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Using ensemble data assimilation to investigate the initial
condition sensitivity of Western Pacific extratropical transitions
Ryan D. Torn
University of Washington
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Satellite EvolutionET – 48 h ET ET + 48 h
Tok
age
(200
4)N
abi (
2005
)
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Effect of Mid-latitude Flow
Harr and Elsberry 2000
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Forecast Sensitivity to TC
Klein et al. 2002
Control Simulation TC displaced 250 km SW
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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?
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Forecast Sensitivity
cov( , )
var( )b
b
b x
x JJ
x
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!
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GFS Forecast of Tokage ET
Courtesy Pat Harr, Naval Postgraduate School
48 hour forecast initialized 12 UTC 19 October 2004
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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.
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Tokage 00 hr ForecastSea-Level Pressure 500 hPa Height
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Tokage 24 hr ForecastSea-Level Pressure 500 hPa Height
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Tokage 48 hr ForecastSea-Level Pressure 500 hPa Height
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Tokage Forecast
Tokage Track
Tokage min. SLP
Initialized 12 UTC 19 October
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12 Hour Forecast SensitivitySea-level Pressure 500 hPa Height
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48 Hour min. SLP Sensitivity500 hPa Height
• Shifting Siberian trough to the east
• Shifting Mongolian trough to the west
• Moving Tokage to the southwest
Min. SLP is increased by:
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48 Hour RMS error sensitivity500 hPa Height
RMS error is decreased by:
• Shifting Siberian trough to the east
• Shifting Mongolian trough to the west
• Moving Tokage to the southwest
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500 hPa Observations
•Lack of sonde observations in critical region
•Sondes were missing during this cycle
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Nabi 00 hr ForecastSea-Level Pressure 500 hPa Height
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Nabi 24 hr ForecastSea-Level Pressure 500 hPa Height
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Nabi 48 hr ForecastSea-Level Pressure 500 hPa Height
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Nabi ForecastInitialized 00 UTC 6 September 2005
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48 Hour min. SLP Sensitivity500 hPa Height Sea-level Pressure
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48 Hour RMS Error Sensitivity500 hPa Height
• Shifting Chinese trough to west
• Amplification of Siberian ridge
• Shifting downstream trough to the east
RMS error is decreased by:
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500 hPa Observations
•Several sondes in the most sensitive regions
•Analysis more confident in trough position, thus less forecast variance.
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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.