Journal Article

Jumpiness in Ensemble Forecasts of Atlantic Tropical Cyclone Tracks

Pappenberger, Florian, Methven, John A., Richardson, David S., Cloke, Hannah L.

2024

Overview

We investigate the run-to-run consistency (jumpiness) of ensemble forecasts of tropical cyclone tracks from three global centers: ECMWF, the Met Office, and NCEP. We use a divergence function to quantify the change in cross-track position between consecutive ensemble forecasts initialized at 12-h intervals. Results for the 2019–21 North Atlantic hurricane season show that the jumpiness varied substantially between cases and centers, with no common cause across the different ensemble systems. Recent upgrades to the Met Office and NCEP ensembles reduced their overall jumpiness to match that of the ECMWF ensemble. The average divergence over the set of cases provides an objective measure of the expected change in cross-track position from one forecast to the next. For example, a user should expect on average that the ensemble mean position will change by around 80–90 km in the cross-track direction between a forecast for 120 h ahead and the updated forecast made 12 h later for the same valid time. This quantitative information can support users’ decision-making, for example, in deciding whether to act now or wait for the next forecast. We did not find any link between jumpiness and skill, indicating that users should not rely on the consistency between successive forecasts as a measure of confidence. Instead, we suggest that users should use ensemble spread and probabilistic information to assess forecast uncertainty, and consider multimodel combinations to reduce the effects of jumpiness.

• Publisher: American Meteorological Society
• Subject: Cyclones / Decision making / Divergence / Ensemble forecasting / Ensembles / Forecast verification/skill / Forecasting skill / Hurricanes / Investigations / Numerical weather forecasting / Position measurement / Prediction models / Tropical cyclone forecasting / Tropical cyclone tracks / Tropical cyclones / Weather forecasting