Sensitivity of ensemble forecasts of extratropical transition to initial perturbations targeted on the tropical cyclone

The reduced predictability associated with the extratropical transition of tropical cyclones presents a severe challenge for numerical weather prediction. Guidance with respect to the uncertainty associated with extratropical transition can be obtained from ensemble prediction systems. In the ECMWF Ensemble Prediction System (EPS) initial perturbations for the ensemble members are calculated for areas targeted around tropical cyclones using singular vectors calculated with linearised diabatic physics. In the recent upgrade to the ECMWF EPS the calculation of these initial perturbations has been modified so that it is now applied to tropical cyclones as they move into the midlatitudes and undergo extratropical transition.

In this paper we present experiments designed to investigate the impact of the new initial perturbations on the predictability of the extratropical transition and the downstream flow. Ensemble forecasts using the current operational configuration but with no stochastic physics are compared with experiments that are identical except that the targeting for a specific tropical cyclone, in this case Typhoon Maemi (2003), is switched off. The sensitivity is quantified using a principal component analysis with clustering of the principal components.