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Generated using version 3 0 of the official AMS LATEX template Initial Condition Sensitivity of Western Pacific Extratropical Transitions determined using Ensemble based Sensitivity Analysis Ryan D Torn and Gregory J Hakim University of Washington Seattle WA Corresponding author address Ryan Torn Department of Earth and Atmospheric Sciences ES 351 University at Albany 1400 Washington Ave Albany NY 12222 E mail torn atmos albany edu 1 ABSTRACT An ensemble Kalman filter EnKF based on the Weather Research and Forecasting WRF model is used to generate ensemble analyses and forecasts for the extratropical transition ET events associated with Typhoons Tokage 2004 and Nabi 2005 Ensemble sensitivity analysis is then used to evaluate the relationship between forecast errors and initial condition errors at the onset of transition and to objectively determine the observations having the largest impact on forecasts of these storms Observations from rawinsondes surface stations aircraft cloud winds and cyclone besttrack position are assimilated every six hours for a period before during and after transition Ensemble forecasts initialized at the onset of transition exhibit skill similar to the operational Global Forecast System GFS forecast and to a WRF forecast initialized from the GFS analysis WRF ensemble forecasts of Tokage Nabi are characterized by relatively large small ensemble variance and greater smaller sensitivity to the initial conditions In both cases the 48 hour cyclone minimum sea level pressure forecast and the RMS error in sealevel pressure are most sensitive to the TC position and to mid latitude troughs that interact with the tropical cyclone during ET Diagnostic perturbations added to the initial conditions based on ensemble sensitivity reduce the error in the storm minimum sea level pressure forecast by 50 Observation impact calculations indicate that assimilating approximately 40 observations in regions of greatest initial condition sensitivity produces a

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