Modelling the Extratropical Transition of Tropical Cyclones and Its Downstream Impact

Tropical cyclones that move polewards may undergo extratropical transition (ET) and interact with the midlatitude flow. The presence of a tropical cyclone undergoing ET is often accompanied by low predictability for the Atlantic-European and Northwest-American sector, where the weather patterns can be significantly altered by an ET event. In this HLRS project we investigate ET and its downstream impact using high-resolution numerical simulations. The investigation of the mechanisms responsible for the reduction in predictability associated with ET was one of the primary aims of the THORPEX Pacific Asian Regional Campaign (T-PARC), which took place in the Northwest Pacific region in summer 2008. In support of the T-PARC campaign, a series of high-resolution (up to 0.025○) numerical weather forecasts using the COSMO model were computed at the Steinbuch Centre for Computing (SCC), KIT. During T-PARC, a strong interaction between the outflow of Typhoon Jangmi and the midlatitude jet was observed. We use COSMO simulations to investigate this interaction in more detail. Trajectory calculations showed that Jangmi advected tropical air into the jet streak. A potential vorticity inversion technique was applied to remove the storm from the forecast and thus to quantify the impact of Jangmi on the acceleration of the midlatitude jet. In this report we present the forecast operation along with a more detailed case study of the ET of Typhoon Jangmi (2008).