Meteorological Applications, 15, 155-162

How much does simplification of probability forecasts reduce forecast quality?

Francisco J. Doblas-Reyes
ECMWF, Shinfield Park
RG2 9AX, Reading, UK

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

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

Probability forecasts 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. An example from the ECMWF operational seasonal ensemble forecast system is used to show that the simplification of the forecast probabilities reduces the Brier skill score by as much as 57% with respect to the skill score obtained with the full set of probabilities issued, especially for a small number of probability categories. This is mainly due to a decrease in forecast resolution of up to 36%. It is therefore important that forecast quality is 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 forecasting system.

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