Section: Software

Polyphemus

Participants : Vivien Mallet, Pierre Tran, Damien Garaud, Anne Tilloy.

Polyphemus (see the web site http://cerea.enpc.fr/polyphemus/ ) is a modeling system for air quality. As such, it is designed to yield up-to-date simulations in a reliable framework: data assimilation, ensemble forecast and daily forecasts. Its completeness makes it suitable for use in many applications: photochemistry, aerosols, radionuclides, etc. It is able to handle simulations from local to continental scales, with several physical models. It is divided into three main parts:

• libraries that gather data processing tools (SeldonData), physical parameterizations (AtmoData) and postprocessing abilities (AtmoPy);

• programs for physical preprocessing and chemistry-transport models (Polair3D, Castor, two Gaussian models, a Lagrangian model);

• drivers on top of the models in order to implement advanced simulation methods such as data assimilation algorithms.

Figure 1 depicts a typical result produced by Polyphemus. Clime is involved in the overall design of the system and in the development of advanced methods in model coupling, data assimilation and ensemble forecast (through drivers and post-processing).

In 2011, Polyphemus was extended for a better integration with the data assimilation library Verdandi. A first (unstable) version of Polyphemus with a complete overhaul of the input/output operations and of the configuration files was provided to the developers. The derivative of Polyphemus that is used at IRSN was used for the first time in a crisis context in order to simulate the transport of radionuclides during the Fukushima nuclear disaster.

Figure 1. Map of the relative standard deviation (or spread, %) of an ensemble built with Polyphemus (ozone simulations, $\mu g{m}^{-3}$). The standard deviations are averaged over the summer of 2001. They provide an estimation of the simulation uncertainties.