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On‐line calibration of the vertical distribution of ensemble spread

The Met Office Global and Regional Ensemble Prediction System (MOGREPS) uses an on‐line inflation factor calculation to calibrate the spread of the ensemble in space and time and counteract the tendency of the Ensemble Transform Kalman Filter (ETKF) to underestimate analysis uncertainty. Sonde measurements are augmented with ATOVS brightness temperature observations to provide sufficient coverage across the globe. This paper describes how the scheme has been extended to improve the vertical distribution of spread. Separate inflation factors are derived for each of three ‘bands’ of model levels, loosely representing the boundary layer, troposphere and stratosphere. A continuous profile is ensured by linearly interpolating between the band centres. The error/spread relationship observed in each ATOVS channel is mapped onto the three bands using a set of weights obtained by integrating the appropriate Jacobians over the range of pressures covered by each band. These weights are found to be approximately constant, thanks to the small number of bands and the fact that near‐surface channels are not used over land. The revised initial perturbations lead to increased near‐surface spread which persists throughout the 72 h forecast and carries through to improved probabilistic scores. However, the near‐surface spread remains too small due to insufficient growth of spread by the forecast model. This suggests perturbation of sea‐surface temperatures, soil initial conditions, soil properties and boundary‐layer physics as priorities for future work.