Per Undén

TL;DR: In this article, the structure functions for the 3D-Var assimilation scheme of the European Centre for Medium-Range Weather Forecasts are evaluated from statistics of the differences between two forecasts valid at the same time.

TL;DR: Stratospheric humidity analyses produced operationally by the European Centre for Medium-Range Weather Forecasts (ECMWF) are discussed for the period since late January 1996 when the practice of resetting the upperlevel specific humidity to a fixed value at each analysis time was abandoned.

TL;DR: In this article, the authors assess the performance of the three-dimensional variational data assimilation scheme, in the light of the results from the extensive pre-operational program of numerical experimentation.

TL;DR: In this article, a comprehensive evaluation of the ECMWF operational data assimilation system has been made, based largely on statistics of its performance over a long period of time, and the evaluation has identified many deficiencies, covering such aspects of the analysis as the optimum interpolation statistics, the data selection algorithms and the quality control criteria applied within the analysis.

TL;DR: In this paper, the authors used the European Centre for Medium Range Weather Forecasts of three cases of explosive cyclogenesis of large magnitude that occurred in the North Atlantic during a 1-week period in January 1986, and results of numerical experiments performed on the three cases are described.