Stratospheric Initial Condition for Skillful Surface Prediction in the ECMWF Model

Abstract

The subseasonal-to-seasonal prediction of boreal-winter sea-level pressure (SLP) is investigated by examining 19-year reforecast data sets from the European Centre for Medium-Range Weather Forecasts ensemble prediction system. Based on a pattern anomaly correlation coefficient of 0.6, the model reliably predicted the extratropical SLP anomalies with a maximum forecast lead time of 9.8 days. This prediction skill, however, varies significantly from one forecast to another. The forecasts with relatively high skills and those with low skills exhibit significant zonal Wavenumber 1 patterns in the stratospheric initial conditions with an opposite sign. The high-skill forecasts present a relatively weak westward Wavenumber 1 phase tilt with height compared with the low-skill forecasts, dampening an upward wave propagation in the stratosphere. This result suggests that not only the zonal mean but also the zonally asymmetric stratospheric conditions play an important role in extended surface prediction by modulating vertical wave propagation or wave reflection.