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 hETET + 48 h Tokage (2004) Nabi (2005)

Effect of Mid-latitude Flow Harr and Elsberry 2000

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

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?

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!

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

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.

Tokage 00 hr Forecast Sea-Level Pressure500 hPa Height

Tokage 24 hr Forecast Sea-Level Pressure500 hPa Height

Tokage 48 hr Forecast Sea-Level Pressure500 hPa Height

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

12 Hour Forecast Sensitivity Sea-level Pressure500 hPa Height

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

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

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

Nabi 00 hr Forecast Sea-Level Pressure500 hPa Height

Nabi 24 hr Forecast Sea-Level Pressure500 hPa Height

Nabi 48 hr Forecast Sea-Level Pressure500 hPa Height

Nabi Forecast Initialized 00 UTC 6 September 2005

48 Hour min. SLP Sensitivity 500 hPa HeightSea-level Pressure

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

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

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.