ensemble sensitivity analysis applied to tropical cyclones: preliminary results from typhoon nuri...
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Ensemble Sensitivity Analysis Applied to Tropical Cyclones: Preliminary Results from Typhoon Nuri (2008)
Rahul Mahajan & Greg HakimUniversity of Washington, Seattle
Third THORPEX International Science Symposium (TTISS)Monterey California, 14 - 18 September, 2009
Outline
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Motivation and Goal• Key issues regarding Tropical Cyclones:• Dynamical understanding• Genesis and Structure
• Predictability• Track and Intensity
• Apply ensemble sensitivity analysis to identify dynamical processes that control genesis
• Diagnostically modify initial conditions to stop or speed-up genesis processes
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Sensitivity Analysis• relates the change to an input (state: x) to changes in
the output (metric: )
adjoint sensitivity to IC
adjoint or transpose of TLM
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Sensitivity Analysis• adjoint approach is deterministic
• consider and as random variables
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Ensemble Sensitivity
• compute sensitivity from ensemble output
• linear regression• dependent variable is forecast metric• independent variable is element of state vector
• can apply confidence testing using ensemble statistics• no adjoint or tangent linear model required• generate an ensemble by cycling an ensemble Kalman filter
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Typhoon Nuri (2008)
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Typhoon Nuri (2008)
Genesis -24 hrs 1230Z 16 August 2008
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Typhoon Nuri (2008)
Genesis -12 hrs 0030Z 17 August 2008
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Typhoon Nuri (2008)
Genesis 1230Z 17 August 2008
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Typhoon Nuri (2008)
Genesis +12 hrs 0030Z 18 August 2008
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Typhoon Nuri (2008)
DBTY TS
TD
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Experiment Setup• Model• Weather Research and Forecasting (WRF)• 27km horizontal resolution domain over west Pacific• 38 vertical levels, 10 hPa model top
• Data Assimilation• Ensemble Kalman square-root filter• 90 ensemble members• assimilate observations every 6 hrs
• Conventional observations: • rawinsondes, ACARS, surface stations, buoys, ships, cloud track
winds
• Field observations from T-PARC / TCS08: • dropsondes, flight-level data, driftsondes
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Procedure• Cycle data assimilation from pre-depression to
maximum intensity of typhoon Nuri
• Generate ensemble forecast out to 24 hours from pre-depression to tropical storm strength
• Compute sensitivity of metrics at forecast time to analysis state at initialization time
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Nuri 00hr forecast
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Nuri 06hr forecast
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Nuri 12hr forecast
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Nuri 18hr forecast
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Nuri 24hr forecast
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Sensitivity Results
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Forecast Sensitivity to 850 hPa height
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Initial Condition Perturbations
• Test statistical prediction with model response
• Perturb initial state via
• independent variable is the forecast metric
• dependent variable is the analysis
• apply for a range of scales
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Nuri 24hr forecast – Perturbed ICInitialization Time Forecast Time
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Perturbed Forecasts
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Summary and Future Directions• Applied ensemble sensitivity analysis to explore tropical
cyclogenesis of typhoon Nuri
• Sensitivity associated with the incipient storm and axis from the North to the West of the storm
• Modified initial conditions with perturbations:• small: model response comparable to ensemble predictions• large: model response less than the ensemble predictions
suggesting non-linear interaction
• Iterate over different choices of forecast metrics and analysis state variables and increased resolution
• Assess the impact of T-PARC / TCS08 field observations