear analysis by
considering the exact magnetic flux {\psi} dependency of the equilibrium
Hamiltonian H_{eq}({\psi}) in the quasi-neutrality equation and equilibrium
gyro-bounce averaged distribution function F_{eq} . Using this spectral method,
linear growth-rates of TIM instability in presence of different temperature
profiles and precession frequencies of trapped ions, with an approximated
constant Hamiltonian and the exact {\psi} dependent equilibrium Hamiltonian,
are investigated. The growth-rate depends on the logarithmic gradient of
temperature \kappa_{T} , density \kappa_{n} and equilibrium Hamiltonian
\kappa_{\Lambda} . With the exact {\psi} dependent Hamiltonian, the growth
rates and potential profiles are modified significantly, compared to the cases
with approximated constant Hamiltonian. All the results from the global linear
analysis agree with a semi-Lagrangian based linear Vlasov solver with a good
accuracy. This spectral method is very fast and requires very less computation
resources compared to a linear version of Vlasov-solver based on a
semi-Lagrangian scheme
Evaluation of Near-Surface Air Temperature from Reanalysis over the United States and Ukraine: Application to Winter Wheat Yield Forecasting
In this work we evaluate the near-surface air temperature datasets from the ERA-Interim, JRA55, MERRA2, NCEP1, and NCEP2 reanalysis projects. Reanalysis data were first compared to observations from weather stations located on wheat areas of the United States and Ukraine, and then evaluated in the context of a winter wheat yield forecast model. Results from the comparison with weather station data showed that all datasets performed well (r2>0.95) and that more modern reanalysis such as ERAI had lower errors (RMSD ~ 0.9) than the older, lower resolution datasets like NCEP1 (RMSD ~ 2.4). We also analyze the impact of using surface air temperature data from different reanalysis products on the estimations made by a winter wheat yield forecast model. The forecast model uses information of the accumulated Growing Degree Day (GDD) during the growing season to estimate the peak NDVI signal. When the temperature data from the different reanalysis projects were used in the yield model to compute the accumulated GDD and forecast the winter wheat yield, the results showed smaller variations between obtained values, with differences in yield forecast error of around 2% in the most extreme case. These results suggest that the impact of temperature discrepancies between datasets in the yield forecast model get diminished as the values are accumulated through the growing season
Solving discrete logarithms on a 170-bit MNT curve by pairing reduction
Pairing based cryptography is in a dangerous position following the
breakthroughs on discrete logarithms computations in finite fields of small
characteristic. Remaining instances are built over finite fields of large
characteristic and their security relies on the fact that the embedding field
of the underlying curve is relatively large. How large is debatable. The aim of
our work is to sustain the claim that the combination of degree 3 embedding and
too small finite fields obviously does not provide enough security. As a
computational example, we solve the DLP on a 170-bit MNT curve, by exploiting
the pairing embedding to a 508-bit, degree-3 extension of the base field.Comment: to appear in the Lecture Notes in Computer Science (LNCS
Land Surface Temperature Product Validation Best Practice Protocol Version 1.1 - January, 2018
Land Surface Temperature Product Validation Best Practice Protocol Version 1.0 - October, 2017
The Global Climate Observing System (GCOS) has specified the need to systematically generate andvalidate Land Surface Temperature (LST) products. This document provides recommendations on goodpractices for the validation of LST products. Internationally accepted definitions of LST, emissivity andassociated quantities are provided to ensure the compatibility across products and reference data sets. Asurvey of current validation capabilities indicates that progress is being made in terms of up-scaling and insitu measurement methods, but there is insufficient standardization with respect to performing andreporting statistically robust comparisons.Four LST validation approaches are identified: (1) Ground-based validation, which involvescomparisons with LST obtained from ground-based radiance measurements; (2) Scene-based intercomparisonof current satellite LST products with a heritage LST products; (3) Radiance-based validation,which is based on radiative transfer calculations for known atmospheric profiles and land surface emissivity;(4) Time series comparisons, which are particularly useful for detecting problems that can occur during aninstrument's life, e.g. calibration drift or unrealistic outliers due to undetected clouds. Finally, the need foran open access facility for performing LST product validation as well as accessing reference LST datasets isidentified
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The surface temperatures of Earth: steps towards integrated understanding of variability and change
Surface temperature is a key aspect of weather and climate, but the term may refer to different quantities that play interconnected roles and are observed by different means. In a community-based activity in June 2012, the EarthTemp Network brought together 55 researchers from five continents to improve the interaction between scientific communities who focus on surface temperature in particular domains, to exploit the strengths of different observing systems and to better meet the needs of different communities. The workshop identified key needs for progress towards meeting scientific and societal requirements for surface temperature understanding and information, which are presented in this community paper. A "whole-Earth" perspective is required with more integrated, collaborative approaches to observing and understanding Earth's various surface temperatures. It is necessary to build understanding of the relationships between different surface temperatures, where presently inadequate, and undertake large-scale systematic intercomparisons. Datasets need to be easier to obtain and exploit for a wide constituency of users, with the differences and complementarities communicated in readily understood terms, and realistic and consistent uncertainty information provided. Steps were also recommended to curate and make available data that are presently inaccessible, develop new observing systems and build capacities to accelerate progress in the accuracy and usability of surface temperature datasets
What controls the isotopic composition of Greenland surface snow?
