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How much does simplification of probability forecasts reduce forecast quality?

  1. F. J. Doblas-Reyes1,*,
  2. C. A. S. Coelho2,
  3. D. B. Stephenson3

Article first published online: 20 MAR 2008

DOI: 10.1002/met.50

Issue

Meteorological Applications

Meteorological Applications

Special Issue: Forecast Verification

Volume 15, Issue 1, pages 155–162, March 2008

Additional Information

How to Cite

Doblas-Reyes, F. J., Coelho, C. A. S. and Stephenson, D. B. (2008), How much does simplification of probability forecasts reduce forecast quality?. Met. Apps, 15: 155–162. doi: 10.1002/met.50

Author Information

  1. 1

    European Centre for Medium-Range Weather Forecasts, Shinfield Park, Reading RG2 9AX, UK

  2. 2

    Centro de Previsão de Tempo e Estudos Climáticos, Rodovia Presidente Dutra, Km 40, SP-RJ 12630-000, Cachoeira Paulista, Brazil

  3. 3

    School of Engineering, Computing and Mathematics, University of Exeter, Harrison Building, North Park Road, Exeter EX4 4QF, UK

*European Centre for Medium-Range Weather Forecasts, Shinfield Park, Reading RG2 9AX, UK.

Publication History

  1. Issue published online: 20 MAR 2008
  2. Article first published online: 20 MAR 2008
  3. Manuscript Accepted: 2 JAN 2008
  4. Manuscript Revised: 18 DEC 2007
  5. Manuscript Received: 21 SEP 2007

Funded by

  • ENSEMBLES. Grant Number: GOCE-CT-2003-505539
  • Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP). Grant Numbers: 2005/05210-7, 2006/02497-6

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Keywords:

  • forecast quality;
  • probability forecasts;
  • ensemble forecasting

Abstract

Probability forecasts from an ensemble are often discretized into a small set of categories before being distributed to the users. This study investigates how such simplification can affect the forecast quality of probabilistic predictions as measured by the Brier score (BS). An example from the European Centre for Medium-Range Weather Forecasts (ECMWF) operational seasonal ensemble forecast system is used to show that the simplification of the forecast probabilities reduces the Brier skill score (BSS) by as much as 57% with respect to the skill score obtained with the full set of probabilities issued from the ensemble. This is more obvious for a small number of probability categories and is mainly due to a decrease in forecast resolution of up to 36%. The impact of the simplification as a function of the ensemble size is also discussed. The results suggest that forecast quality should be made available for the set of probabilities that the forecast user has access to as well as for the complete set of probabilities issued by the ensemble forecasting system. Copyright © 2008 Royal Meteorological Society

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