Publications

2010

• Modélisation du brouillard durant la campagne ParisFog : approche prédictive et étude de l'effet des hétérogénéités spatiales
  
  • Zhang Xiaojing
  , 2010. Dans le domaine de la modélisation du brouillard ou des nuages bas, la compréhension précise des interactions complexes entre la turbulence, la microphysique, et le rayonnement reste un enjeu majeur pour améliorer la prévision numérique. Si l'amélioration de la modélisation du brouillard est un enjeu important pour la prévision dans le domaine du transport, elle l'est aussi dans le domaine industriel en raison de ses rejets atmosphériques (aéroréfrigérants industriels, brouillards pollués . . . ). La version 1D de Code_Saturne a été utilisée pour la simulation du brouillard sur les données de la campagne ParisFog réalisée sur le site du SIRTA pendant l'hiver 2006-2007. La simulation des cas observés a permis de montrer que le modèle était capable de reproduire correctement les principaux processus mis en jeu dans le brouillard depuis sa formation jusqu'à sa dissipation. Une étude de sensibilité aux différentes paramétrisations physiques choisies a montré que la dynamique du brouillard est particulièrement sensible à la fermeture turbulente, la teneur en eau à la sédimentation, et le spectre des gouttes au schéma de nucléation. La mise en oeuvre de Code_Saturne 1D sur une longue période en mode prédictif a montré la robustesse de nos choix et l'apport du couplage par nudging avec un modèle de méso-échelle à 36 heures d'échéance. La version 3D de Code_Saturne nous a permis d'étudier l'effet des hétérogénéités spatiales du site SIRTA sur la formation du brouillard. Dans un premier temps, les calculs ont été effectués sur le site considéré comme homogène horizontalement, sans la prise en compte des hétérogénéités. Il s'agissait ici de comparer la version 3D de Code_Saturne en mode RANS avec l'ensemble des paramétrisations physiques concernant le brouillard avec la version 1D. Ensuite, les effets de la prise en compte de la rugosité, des zones boisées et des bâtiments ont été étudiés.
• Automatic generation of large ensembles for air quality forecasting using the Polyphemus system
  
  • Garaud Damien
  • Mallet Vivien
  Geoscientific Model Development, European Geosciences Union, 2010, 3 (1), pp.69-85. This paper describes a method to automatically generate a large ensemble of air quality simulations. Such an ensemble may be useful for quantifying uncertainty, improving forecasts, evaluating risks, identifying process weaknesses, etc. The objective is to take into account all sources of uncertainty: input data, physical formulation and numerical formulation. The leading idea is to build different chemistry-transport models in the same framework, so that the ensemble generation can be fully controlled. Large ensembles can be generated with a Monte Carlo simulations that address at the same time the uncertainties in the input data and in the model formulation. This is achieved using the Polyphemus system, which is flexible enough to build various different models. The system offers a wide range of options in the construction of a model: many physical parameterizations, several numerical schemes and different input data can be combined. In addition, input data can be perturbed. In this paper, some 30 alternatives are available for the generation of a model. For each alternative, the options are given a probability, based on how reliable they are supposed to be. Each model of the ensemble is defined by randomly selecting one option per alternative. In order to decrease the computational load, as many computations as possible are shared by the models of the ensemble. As an example, an ensemble of 101 photochemical models is generated and run for the year 2001 over Europe. The models' performance is quickly reviewed, and the ensemble structure is analyzed. We found a strong diversity in the results of the models and a wide spread of the ensemble. It is noteworthy that many models turn out to be the best model in some regions and some dates. (10.5194/gmd-3-69-2010)
  DOI : 10.5194/gmd-3-69-2010
• Assessment of the detection abilities of monitoring networks for passive tracers at local and regional scales
  
