|Title||Direct 4D-Var assimilation of all-sky radiances. Part I: Implementation|
|Publication Type||Technical memorandum|
|Secondary Title||ECMWF Technical Memorandum|
|Authors||Bauer, P, Geer, AJ, Lopez, P, Salmond, D|
This paper describes a new radiance assimilation scheme for microwave imager observations which unifies the treatment of clear-sky, cloud and precipitation-affected situations, giving an all-sky' approach. The previous approach assimilated radiances in clear skies and retrievals of total column water vapour in clouds and rain. In March 2009, the new approach became operational in the Four-Dimensional Variational Assimilation (4D-Var) system of the European Centre for Medium-Range Weather Forecasts. This approach employs moist physics parameterizations and a full 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 where the observations are given the same weight as before, the all-sky system is able to produce 4D-Var analyses that are closer to the observations than the previous approach. Despite the nonlinearity of rain and cloud processes, 4D-Var minimises successfully through the use of an incremental technique. Overall the quality of the 4D-Var minimisation, in terms of number of iterations and conditioning, is unaffected by the new approach.