Investigating the downstream impact of tropical cyclones using ensemble sensitivity analysis

The extratropical transition (ET) of a tropical cyclone (TC) may strongly influence the mid-latitude wave pattern by amplifying or triggering a Rossby wave train and thus may even facilitate high impact weather events in regions lying far downstream of the transitioning TC. The processes involved during the transitioning and interaction process are often not well represented in numerical weather prediction (NWP) systems. As a result, predictability for downstream regions is typically reduced during ET. A better understanding of the processes involved and their representation in NWP systems will help to overcome those issues in predictability.

The processes involved during the interaction of a TC with the midlatitude flow can be studied in detail using an analysis of the eddy kinetic energy (Ke) budget. In the present study we determine the Ke budget for ensemble forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) that were initialised prior to the ET of a TC. The ensemble forecast provides us with multiple solutions for the interaction between the transitioning TC and the midlatitude flow. Ensemble sensitivity analysis is then employed to identify the contribution of energy fluxes emanating from the transitioning TC or from the upstream midlatitudes on the amplification of the downstream wave pattern. Significant correlations are found for specific flow features that point on the impact of the TC on the midlatitude flow. We further aim on identifying links between the Ke budget terms and the reduction of predictability in downstream regions.