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Another Look at the Use of Maximum Potential Intensity of Tropical Cyclones

Another Look at the Use of Maximum Potential Intensity of Tropical Cyclones. Andrea Schumacher, CIRA/CSU, Fort Collins, CO Mark DeMaria and John Knaff, NOAA/NESDIS/ StAR , Fort Collins, CO NCAR/NOAA/CSU Tropical Cyclone Workshop 16 November 2011 CIRA, Fort Collins, CO. Introduction.

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Another Look at the Use of Maximum Potential Intensity of Tropical Cyclones

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  1. Another Look at the Use of Maximum Potential Intensity of Tropical Cyclones Andrea Schumacher, CIRA/CSU, Fort Collins, CO Mark DeMaria and John Knaff, NOAA/NESDIS/StAR, Fort Collins, CO NCAR/NOAA/CSU Tropical Cyclone Workshop 16 November 2011 CIRA, Fort Collins, CO

  2. Introduction • Maximum potential intensity (MPI) • Upper bound to TC intensity • Important quantity to know when predicting TC intensity • Approaches • Empirical • Theory • Modeling (not discussed here)

  3. Empirical MPI • Atlantic (DeMaria and Kaplan 1994) • Clim SST/Vmax sample from 1962-1992 • Fit upper bound of Vmax (least squares, exponential) • A = 28.2, B = 55.8, C = 0.1813,T0 = 30.0 °C • T in °C • V given in ms-1 • E. Pacific (Whitney and Hobgood 1997) • Clim SST/Vmax sample from 1963-1993 • Fit upper bound of Vmax (least squares, linear) • C0 = -79.17 ms-1, C1 = 5.36 ms-1 • SST in °C • EPMPI given in ms-1

  4. MPI Theory • Two main thermodynamic theories • Carnot heat engine (Emanuel 1986; 1991) • Acquire enthalpy from the ocean • Convert to wind energy • Function of sea surface and storm outflow temperature • Calculates MPI as a maximum surface wind speed • Eye subsidence (Miller 1958 and Holland 1996) • Requires SST, atmospheric sounding and surface pressure • Calculates MPI as a minimum central pressure

  5. Motivation – Predicting Intensity • Current version of Statistical Hurricane Intensity Prediction Scheme (SHIPS) uses empirical MPI as a predictor • Pros: • Availability (function of SST only) • Cons: • Small sample set – 31 years • Doesn’t include atmospheric variables • Only uses SST – no ocean structure (e.g., OHC) • Doesn’t always serve as true “upper bound” to Vmax…

  6. Motivation (Cont…) • Sometimes, Vmax > theoretical MPI • Example: Hurricane Celia 2010

  7. Project Goals • Re-examine empirical formulas using 1982-2010 SHIPS sample set – valid? • Examine Emanuel’s theoretical MPI (1995) over the SHIPS dataset • Identify when valid / not valid • Create a combined theoretical / empirical formula for MPI in SHIPS

  8. SHIPS Empirical MPI Validation1982-2010 10653 cases total Vmax > MPI for 30 cases (0.3%)  Good upper bound for sample 13003 cases total Vmax > MPI for 8 cases (0.1%)  Good upper bound for sample

  9. Emanuel MPI* in SHIPS1982-2010 Vmax > MPI 12% of cases Vmax > MPI 16% of cases First, try a few simple improvements... *Note: Atmospheric sounding comes from GFS model.

  10. Asymmetry Adjustment • Vmax includes asymmetric adjustment, MPI does not • For proper comparison, remove asymmetric component from storm motion (Schewerdt): Vmax(adjusted) = Vmax – 1.5*(TC speed)0.63 TC Motion Symmetric Vortex Asymmetric Vortex

  11. Emanuel MPI+ Asymmetry Adjustment Vmax > MPI 10% of cases Vmax > MPI 14% of cases

  12. Improved Reduction Factor • Emanuel MPI assumes 700-mb winds reduced by factor of 0.8 at surface (friction) • Franklin et al. (2002) used dropwinsonde data to show reduction factor is closer to 0.9 • Multiplied Emanuel MPI by 0.9/0.8

  13. Emanuel MPI + Asymmetry Adjustment + Improved Reduction Factor Vmax > MPI 9% of cases Vmax > MPI 11% of cases

  14. Observed VmaxExceeds Theoretical MPI. Why? When?

  15. Environmental Differences - Atlantic “Violators” have… • Higher intensity, weakening • Lower MPI, decreasing • Lower SST, decreasing • Larger low-level temperature gradient • Lower OHC • Higher latitude

  16. Standardized differences

  17. Standardized differences (12-hr tendency)

  18. Summary • Empirical MPI provides good upper bound for intensity • SHIPS parameter based on Emanuel theoretical MPI improved by • Adding asymmetry adjustment due to storm motion • Using more appropriate reduction factor when converting 700-mb winds to surface

  19. Future Work • Objectively identifying conditions where Emanuel MPI should be adjusted or replaced with empirical • Fast-moving TCs moving over cooler SST/OHC, lack of equilibrium • Extratropical cases (Atlantic) • Annular cases (E Pacific) • Creating combination empirical/theoretical MPI for testing in SHIPS • Parameterizing SST change under inner core • Decrease MPI

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