92 research outputs found
Implementing northern peatlands in a global land surface model: description and evaluation in the ORCHIDEE high-latitude version model (ORC-HL-PEAT)
Widely present in boreal regions, peatlands contain large carbon stocks
because of their hydrologic properties and high water content, which makes
primary productivity exceed decomposition rates. We have enhanced the global
land surface model ORCHIDEE by introducing a hydrological representation of
northern peatlands. These peatlands are represented as a new plant functional
type (PFT) in the model, with specific hydrological properties for peat soil.
In this paper, we focus on the representation of the hydrology of northern
peatlands and on the evaluation of the hydrological impact of this
implementation. A prescribed map based on the inventory of Yu et al. (2010)
defines peatlands as a fraction of a grid cell represented as a PFT
comparable to C3 grasses, with adaptations to reproduce shallow roots and
higher photosynthesis stress. The treatment of peatland hydrology differs
from that of other vegetation types by the fact that runoff from other soil
types is partially directed towards the peatlands (instead of directly to the
river network). The evaluation of this implementation was carried out at
different spatial and temporal scales, from site evaluation to larger scales
such as the watershed scale and the scale of all northern latitudes. The
simulated net ecosystem exchanges agree with observations from three FLUXNET
sites. Water table positions were generally close to observations, with some
exceptions in winter. Compared to other soils, the simulated peat soils have
a reduced seasonal variability in water storage. The seasonal cycle of the
simulated extent of inundated peatlands is compared to flooded area as
estimated from satellite observations. The model is able to represent more
than 89.5 % of the flooded areas located in peatland areas, where the
modelled extent of inundated peatlands reaches 0.83×106 km2.
However, the extent of peatlands in northern latitudes is too small to
substantially impact the large-scale terrestrial water storage north of
45° N. Therefore, the inclusion of peatlands has a weak impact on
the simulated river discharge rates in boreal regions.</p
Additive energy forward curves in a Heath-Jarrow-Morton framework
One of the peculiarities of power and gas markets is the delivery mechanism
of forward contracts. The seller of a futures contract commits to deliver, say,
power, over a certain period, while the classical forward is a financial
agreement settled on a maturity date. Our purpose is to design a
Heath-Jarrow-Morton framework for an additive, mean-reverting, multicommodity
market consisting of forward contracts of any delivery period. The main
assumption is that forward prices can be represented as affine functions of a
universal source of randomness. This allows us to completely characterize the
models which prevent arbitrage opportunities: this boils down to finding a
density between a risk-neutral measure , such that the prices of
traded assets like forward contracts are true -martingales, and the
real world probability measure , under which forward prices are
mean-reverting. The Girsanov kernel for such a transformation turns out to be
stochastic and unbounded in the diffusion part, while in the jump part the
Girsanov kernel must be deterministic and bounded: thus, in this respect, we
prove two results on the martingale property of stochastic exponentials. The
first allows to validate measure changes made of two components: an
Esscher-type density and a Girsanov transform with stochastic and unbounded
kernel. The second uses a different approach and works for the case of
continuous density. We apply this framework to two models: a generalized
Lucia-Schwartz model and a cross-commodity cointegrated market.Comment: 28 page
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Reliability and Consistency of Surface Contamination Measurements
Surface contamination evaluation is a tough problem since it is difficult to isolate the radiations emitted by the surface, especially in a highly irradiating atmosphere. In that case the only possibility is to evaluate smearable (removeable) contamination since ex-situ countings are possible. Unfortunately, according to our experience at CEA, these values are not consistent and thus non relevant. In this study, we show, using in-situ Fourier Transform Infra Red spectrometry on contaminated metal samples, that fixed contamination seems to be chemisorbed and removeable contamination seems to be physisorbed. The distribution between fixed and removeable contamination appears to be variable. Chemical equilibria and reversible ion exchange mechanisms are involved and are closely linked to environmental conditions such as humidity and temperature. Measurements of smearable contamination only give an indication of the state of these equilibria between fixed and removeable contamination at the time and in the environmental conditions the measurements were made
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Impact of the representation of the infiltration on the river flow during intense rainfall events in JULES
Intense rainfall can lead to flash flooding and may cause disruption, damage and loss of life. Since flooding from intense rainfall (FFIR) events are of a short duration and occur within a limited area, they are generally poorly predicted by Numerical Weather Prediction (NWP) models. This is because of the high spatio-temporal resolution required and because of the way the convective rainfall is described in the model. Moreover, the hydrological process descriptions of Land Surface Models (LSMs) are not necessarily suitable to deal with cases of intense rainfall.
