Medium Range Lagged Average Forecasts

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Abstract
In this work we report the application of the lagged average forecasting (LAF) technique to operational forecasts of the ECMWF. The ECMWF data consist of two 100-day samples of 10-day forecasts of 500 mb geopotential height for winter 1980/81 and summer 1981. The LAF ensemble includes the latest operational forecast, and also forecasts for the same verification time stated one or more days earlier than the latest one. We focus on the following two issues: 1) Does ensemble averaging improve forecast skill and 2) Is the dispersion of the ensemble useful in predicting forecast skill. We used the LAF technique to produce 3, 5, 7, 8 and 9 day forecasts of the 500 mb height field. The results show the statistically filtered LAF is a marked improvement upon the operational forecast after 5 days. We find that on a global scale forecast skill is weakly correlated with the dispersion of the ensemble, as measured by the rms difference between the operational forecast and the statistically filtered LAF.

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