1. Institut für Theorie Elektromagnetischer Felder, Technische Universität Darmstadt, 64289 Darmstadt, Germany
2. Department of Electrical and Electronics Engineering, Dokuz Eylul University, 35160 İzmir, Turkey
3. Department of Mathematics, University of Reading, Whiteknights P.O. Box 220, Reading RG6 6AX, UK
4. German Meteorological Service, Deutscher Wetterdienst Research and Development, Head Division FE 12 (Data Assimilation) Frankfurter Straße 135, 63067 Offenbach, Germany