International audienceWater stable isotopes in Greenland ice core data provide key paleoclimatic information, and have been compared with precipitation isotopic composition simulated by isotopically enabled atmospheric models. However, post-depositional processes linked with snow metamorphism remain poorly documented. For this purpose, monitoring of the isotopic composition (d18O, dD) of near-surface water vapor, precipitation and samples of the top (0.5 cm) snow surface has been conducted during two summers (2011-2012) at NEEM, NW Greenland. The samples also include a subset of 17O-excess measurements over 4 days, and the measurements span the 2012 Greenland heat wave. Our observations are consistent with calculations assuming isotopic equilibrium between surface snow and water vapor. We observe a strong correlation between near-surface vapor d18O and air temperature (0.85 ± 0.11‰ °C-1 (R = 0.76) for 2012). The correlation with air temperature is not observed in precipitation data or surface snow data. Deuterium excess (d-excess) is strongly anti-correlated with d18O with a stronger slope for vapor than for precipitation and snow surface data. During nine 1-5-day periods between precipitation events, our data demonstrate parallel changes of d18O and d-excess in surface snow and near-surface vapor. The changes in d18O of the vapor are similar or larger than those of the snow d18O. It is estimated using the CROCUS snow model that 6 to 20% of the surface snow mass is exchanged with the atmosphere. In our data, the sign of surface snow isotopic changes is not related to the sign or magnitude of sublimation or deposition. Comparisons with atmospheric models show that day-to-day variations in near-surface vapor isotopic composition are driven by synoptic variations and changes in air mass trajectories and distillation histories. We suggest that, in between precipitation events, changes in the surface snow isotopic composition are driven by these changes in near-surface vapor isotopic composition. This is consistent with an estimated 60% mass turnover of surface snow per day driven by snow recrystallization processes under NEEM summer surface snow temperature gradients. Our findings have implications for ice core data interpretation and model-data comparisons, and call for further process studies. © Author(s) 2014
High spatial resolution imaging of methane and other trace gases with the airborne Hyperspectral Thermal Emission Spectrometer (HyTES)
Currently large uncertainties exist associated with the
attribution and quantification of fugitive emissions of criteria pollutants
and greenhouse gases such as methane across large regions and key economic
sectors. In this study, data from the airborne Hyperspectral Thermal
Emission Spectrometer (HyTES) have been used to develop robust and reliable
techniques for the detection and wide-area mapping of emission plumes of
methane and other atmospheric trace gas species over challenging and diverse
environmental conditions with high spatial resolution that permits direct
attribution to sources. HyTES is a pushbroom imaging spectrometer with high
spectral resolution (256 bands from 7.5 to 12 µm), wide swath (1–2 km),
and high spatial resolution (∼ 2 m at 1 km altitude) that
incorporates new thermal infrared (TIR) remote sensing technologies. In this
study we introduce a hybrid clutter matched filter (CMF) and plume dilation
algorithm applied to HyTES observations to efficiently detect and
characterize the spatial structures of individual plumes of CH4,
H2S, NH3, NO2, and SO2 emitters. The sensitivity and
field of regard of HyTES allows rapid and frequent airborne surveys of large
areas including facilities not readily accessible from the surface. The
HyTES CMF algorithm produces plume intensity images of methane and other
gases from strong emission sources. The combination of high spatial
resolution and multi-species imaging capability provides source attribution
in complex environments. The CMF-based detection of strong emission sources
over large areas is a fast and powerful tool needed to focus on more
computationally intensive retrieval algorithms to quantify emissions with
error estimates, and is useful for expediting mitigation efforts and
addressing critical science questions
Using continuous measurements of near-surface atmospheric water vapour isotopes to document snow-air interactions
Water stable isotope data from Greenland ice cores provide key paleoclimatic information. However, postdepositional processes linked with snow metamorphism remain poorly documented. For this purpose, a monitoring of the isotopic composition δ18O and δD at several height levels (up to 13 meter) of near-surface water vapor, precipitation and snow in the first 0.5 cm from the surface has been conducted during three summers (2010-2012) at NEEM, NW Greenland. We observe a clear diurnal cycle in both the value and gradient of the isotopic composition of the water vapor above the snow surface. The diurnal amplitude in δD is found to be ~15‰. The diurnal isotopic composition follows the absolute humidity cycle. This indicates a large flux of vapor from the snow surface to the atmosphere during the daily warming and reverse flux during the daily cooling. The isotopic measurements of the flux of water vapor above the snow give new insights into the post depositional processes of the isotopic composition of the snow. During nine 1-5 days periods between precipitation events, our data demonstrate parallel changes of δ18O and d-excess in surface snow and near-surface vapor. The changes in δ18O of the vapor are similar or larger than those of the snow δ18O. It is estimated using the CROCUS snow model that 6 to 20% of the surface snow mass is exchanged with the atmosphere. In our data, the sign of surface snow isotopic changes is not related to the sign or magnitude of sublimation or deposition. Comparisons with atmospheric models show that day-to-day variations in near-surface vapor isotopic composition are driven by synoptic variations and changes in air mass trajectories and distillation histories. We suggest that, in-between precipitation events, changes in the surface snow isotopic composition are driven by these changes in near-surface vapor isotopic composition. This is consistent with an estimated 60% mass turnover of surface snow per day driven by snow recrystallization processes associated with temperature gradients near the snow surface. Our findings have implications for ice core data interpretation and model-data comparisons, and call for further process studies
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