3.5 The Impacts of Recurving Tropical Cyclone on Extended-Range Predictability of Midlatitude Weather Patterns

Thursday, 26 January 2017: 2:30 PM
2AB (Washington State Convention Center )
Patrick A. Harr, NPS, Monterey, CA; and H. M. Archambault

Several case studies have identified recurving tropical cyclones that move into the midlatitudes and transition to extratropical cyclones as sources of error and uncertainty downstream of the recurvature location. In this study, the impact of recurving tropical cyclones on midlatitude predictability is examined via a comprehensive climatology of forecast error and uncertainty to establish the overall significance of such impacts.

The downstream impacts of recurving tropical cyclones on ensemble and deterministic extended-range predictions are determined using the National Centers for Environmental Prediction Global Ensemble Forecast System (NCEP/GEFS) Reforecast-2. The once-daily (at 0000 UTC) Re-forecast-2 data are available from 1985 and contain a deterministic forecast, control forecast and 10 perturbation members. This data set allows for a general examination of predictability because it is produced with a fixed model and uses reanalysis data for initial conditions.

Using the long period of forecasts provided in the reforecast data set, climatological values of forecast error (anomaly correlation scores) and uncertainty (ensemble spread) are computed. The forecast attributes associated with recurving tropical cyclones are statistically compared to climatological values to establish that recurving tropical cyclones are associated with periods of reduced predictability in downstream midlatitude weather patterns. To refine the linkages between recurving tropical cyclones, forecast accuracy, and forecast uncertainty downstream, the tendency for predictability to be reduced is examined relative to specific regions or flow patterns. Finally, the character of the reduced accuracy and increased uncertainty is examined relative to errors in location and amplitude of key circulation features downstream of recurving tropical cyclones.

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