  • Korsakissok I.
  • Mallet Vivien
  • Sportisse Bruno
  Atmospheric Environment, Elsevier, 2010, 44 (3), pp.408-420. We propose a method to evaluate the detection abilities of networks used for protection purposes. Such networks are designed for the detection of nuclear, biological or gaseous emissions, without constraint on the source location. Their assigned goal is to have the best chance to detect a threatening emission located anywhere in the vicinity of a domain to protect. Two sensors siting applications are addressed: sensors placed in the surroundings of a facility to protect, or sensors carried by people scattered within a small area. A network protection ability is related both to its detection scope, and to its response time. To assess the performance of such networks, two statistical indicators are therefore designed: the detection probability, computed on a large number of possible source locations, and the saturation time, which is the time when the maximum detection probability has been reached. Simulations are then carried out with the Polyphemus air quality modeling system for many emission scenarios, including 961 possible source locations, various emitted species, and a few representative meteorological situations. This allows to assess the performance of single sensors as well as full networks, and their sensitivity to parameters like meteorological conditions and source characteristics. The emitted quantity and meteorological dispersion are found to be important parameters, whereas the species type does not significantly influence the results. Two network design methods are considered: (1) networks composed of a given number of the “best” sensors according to an indicator, and (2) sensors placed in circles around the protected domain. The networks built with respect to the detection probability show good results with a limited number of sensors, while the saturation time is not reliable enough to build networks. The networks based on circles also show a good performance in the studied cases, provided there is a sufficient number of sensors. (10.1016/j.atmosenv.2009.10.022)
  DOI : 10.1016/j.atmosenv.2009.10.022
• Meteorology and air-quality in a mega-city: application to Tehran, Iran
  
  • Malakooti Hossein
  , 2010. The influence of a mega-city on the atmospheric boundary layer and surface conditions was examined in the complex-terrain, semi-arid Tehran region using the Pennsylvania State University/National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) during a high pollution period. In addition, model sensitivity studies were conducted to evaluate the performance of the urban canopy and urban soil model "SM2-U (3D)" parameterization on the meteorological fields and ground level air pollutant concentrations in this area. The topographic flows and urban effects were found to play important roles in modulating the wind and temperature fields, and the urbanized areas exerted important local effects on the boundary layer meteorology. An emission inventory of air pollutants and an inventory of heat generation were developed and updated for 2005 in this work. Emissions from on-road motor vehicles constitute a major portion of the emission inventory and play the most important role in terms of contributions of air pollutants to the atmosphere in Tehran. By using a detailed methodology, we calculated spatial and temporal distributions of the anthropogenic heat flux (Qf) for Tehran during 2005. Wintertime Qf is larger than summertime Qf, which reflects the importance of heating emissions from buildings and traffic during cold and warm period respectively. Different urban parameterizations were used as a tool to investigate the modifications induced by the presence of an urban area in the area of interest. It was found that, for local meteorological simulations, the drag-force approach coupled with an urban soil model (DA-SM2-U) is preferable to the roughness approach (RA-SLAB). The comparisons indicated that the most important features of the wind, temperature and turbulent fields in urban areas are well reproduced by the DA-SM2-U configuration with the anthropogenic heat flux being taken into account (i.e., "DA-SM2-U Qf: On" option). This modeling option showed that the suburban part of the city is dominated by topographic flows whereas the center and south of Tehran are more affected by urban heat island (UHI) forcing especially during the night. The chemical transport modeling, including a model sensitivity study, was used to investigate the impact of the different urban parameterization on the dispersion and formation of pollutants over the Tehran region. Results show that applying DA approaches leads to significant improvements in the simulated spatial and temporal distribution of air pollutant concentrations in the city area and affects significantly the size of the urban plumes.
• Optimal Reduction of the Ozone Monitoring Network over France
  
