Data assimilation: Comparison and hybridization between ensemble and variational methods

The accuracy of numerical weather prediction (NWP) depends critically on the qualities of the initial conditions and the forecast model. The initial conditions of an NWP model usually come from data assimilation (DA), a procedure that aims to estimate the state and uncertainty of the atmosphere as accurately as possible by combining all available atmospheric information. In other words, DA is known as the process of creating the best estimate of the initial state for NWP models through combining all sources of information, including the first guess from previous short-term model forecasts and observations, along with the associated uncertainties in each source of information.