1. Institut für Theorie Elektromagnetischer Felder, Technische Universität, 64289 Darmstadt, Germany
2. Department of Electrical and Electronics Engineering, Dokuz Eylul University, 35160 Izmir, 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 Straβe 135, 63067 Offenbach, Germany