  • Wu Lin
  • Bocquet Marc
  • Chevallier Matthieu
  Atmospheric Environment, Elsevier, 2010 (44). Ozone is a harmful air pollutant at ground level, and its concentrations are measured with routine monitoring networks. Due to the heterogeneous nature of ozone fields, the spatial distribution of the ozone concentration measurements is very important. Therefore, the evaluation of distributed monitoring networks is of both theoretical and practical interests. In this study, we assess the efficiency of the ozone monitoring network over France (BDQA) by investigating a network reduction problem. We examine how well a subset of the BDQA network can represent the full network. The performance of a subnetwork is taken to be the root mean square error (RMSE) of the hourly ozone mean concentration estimations over the whole network given the observations from that subnetwork. Spatial interpolations are conducted for the ozone estimation taking into account the spatial correlations. Several interpolation methods, namely ordinary kriging, simple kriging, kriging about the means, and consistent kriging about the means, are compared for a reliable estimation. Exponential models are employed for the spatial correlations. It is found that the statistical information about the means improves significantly the kriging results, and that it is necessary to consider the correlation model to be hourly-varying and daily stationary. The network reduction problem is solved using a simulated annealing algorithm. Significant improvements can be obtained through these optimizations. For instance, removing optimally half the stations leads to an estimation error of the order of the standard observational error (10 μgm−3). The resulting optimal subnetworks are dense in urban agglomerations around Paris (Île-de-France) and Nice (Côte d'Azur), where high ozone concentrations and strong heterogeneity are observed. The optimal subnetworks are probably dense near frontiers because beyond these frontiers there is no observation to reduce the uncertainty of the ozone field. For large rural regions, the stations are uniformly distributed. The fractions between urban, suburban and rural stations are rather constant for optimal subnetworks of larger size (beyond 100 stations). By contrast, for smaller subnetworks, the urban stations dominate. (10.1016/j.atmosenv.2010.04.012)
  DOI : 10.1016/j.atmosenv.2010.04.012
• Cross-hemispheric transport of central African biomass burning pollutants: implications for downwind ozone production
  
  • Real Elsa
  • Orlandi E.
  • Law Kathy S.
  • Fierli Federico
  • Josset Damien
  • Cairo F.
  • Schlager H.
  • Borrmann S.
  • Kunkel D.
  • Volk C. M.
  • Mcquaid J. B.
  • Stewart D. J.
  • Lee J. R.
  • Lewis A.
  • Hopkins J. R.
  • Ravegnani F.
  • Ulanovski A.
  • Liousse C.
  Atmospheric Chemistry and Physics, European Geosciences Union, 2010, 10 (6), pp.3027-3046. Pollutant plumes with enhanced concentrations of trace gases and aerosols were observed over the southern coast of West Africa during August 2006 as part of the AMMA wet season field campaign. Plumes were observed both in the mid and upper troposphere. In this study we examined the origin of these pollutant plumes, and their potential to photochemically produce ozone (O3) downwind over the Atlantic Ocean (10.5194/acp-10-3027-2010)
  DOI : 10.5194/acp-10-3027-2010
• PARISFOG: Shedding New Light on Fog Physical Processes
  
  • Haeffelin M.
  • Bergot Thierry
  • Elias T.
  • Tardiff R.
  • Carrer Dominique
  • Chazette P.
  • Colomb M.
  • Drobinski P.
  • Dupont E.
  • Dupont J.-C.
  • Gomes Laurent
  • Musson-Genon L.
  • Pietras C.
  • Plana-Fattori Artemio
  • Protat Alain
  • Rangognio Jerome
  • Raut Jean-Christophe
  • Rémy S.
  • Richard D.
  • Sciare J.
  • Zhang X.
  Bulletin of the American Meteorological Society, American Meteorological Society, 2010, 91, pp.767-783. Fog is a weather condition with significant socioeconomic impacts associated with increased hazards and constraints in road, maritime, and air traffic. While current numerical weather prediction models are able to forecast situations that are favorable to fog events, it is very difficult to determine the exact location and time of formation or dissipation. One-dimensional assimilation-forecast models have been implemented at a few airports and provide improved local predictions of fog events, but this approach is limited to specific locations. The occurrence, development, and dissipation of fog result from multiple processes (thermodynamical, radiative, dynamical, microphysical) that occur simultaneously, through a wide range of conditions, and that interact nonlinearly with each other. Hence, to advance our ability to forecast fog processes, we must gain a better understanding of how critical physical processes feed back on each other and improve their parametric representations in models. To provide a dataset suitable to study these processes simultaneously in continental fog, a suite of active and passive remote sensing instruments and in situ sensors was deployed at the Site Instrumental de Recherche en Télédétection Atmosphérique (SIRTA; instrumented site for atmospheric remote sensing research) observatory, near Paris, France, for 6 months (winter 2006/07) to monitor profiles of wind, turbulence, microphysical, and radiative properties as well as temperature, humidity, aerosol, and fog droplet microphysics and chemical composition in the surface layer. This field experiment, called ParisFog, provides a comprehensive characterization of over 100 fog and near-fog events. The ParisFog dataset contains contrasted events of stratus-lowering fog and radiative cooling fog as well as a large number of situations considered as favorable to fog formation but where fog droplets did not materialize. The effect of hydrated aerosols on visibility, the role of aerosols' microphysical and chemical properties on supersaturation and droplet activation, and the role of turbulence and sedimentation on fog life cycles have been investigated using the ParisFog dataset. The interactions between these processes, however, remain to be explored. (10.1175/2009BAMS2671.1)
  DOI : 10.1175/2009BAMS2671.1
• Impact of dry deposition of semi-volatile organic compounds on secondary organic aerosols
  
