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Ryan D. Torn University of Washington

Using ensemble data assimilation to investigate the initial condition sensitivity of Western Pacific extratropical transitions. Ryan D. Torn University of Washington. Satellite Evolution. ET – 48 h. ET. ET + 48 h. Tokage (2004). Nabi (2005). Effect of Mid-latitude Flow.

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Ryan D. Torn University of Washington

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  1. Using ensemble data assimilation to investigate the initialcondition sensitivity of Western Pacific extratropical transitions Ryan D. Torn University of Washington

  2. Satellite Evolution ET – 48 h ET ET + 48 h Tokage (2004) Nabi (2005)

  3. Effect of Mid-latitude Flow Harr and Elsberry 2000

  4. Forecast Sensitivity to TC Control SimulationTC displaced 250 km SW Klein et al. 2002

  5. Overview • Want to determine the initial condition sensitivity of Western Pacific ET events • Use EnKF data assimilation as a tool to answer the following questions about ET events: • What analysis features is the ET forecast most sensitive to? • Are observations available in the area where the forecast is most sensitive to the analysis? • Are these results generic or case dependent?

  6. Forecast Sensitivity EnKF offers an alternative way to calculate the sensitivity of a forecast metric (J) to the analysis using the ensemble of analyses and forecasts: No tangent linear model necessary, only linear regression!

  7. GFS Forecast of Tokage ET 48 hour forecast initialized 12 UTC 19 October 2004 Courtesy Pat Harr, Naval Postgraduate School

  8. Experiment Setup • WRF model, 45 km resolution, 30 vertical levels • 90 ensemble members • observation assimilation every 6 hours • rawindsondes • ACARS • cloud track winds • surface stations • buoys, ships • ~10,000 obs.

  9. Tokage 00 hr Forecast Sea-Level Pressure 500 hPa Height

  10. Tokage 24 hr Forecast Sea-Level Pressure 500 hPa Height

  11. Tokage 48 hr Forecast Sea-Level Pressure 500 hPa Height

  12. Tokage Forecast Initialized 12 UTC 19 October Tokage min. SLP Tokage Track

  13. 12 Hour Forecast Sensitivity Sea-level Pressure 500 hPa Height

  14. 48 Hour min. SLP Sensitivity 500 hPa Height Min. SLP is increased by: • Shifting Siberian trough to the east • Shifting Mongolian trough to the west • Moving Tokage to the southwest

  15. 48 Hour RMS error sensitivity 500 hPa Height RMS error is decreased by: • Shifting Siberian trough to the east • Shifting Mongolian trough to the west • Moving Tokage to the southwest

  16. 500 hPa Observations • Lack of sonde observations in critical region • Sondes were missing during this cycle

  17. Nabi 00 hr Forecast Sea-Level Pressure 500 hPa Height

  18. Nabi 24 hr Forecast Sea-Level Pressure 500 hPa Height

  19. Nabi 48 hr Forecast Sea-Level Pressure 500 hPa Height

  20. Nabi Forecast Initialized 00 UTC 6 September 2005

  21. 48 Hour min. SLP Sensitivity 500 hPa Height Sea-level Pressure

  22. 48 Hour RMS Error Sensitivity 500 hPa Height RMS error is decreased by: • Shifting Chinese trough to west • Amplification of Siberian ridge • Shifting downstream trough to the east

  23. 500 hPa Observations • Several sondes in the most sensitive regions • Analysis more confident in trough position, thus less forecast variance.

  24. Summary • Extratropical Transitions can be a significant predictability problem for NWP models • Described set of experiments to understand the sensitivity of the ET forecast to analysis features • Tokage and Nabi results suggest that largest forecast sensitivities are associated with upper-level troughs upstream of TC. Stronger westerlies may lead to more sensitivity. • Future work will include additional ET events and an assessment of observation impact.

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