Analysis differences and error variance estimates from multi-centre analysis data

NWP forecast performance has made great progress during the past decade due to a few important factors. First, the numerical forecast models at major numerical weather prediction (NWP) centres have improved tremendously due to more accurate physics parametrisation schemes and increased computing power, which permits the use of higher resolution forecast models. Second, more observations, more accurate observing systems and improved data assimilation (DA) methods have been developed, such as 4D-Var (Rabier et al. 2000) and ensemble Kalman filters (Whitaker and Hamill 2002; Tippett et al. 2003; Whitaker et al. 2007). More accurate DA systems have played a key role in providing more accurate initial conditions for the NWP models, which have improved weather forecasts, particularly over the short and medium ranges. Analysis differences and error variance estimates from multi-centre analysis data