  • Bessagnet Bertrand
  • Seigneur Christian
  • Menut Laurent
  Atmospheric Environment, Elsevier, 2010, 44, pp.1781-1787. Dry deposition of semi-volatile organic compounds (SVOC) is not currently treated in most chemical transport models of air quality and this omission has been identified as a possible major source of uncertainty. The effect of dry deposition of SVOC on the concentration of secondary organic aerosols (SOA) is investigated in summertime with the chemical transport model CHIMERE that simulates SOA concentrations by means of molecular SOA surrogate species. Omitting dry deposition could overestimate SOA concentrations by as much as 50%. This overestimation is larger during nighttime due to higher relative humidity. (10.1016/j.atmosenv.2010.01.027)
  DOI : 10.1016/j.atmosenv.2010.01.027
• Diagnosis and impacts of non-Gaussianity of innovations in data assimilation
  
  • Pires Carlos
  • Talagrand Olivier
  • Bocquet Marc
  Physica D: Nonlinear Phenomena, Elsevier, 2010, 239. The Best Linear Unbiased Estimator (BLUE) has widely been used in atmospheric and oceanic data assimilation (DA). However, when the errors from data (observations and background forecasts) have non-Gaussian probability density functions (pdfs), the BLUE diers from the absolute Minimum Variance Unbiased Estimator (MVUE), minimizing the mean square a posteriori error. The non-Gaussianity of errors can be due to the inherent statistical skewness and positiveness of some physical observables (e.g. moisture, chemical species) or because of the nonlinearity of the data assimilation models and observation operators acting on Gaussian errors. Non-Gaussianity of assimilated data errors can be justied from a priori hypotheses or inferred from statistical diagnostics of innovations (observation minus background). ollowing this rationale, we compute measures of innovation non-Gaussianity, namely its skewness and kurtosis, relating it to: a) the non-Gaussianity of the individual error themselves, b) the correlation between nonlinear functions of errors, and c) the heteroscedasticity of errors within diagnostic samples. Those relationships impose bounds for skewness and kurtosis of errors which are critically dependent on the error variances, thus leading to a necessary tuning of error variances in order to accomplish consistency with innovations. We evaluate the sub-optimality of the BLUE as compared to the MVUE, in terms of excess of error variance, under the presence of non-Gaussian errors. The error pdfs are obtained by the maximum entropy method constrained by error moments up to fourth order, from which the Bayesian probability density function and the MVUE are computed. The impact is higher for skewed extreme innovations and grows in average with the skewness of data errors, especially if those skewnesses have the same sign. Application has been performed to the quality-accepted ECMWF innovations of brightness temperatures of a set of High Resolution Infrared Sounder (HIRS) channels. In this context, the MVUE has led in some extreme cases to a potential reduction of 20-60% error variance as compared to the BLUE. (10.1016/j.physd.2010.05.006)
  DOI : 10.1016/j.physd.2010.05.006
• Air mass origins influencing TTL chemical composition over West Africa during 2006 summer monsoon
  