In this study different representations of infiltration into the soil were developed in the JULES land surface scheme with the aim of improving prediction of the amount of surface runoff, and thus ultimately river flow. Infiltration and surface runoff are explored in a test case of intense rainfall with a variable maximum infiltration. The modelled hydraulic conductivity profile is modified with depth to reduce the rate of outgoing fluxes. The new infiltration scheme is then applied to different UK catchments. The resulting river flow is evaluated against a benchmark river flow calculated using default infiltration in JULES and also observations. The results demonstrate improved representation of the highest flows with this new variable maxiumum infiltration scheme in some catchments but limited improvement elsewhere. This scheme shows best improvement in the wettest areas of the UK where the annual mean precipitation is above 1200 mm. This work highlights the requirement for substantial further work on the hydrological process representation in JULES
Oceanic forcing of Antarctic climate change: a study using a stretched-grid atmospheric general circulation model
A variable-resolution atmospheric general circulation model (AGCM) is used for climate change projections over the Antarctic. The present-day simulation uses prescribed observed sea surface conditions, while a set of five simulations for the end of the twenty-first century (2070-99) under the Special Report on Emissions Scenarios (SRES) A1B scenario uses sea surface condition anomalies from selected coupled ocean atmosphere climate models from phase 3 of the Coupled Model Intercomparison Project (CMIP3). Analysis of the results shows that the prescribed sea surface condition anomalies have a very strong influence on the simulated climate change on the Antarctic continent, largely dominating the direct effect of the prescribed greenhouse gas concentration changes in the AGCM simulations. Complementary simulations with idealized forcings confirm these results. An analysis of circulation changes using self-organizing maps shows that the simulated climate change on regional scales is not principally caused by shifts of the frequencies of the dominant circulation patterns, except for precipitation changes in some coastal regions. The study illustrates that in some respects the use of bias-corrected sea surface boundary conditions in climate projections with a variable-resolution atmospheric general circulation model has some distinct advantages over the use of limited-area atmospheric circulation models directly forced by generally biased coupled climate model output
Learning Pretopological Spaces for Lexical Taxonomy Acquisition
International audienceIn this paper, we propose a new methodology for semi-supervised acquisition of lexical taxonomies from a list of existing terms. Our approach is based on the theory of pretopology that offers a powerful formalism to model semantic relations and transform a list of terms into a structured term space by combining different discriminant criteria. In order to learn a parameterized pretopological space, we define the Learning Pretopological Spaces strategy based on genetic algorithms. The rare but accurate pieces of knowledge given by an expert (semi-supervision) or automatically extracted with existing linguistic patterns (auto-supervision) are used to parameterize the different features defining the pretopological term space. Then, a structuring algorithm is used to transform the pretopological space into a lexical taxonomy, i.e. a direct acyclic graph. Results over three standard datasets (two from WordNet and one from UMLS) evidence improved performances against existing associative and pattern-based state-of-the-art approaches
GATE : a simulation toolkit for PET and SPECT
Monte Carlo simulation is an essential tool in emission tomography that can
assist in the design of new medical imaging devices, the optimization of
acquisition protocols, and the development or assessment of image
reconstruction algorithms and correction techniques. GATE, the Geant4
Application for Tomographic Emission, encapsulates the Geant4 libraries to
achieve a modular, versatile, scripted simulation toolkit adapted to the field
of nuclear medicine. In particular, GATE allows the description of
time-dependent phenomena such as source or detector movement, and source decay
kinetics. This feature makes it possible to simulate time curves under
realistic acquisition conditions and to test dynamic reconstruction algorithms.
A public release of GATE licensed under the GNU Lesser General Public License
can be downloaded at the address http://www-lphe.epfl.ch/GATE/
ORCHIDEE-PEAT (revision 4596), a model for northern peatland CO2, water, and energy fluxes on daily to annual scales
Peatlands store substantial amounts of carbon and are vulnerable to climate change. We present a modified version of the Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface model for simulating the hydrology, surface energy, and CO2 fluxes of peatlands on daily to annual timescales. The model includes a separate soil tile in each 0.5 degrees grid cell, defined from a global peatland map and identified with peat-specific soil hydraulic properties. Runoff from non-peat vegetation within a grid cell containing a fraction of peat is routed to this peat soil tile, which maintains shallow water tables. The water table position separates oxic from anoxic decomposition. The model was evaluated against eddy-covariance (EC) observations from 30 northern peatland sites, with the maximum rate of carboxylation (V-cmax) being optimized at each site. Regarding short-term day-to-day variations, the model performance was good for gross primary production (GPP) (r(2) = 0.76; Nash-Sutcliffe modeling efficiency, MEF = 0.76) and ecosystem respiration (ER, r(2) = 0.78, MEF = 0.75), with lesser accuracy for latent heat fluxes (LE, r(2) = 0.42, MEF = 0.14) and and net ecosystem CO2 exchange (NEE, r(2) = 0.38, MEF = 0.26). Seasonal variations in GPP, ER, NEE, and energy fluxes on monthly scales showed moderate to high r(2) values (0.57-0.86). For spatial across-site gradients of annual mean GPP, ER, NEE, and LE, r(2) values of 0.93, 0.89, 0.27, and 0.71 were achieved, respectively. Water table (WT) variation was not well predicted (r(2) <0.1), likely due to the uncertain water input to the peat from surrounding areas. However, the poor performance of WT simulation did not greatly affect predictions of ER and NEE. We found a significant relationship between optimized V-cmax and latitude (temperature), which better reflects the spatial gradients of annual NEE than using an average V-cmax value.Peer reviewe
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