Publications

2017

• Online learning with the Continuous Ranked Probability Score for ensemble forecasting
  
  • Thorey J.
  • Mallet V.
  • Baudin P.
  Quarterly Journal of the Royal Meteorological Society, Wiley, 2017, 143 (702), pp.521-529. Ensemble forecasting resorts to multiple individual forecasts to produce a discrete probability distribution which accurately represents the uncertainties. Before every forecast, a weighted empirical distribution function is derived from the ensemble, so as to minimize the Continuous Ranked Probability Score (CRPS). We apply online learning techniques, which have previously been used for deterministic forecasting, and we adapt them for the minimization of the CRPS. The proposed method theoretically guarantees that the aggregated forecast competes, in terms of CRPS, against the best weighted empirical distribution function with weights constant in time. This is illustrated on synthetic data. Besides, our study improves the knowledge of the CRPS expectation for model mixtures. We generalize results on the bias of the CRPS computed with ensemble forecasts, and propose a new scheme to achieve fair CRPS minimization , without any assumption on the distributions. (10.1002/qj.2940)
  DOI : 10.1002/qj.2940
• Large eddy simulation of radiation fog: impact of dynamics on the fog life cycle
  
  • Mazoyer Marie
  • Lac Christine
  • Thouron Odile
  • Bergot Thierry
  • Masson Valéry
  • Musson-Genon Luc
  Atmospheric Chemistry and Physics, European Geosciences Union, 2017, 17 (21), pp.13017-13035. Large eddy simulations (LESs) of a radiation fog event occurring during the ParisFog experiment are studied with a view to analyse the impact of the dynamics of the boundary layer on the fog life cycle. The LES, performed with the Meso-NH model at 5m resolution horizontally and 1m vertically, and with a 2-moment microphysical scheme, includes the drag effect of a tree barrier and the deposition of droplets on vegetation. The model shows good agreement with measurements of near-surface dynamic and thermodynamic parameters and liquid water path. The blocking effect of the trees induces elevated fog formation, as actually observed, and horizontal heterogeneities during the formation. It also limits cooling and cloud water production. Deposition is found to exert the most significant impact on fog prediction as it not only erodes the fog near the surface but also modifies the fog life cycle and induces vertical heterogeneities. A comparison with the 2m horizontal resolution simulation reveals small differences, meaning that grid convergence is achieved. Conversely, increasing numerical diffusion through a wind advection operator of lower order leads to an increase in the liquid water path and has a very similar effect to removing the tree barrier. This study allows us to establish the major dynamical ingredients needed to accurately represent the fog life cycle at very high-resolution. (10.5194/acp-17-13017-2017)
  DOI : 10.5194/acp-17-13017-2017
• Statistical atmospheric inversion of local gas emissions by coupling the tracer release technique and local-scale transport modelling: a test case with controlled methane emissions
  
  • Ars Sébastien
  • Broquet Grégoire
  • Yver Kwok Camille
  • Roustan Yelva
  • Wu Lin
  • Arzoumanian Emmanuel
  • Bousquet Philippe
  Atmospheric Measurement Techniques, European Geosciences Union, 2017, 10 (12), pp.5017 - 5037. This study presents a new concept for estimating the pollutant emission rates of a site and its main facilities using a series of atmospheric measurements across the pollutant plumes. This concept combines the tracer release method, local-scale atmospheric transport modelling and a statistical atmospheric inversion approach. The conversion between the controlled emission and the measured atmospheric concentrations of the released tracer across the plume places valuable constraints on the atmospheric transport. This is used to optimise the configuration of the transport model parameters and the model uncertainty statistics in the inversion system. The emission rates of all sources are then inverted to optimise the match between the concentrations simulated with the transport model and the pollutants' measured atmospheric concentrations, accounting for the transport model uncertainty. In principle, by using atmospheric transport modelling, this concept does not strongly rely on the good colocation between the tracer and pollutant sources and can be used to monitor multiple sources within a single site, unlike the classical tracer release technique. The statistical inversion framework and the use of the tracer data for the configuration of the transport and inversion modelling systems should ensure that the transport modelling errors are correctly handled in the source estimation. The potential of this new concept is evaluated with a relatively simple practical implementation based on a Gaussian plume model and a series of inversions of controlled methane point sources using acetylene as a tracer gas. The experimental conditions are chosen so that they are suitable for the use of a Gaussian plume model to simulate the atmospheric transport. In these experiments, different configurations of methane and acetylene point source locations are tested to assess the efficiency of the method in comparison to the classic tracer release technique in coping with the distances between the different methane and acetylene sources. The results from these controlled experiments demonstrate that, when the targeted and tracer gases are not well collocated, this new approach provides a better estimate of the emission rates than the tracer release technique. As an example, the relative error between the estimated and actual emission rates is reduced from 32 % with the tracer release technique to 16 % with the combined approach in the case of a tracer located 60 m upwind of a single methane source. Further studies and more complex implementations with more advanced transport models and more advanced optimisations of their configuration will be required to generalise the applicability of the approach and strengthen its robustness. (10.5194/amt-10-5017-2017)
  DOI : 10.5194/amt-10-5017-2017
• Modelling organic aerosol concentrations and properties during ChArMEx summer campaigns of 2012 and 2013 in the western Mediterranean region
  
