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Multi Scale VarEnKF Localisation using Wavelets

Andreas Rhodin (German Weather Service)

A new global forecast model (ICON, 13km) will become operational at the German Weather Service (DWD) in January 2015. ICON is formulated on an icosahedral grid with local refinements (6km) and replaces the current global (GME, 20km) and later on the local (COSMO-EU, 7km) model as well. The global 3D-Var data assimilation system is further developed towards a VarEnKF. For a variable-resolution model like ICON, multi-scale localisation and analysis is an issue.

Localisation usually is achieved by application of a localisation matrix (Schur product) do the ensemble background error covariance matrix in spatial representation. Multi-scale localisation is achieved by application of a localisation matrix in transformed representation. Wavelet transforms based on the lifting scheme are explored for this purpose.

The VarEnKF scheme under implementation at DWD is described and Multi-scale localisation issues are discussed.