  • Law Kathy S.
  • Fierli Federico
  • Cairo F.
  • Schlager H.
  • Borrmann S.
  • Streibel M.
  • Real Elsa
  • Kunkel D.
  • Schiller C.
  • Ravegnani F.
  • Ulanovsky A.
  • d'Amato F.
  • Viciani S.
  • Volk C. M.
  Atmospheric Chemistry and Physics, European Geosciences Union, 2010, 10 (22), pp.10753-10770. Trace gas and aerosol data collected in the tropical tropopause layer (TTL) between 12–18.5 km by the M55 Geophysica aircraft as part of the SCOUT-AMMA campaign over West Africa during the summer monsoon in August 2006 have been analysed in terms of their air mass origins. Analysis of domain filling back trajectories arriving over West Africa, and in the specific region of the flights, showed that the M55 flights were generally representative of air masses arriving over West Africa during the first 2 weeks of August, 2006. Air originating from the mid-latitude lower stratosphere was under-sampled (in the mid-upper TTL) whilst air masses uplifted from central Africa (into the lower TTL) were over-sampled in the latter part of the campaign. Signatures of recent (previous 10 days) origins were superimposed on the large-scale westerly flow over West Africa. In the lower TTL, air masses were impacted by recent local deep convection over Africa at the level of main convective outflow (350 K, 200 hPa) and on certain days up to 370 K (100 hPa). Estimates of the fraction of air masses influenced by local convection vary from 10 to 50% depending on the method applied and from day to day during the campaign. The analysis shows that flights on 7, 8 and 11 August were more influenced by local convection than on 4 and 13 August allowing separation of trace gas and aerosol measurements into ''convective'' and ''non-convective'' flights. Strong signatures, particularly in short-lived species like CO, NO and fine-mode aerosols were seen during flights most influenced by convection up to 350–365 K. Observed profiles were also constantly perturbed by uplift (as high as 39%) of air masses from the mid to lower troposphere over Asia, India, and oceanic regions resulting in import of clean oceanic (e.g., O3-poor) or polluted air masses from Asia (high O3, CO, CO2) into West Africa. Thus, recent uplift of CO2 over Asia may contribute to the observed positive CO2 gradients in the TTL over West Africa. This suggests a more significant fraction of younger air masses in the TTL making it difficult to derive mean age of air from average gradients. Transport of air masses from the mid-latitude lower stratosphere had an impact from the mid-TTL upwards (20–40% above 370 K) during the campaign period importing air masses with high O3 and NOy. Ozone profiles show a less pronounced lower TTL minimum than observed previously by regular ozonesondes at other tropical locations. Concentrations are less than 100 ppbv in the lower TTL and vertical gradients less steep than in the upper TTL. The air mass origin analysis and simulations of in-situ net photochemical O3 production, initialised with observations, suggest that the lower TTL is significantly impacted by uplift of O3 precursors (over Africa and Asia) leading to positive production rates (up to 2 ppbv per day) in the lower and mid TTL even at moderate NOx levels. Photochemical O3 production increases with higher NOx and H2O in air masses with O3 less than 150 ppbv. (10.5194/acp-10-10753-2010)
  DOI : 10.5194/acp-10-10753-2010
• Some attempts to couple distinct fluid models
  
  • Hérard Jean-Marc
  • Hurisse Olivier
  Networks and Heterogeneous Media, American Institute of Mathematical Sciences, 2010, 5 (3), pp.649-660. We present in this paper a review of some recent works dedicated to the numerical interfacial coupling of fluid models. One main motivation of the whole approach is to provide some meaningful methods and tools in order to compute unsteady patterns, while using distinct existing CFD codes in the nuclear industry. Thus, the main objective is to derive suitable boundary conditions for the codes to be coupled. A first section is devoted to a review of some attempts to couple: (i) 1D and 3D codes, (ii) distinct homogeneous two-phase flow models, (iii) fluid and porous models. More details on numerical procedures described in this section can be found in companion papers. Then we detail in a second section a way to couple a two-fluid hyperbolic model and an homogeneous relaxation model. (10.3934/nhm.2010.5.649)
  DOI : 10.3934/nhm.2010.5.649
• Corrigendum to "Air mass origins influencing TTL chemical composition over West Africa during 2006 summer monsoon" published in Atmos. Chem. Phys., 10, 10753–10770, 2010
  
  • Law Kathy S.
  • Fierli Federico
  • Cairo F.
  • Schlager H.
  • Borrmann S.
  • Streibel M.
  • Real Elsa
  • Kunkel D.
  • Schiller C.
  • Ravegnani F.
  • Ulanovsky A.
  • d'Amato F.
  • Viciani S.
  • Volk C. M.
  Atmospheric Chemistry and Physics, European Geosciences Union, 2010, 10 (22), pp.10939-10940. A problem occurred in the final production process regarding Fig. 10b. Please find the correct Fig.10b (10.5194/acp-10-10939-2010)
  DOI : 10.5194/acp-10-10939-2010