Sensitivity experiments for ensemble forecasts of extratropical transition

The extratropical transition (ET) of tropical cyclones often has a negative impact on the predictability of the atmospheric situation both around the ET event and further downstream. Guidance with respect to this uncertainty can be obtained from ensemble prediction systems.

In the ECMWF Ensemble Prediction System (EPS) initial perturbations for the ensemble members are obtained for areas targeted around tropical cyclones using singular vectors (SVs) calculated with linearized diabatic physics. Sensitivity experiments are presented using the ECMWF EPS to investigate the impact of the perturbations targeted around tropical cyclones on the predictability of the extratropical transition and the downstream flow. Similarly, the impact of the stochastic physics is examined. Finally, the impact of higher horizontal resolution in the EPS is investigated. It is found that the targeted perturbations account for the correct representation of the spread closer around ET time while the stochastic physics is rather responsible for the later forecast times. The targeted perturbations have more influence on the track spread while the stochastic physics yields more spread in the intensity in terms of mean sea level pressure. Without the targeted perturbations the analysis would lie outside of the ensemble spread. The high resolution runs show a better representation of the landfall but still at the time of the filling the analysis central pressure is outside of the ensemble spread. It is shown that the errors introduced into the forecast during ET affect the predictability far downstream.