Jones, S.C.

Anwender, Doris

The extratropical transition (ET) of tropical cyclones (TC) often reduces the predictability of the atmospheric situation around the ET and downstream. Guidance with respect to this uncertainty can be obtained from ensemble prediction systems (EPS). The representation of several TCs that underwent ET is investigated in the EPS of the European Centre for Medium Range Weather Forecasts (ECMWF). The variability in association with an ET is investigated performing case studies. A trough-ridge-trough pattern associated with the ET cases is identified. With the aid of an analysis and clustering method two characteristic variability patterns associated with uncertainty in the shift and the amplitude of the trough-ridge-trough pattern are found. Relations between the ET development in individual clusters and their contribution to these patterns of variability are seen. Sensitivity experiments are presented using the ECMWF EPS to investigate the impact of the perturbations targeted around TCs, of the stochastic physics and of higher horizontal resolution in the EPS. It is found that the targeted perturbations account for the correct amount of spread around ET time while the stochastic physics is responsible for the later forecast times. The targeted perturbations are essential for including the analysis in the spread. High resolution runs yield a better representation of the landfall. The errors introduced into the forecast during ET affect the predictability far downstream.