Section: New Results

Monitoring network design

In this section, we report studies that are related to the evaluation of monitoring network and to new monitoring strategies. As opposed to last year's report, they may not be strictly addressing optimal network design.

Eyjafjallajökull ash concentrations derived from both Lidar and modeling

Participants : Patrick Chazette [LSCE] , Marc Bocquet, Philippe Royer [LSCE] , Victor Winiarek, Jean-Chistophe Raut [ATMOS] , Philippe Labazuy [OPG/LMV] , Mathieu Gouhier [OPG/LMV] , Mélody Lardier [LEOSPHERE] , Jean-Pierre Cariou [LEOSPHERE] .

Following the eruption of the Icelandic volcano Eyjafjallajökull on April 14 2010, ground-based N2-Raman lidar (GBL) measurements were used to trace the temporal evolution of the ash plume from April 16 to April 20 2010, above the southwestern suburb of Paris. The nighttime overpass of the Cloud-Aerosol LIdar with Orthogonal Polarization onboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation satellite (CALIPSO/CALIOP), on April 17 2010, was an opportunity to complement GBL observations. The plume shape retrieved from GBL has been used to assess the size range of the particles size. The lidar-derived aerosol mass concentrations (PM) have been compared with model-derived PM concentrations held in the Eulerian model Polair3D transport model, driven by a source term inferred from the SEVIRI sensor on board the satellite Meteosat. The consistency between model and ground- based wind lidar and CALIOP observations has been checked. The spatial and temporal structures of the ash plume as estimated by each instrument and by the Polair3D simulations are in good agreement.

Potential of the International Monitoring System radionuclide network for inverse modeling

Participants : Mohammad Reza Koohkan, Marc Bocquet, Lin Wu, Monika Krysta [The Preparatory Commission for the Comprehensive Nuclear Test-Ban Treaty Organization] .

The International Monitoring System (IMS) radionuclide network enforces the Comprehensive Nuclear-Test-Ban Treaty, which bans nuclear explosions. We have evaluated the potential of the IMS radionuclide network for inverse modeling of the source, whereas it is usually assessed by its detection capability. To do so, we have chosen the Degrees of Freedom for the Signal (DFS), a well established criterion in remote sensing, in order to assess the performance of an inverse modeling system. Using a recent multiscale data assimilation technique, we have computed optimal adaptive grids of the source parameter space by maximizing the DFS. This optimization takes into account the monitoring network, the meteorology over one year (2009) and the relationship between the source parameters and the observations derived from the FLEXPART Lagrangian transport model. Areas of the domain where the grid-cells of the optimal adaptive grid are large emphasize zones where the retrieval is more uncertain, whereas areas where the grid-cells are smaller and denser stress regions where more source variables can be resolved.

The observability of the globe through inverse modeling is studied in strong, realistic and small model error cases. The strong error and realistic error cases yield heterogeneous adaptive grids, indicating that information does not propagate far from the monitoring stations, whereas in the small error case, the grid is much more homogeneous. In all cases, several specific continental regions remain poorly observed such as Africa as well as the tropics, because of the trade winds. The northern hemisphere is better observed through inverse modeling (more than 60% of the total DFS) mostly because it contains more IMS stations. This unbalance leads to a better performance of inverse modeling in the northern hemisphere winter. The methodology is also applied to the subnetwork composed of the stations of the IMS network that measure noble gases.

Optimal redistribution of the background ozone monitoring stations over France

Participants : Lin Wu, Marc Bocquet.

In this study, the BDQA background stations are partially redistributed over France under a set of design objectives, which are defined on a regular grid that covers France. Spatial interpolations are used to extrapolate simulated concentrations (of chemistry-transport models or assimilation results) to these grid nodes. Three types of criteria are considered: the geostatistical, geometrical, and physical ones. Simulated annealing is employed to select optimally the stations. Significant improvement with all the proposed criteria has been found for the optimally redistributed network against the original background BDQA network. For complex objectives, e.g. that addressing the heterogeneity of ozone field, the physical criteria are more appropriate.