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TC-Relative Helicity and Its Impact on Intensity and Development

TC-Relative Helicity and Its Impact on Intensity and Development. Matt Onderlinde Student Seminar Wed. Sept. 25, 2013. Helicity Refresher. Motivation. 850-200 mb wind shear is commonly used to diagnose the favorability of TC environments

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TC-Relative Helicity and Its Impact on Intensity and Development

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  1. TC-Relative Helicity and Its Impact on Intensity and Development Matt Onderlinde Student Seminar Wed. Sept. 25, 2013

  2. Helicity Refresher

  3. Motivation • 850-200 mb wind shear is commonly used to diagnose the favorability of TC environments • For the same value of shear, helicity can change significantly

  4. Research Questions • What are the implications of different values of helicity? • Does environmental helicity have as much predictive value as shear? • How do observational estimates compare to the WRF simulations? i.e., do we see any signal in nature?

  5. Background • Many studies show vertical wind shear is important for TC intensity change (e.g., DeMaria and Kaplan 1999) • Nolan (2011) suggests that helicity may be important • Point-Downscaling (PDS) developed by Nolan (2011) as a framework to control the environment around a TC

  6. Background – Point-Downscaling The circled term is an artificial force that is equal in magnitude to the pressure gradient that would exist if temperature gradients were allowed to exist.

  7. Methodology – Model Setup • Use PDS in WRF to simulate the effects of helicity on TCs • Idealized nudging is also applied to further restrict changes to this prescribed environment.

  8. Modeling Results – WRF Simulations

  9. Methodology – Reanalysis Data • Calculate TC-relative helicity (TCSRH) from ERA-Interim reanalyses and GFS analyses • TCSRH computed from the average wind profile in annuli around TCs Hugo 1989

  10. Methodology – Best Track Data

  11. Results – ERA-Interim Reanalysis Data

  12. Modulation of local helicity by environmental helicity through TC tilt

  13. Methodology – Idealized TC Tilt

  14. Summary • WRF simulations suggest helicity is important when it comes to TC intensification • Reanalysis data suggests that this signal is weak in nature • Idealized shifting of wind fields shows how environmental helicity can modulate the impact of local storm helicity • In nature, it is very uncommon for the environment to remain constant for a long enough period during which helicity could make an impact on intensity. • Shear still has more predictive ability than helicity… but this isn’t surprising.

  15. Future Work • Currently developing a new WRF modeling tool to allow for time varying of environmental wind profile while still using BOTH nudging and PDS simultaneously. • This will allow for simulations where shear and helicity can be gradually (realistically) ramped up or down.

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