  • Chrit Mounir
  • Sartelet Karine
  • Sciare Jean
  • Pey Jorge
  • Marchand Nicolas
  • Couvidat Florian
  • Sellegri Karine
  • Beekmann Matthias
  Atmospheric Chemistry and Physics, European Geosciences Union, 2017, 17 (20), pp.12509-12531. In the framework of the Chemistry-Aerosol Mediterranean Experiment, a measurement site was set up at a remote site (Ersa) on Corsica Island in the northwestern Mediterranean Sea. Measurement campaigns performed during the summers of 2012 and 2013 showed high organic aerosol concentrations, mostly from biogenic origin. This work aims to represent the organic aerosol concentrations and properties (oxidation state and hydrophilicity) using the air-quality model Polyphemus with a surrogate approach for secondary organic aerosol (SOA) formation. Biogenic precursors are isoprene, monoterpenes and sesquiterpenes. In this work, the following model oxidation products of monoterpenes are added: (i) a carboxylic acid (MBTCA) to represent multi-generation oxidation products in the low-NOx regime, (ii) organic nitrate chemistry and (iii) extremely low-volatility organic compounds (ELVOCs) formed by ozonolysis. The model shows good agreement of measurements of organic concentrations for both 2012 and 2013 summer campaigns. The modelled oxidation property and hydrophilic organic carbon properties of the organic aerosols also agree reasonably well with the measurements. The influence of the different chemical processes added to the model on the oxidation level of organics is studied. Measured and simulated water-soluble organic carbon (WSOC) concentrations show that even at a remote site next to the sea, about 64 % of the organic carbon is soluble. The concentrations of WSOC vary with the origins of the air masses and the composition of organic aerosols. The marine organic emissions only contribute to a few percent of the organic mass in PM$_1$, with maxima above the sea. (10.5194/acp-17-12509-2017)
  DOI : 10.5194/acp-17-12509-2017
• A sensitivity study of road transportation emissions at metropolitan scale
  
  • Chen Ruiwei
  • Aguilera Vincent
  • Mallet Vivien
  • Cohn Florian
  • Poulet David
  • Brocheton Fabien
  Journal of Earth Sciences and Geotechnical Engineering, Scienpress LTD, 2017, 7 (1). Road traffic transportation emissions depend on both the traffic flow and the vehicles emission factors. They are very sensitive to the input data of both traffic assignment models and emissions calculation methods. In this study, we investigate the influences of the input data on a simulation chain from traffic flow assignment to emissions calculations, based on a case study in the agglomeration of Clermont- Ferrand in France. In order to better represent the congestion phenomenon and the temporal and spatial evolution of traffic flow, we use a dynamic traffic assignment model, LADTA, to compute the traffic flow at street resolution. The model is evaluated by comparison between model predictions and traffic flow observations captured by inductive loop traffic detectors. The traffic flow outputs of LADTA are then coupled with COPERT model to calculate emissions of air pollutants (nitrogen oxides NOx for example). A sensitivity study is then carried out by varying the input parameters in the traffic emission modeling chain: the total traffic volume injected into the network, the average speed, the vehicle fleet composition, etc. The study shows that the emissions are very sensitive to these factors, especially during the transition from a free traffic network to a congested one.