Direct 4D-Var assimilation of all-sky radiances. Part I: Implementation

This article describes a new radiance-assimilation scheme for microwave-imager observations, which unifies the treatment of clear-sky, cloudy and precipitation-affected situations, giving an all-sky approach. This became operational in the four-dimensional variational assimilation (4D-Var) system of the European Centre for Medium-Range Weather Forecasts in March 2009, replacing a previous approach that assimilated radiances in clear skies and 1D-Var retrievals of total column water vapour in clouds and precipitation. The new approach employs moist-physics parametrizations and a multiple-scattering radiative-transfer model in the observation operator for all microwave-imager observations. Observation-operator accuracy, observation-error definition and bias correction, basic observational impact, 4D-Var linearity and stability as well as computational cost are described. Because of careful quality control and relatively large observation errors, the all-sky system produces a weaker observational constraint on moisture analysis than the previous system. However, in single-observation experiments in precipitating areas, using the same observation errors as in the previous 1D-Var retrieval approach, the all-sky system is able to produce 4D-Var analyses that are slightly closer to the observations than before. Despite the nonlinearity of rain and cloud processes, 4D-Var minimizes successfully through the use of an incremental technique. Overall, the quality of the 4D-Var minimization, in terms of number of iterations and conditioning, is unaffected by the new approach. Copyright (C) 2010 Royal Meteorological Society