The solution: Ensemble Kalman Filtering
		1) Perturbed observations and ensembles of data assimilation • Evensen, 1994 • Houtekamer and Mitchell, 1998 2) Square root filter, no need for perturbed observations: • Tippett, Anderson, Bishop, Hamill, Whitaker, 2003 • Anderson, 2001 • Whitaker and Hamill, 2002 • Bishop, Etherton and Majumdar, 2001 3) Local Ensemble Kalman Filtering: done in local patches • Ott et al, 2003, MWR under review • Hunt et al, 2003, Tellus: 4DEnKF