Wei, M., Z. Toth, R. Wobus, and Y. Zhu, 2006: Initial Perturbations Based on the Ensemble Transform (ET) Technique in the NCEP Global Ensemble Forecast System. NCEP Office Note 432, September 2006,

Tracton S., R. Kistler, Y. Zhu, J. Giraytys, 2001: NCEP/EMC observaing system experiment (OSE) on the effect of loss of Russian RAOB data. NCEP Office Note 432, May 2001