Spread Calibration of Ensemble MOS Forecasts

Ensemble forecasting systems often contain systematic biases and spread deficiencies that can be corrected by statistical postprocessing. This study presents an improvement to an ensemble statistical postprocessing technique, called ensemble kernel density model output statistics (EKDMOS). EKDMOS uses model output statistics (MOS) equations and spread-skill relationships to generate calibrated probabilistic forecasts. The MOS equations are multiple linear regression equations developed by relating observations to ensemble mean-based predictors. The spread-skill relationships are one-term linear regression equations that predict the expected accuracy of the ensemble mean given the ensemble spread. To generate an EKDMOS forecast, the MOS equations are applied to each ensemble member. Kernel density fitting is used to create a probability density function (PDF) from the ensemble MOS forecasts. The PDF spread is adjusted to match the spread predicted by the spread-skill relationship, producing a calibrated forecast. The improved EKDMOS technique was used to produce probabilistic 2-m temperature forecasts from the North American Ensemble Forecast System (NAEFS) over the period 1 October 2007-31 March 2010. The results were compared with an earlier spread adjustment technique, as well as forecasts generated by rank sorting the bias-corrected ensemble members. Compared to the other techniques, the new EKDMOS forecasts were more reliable, had a better calibrated spread-error relationship, and showed increased day-to-day spread variability.