Improving Intensity Estimates Using Operational Information

1. Improving Intensity Estimates Using Operational Information John Knaff NOAA/NESDIS Regional and Mesoscale Meteorology Branch Fort Collins, CO

2. Acknowledgements Significant Work: • Joe Courtney (BOM) • Dan Brown (NHC) • Jack Bevin (NHC) • Gregg Gallina (SAB) Manuscript Comments: • Chris Landsea (NHC) • Hugh Cobb (NHC) • Ray Zehr (Retired) • Mark DeMaria (RAMMB)

3. Outline • Updates on the Knaff and Zehr wind-pressure relationship (WPR) • Lessons learned since publication • Increasing operational applicability (i.e., Courtney and Knaff 2009) • Preliminary evaluations from RSMC La Reunion • Improving the calibration of Dvorak Intensity Estimates • Combining results to provide objective guidance

4. Knaff and Zehr (2007) • Statistical method to estimate MSLP from maximum winds / max winds from MSLP • Accounts for translation • … latitude (φ) • … size (S) – calculated from numerical analyses • … environmental pressure (Penv) – calculated from numerical analyses *Issues

5. Wind from MSLP

6. MSLP from Wind

7. Lessons Learned (i.e., Knaff and Zehr 2008) • The method did not mesh with operations; required extra effort to calculate parameters S (TC size) and Penv (environmental pressure) • There was a desire by some forecast centers to use quantities already routinely available/estimated in operations • There was an issue with very low latitude storms (that were not in the developmental dataset) • Using the low-level winds to estimate V500 ( for S) did not account for land exposures, resulting in an erroneously estimate of S when land was within 500 km. • Eye size / radius of maximum wind still matters - remains a problem.

9. The Courtney & Knaff (2009) Modification • Low latitude issue addressed • TC size (S) is estimated from R34 • Environmental Pressure (Penv) estimated from the Pressure of the Outer Closed Isobar (POCI).

10. Low Latitudes (<18 degrees) No dependence of latitude… The equation for Vmax has to be iterated because S is a function of Vmax

11. TC Size The tangential wind at 500 km is estimated using a simple relationship involving the average radius of gales, R34 Where r34 is the average of the non-zero quadrants The rest of the calculation of size remains the same Where V500c is an Atlantic climatological V500 based on max wind, latitude.

12. Environmental Pressure • Environmental pressure (Penv) is estimated from the Pressure of Outer Closed Isobar (POCI)

13. Operational Constraints at BoM • S has a minimum value of 0.4 • Dvorak intensities are used • 10-minute wind is converted to 1-minute equivalent using 0.88 • MSLP is estimated to the nearest hPa above 980 hPa • MSLP is estimated to the nearest 5 hPa below 980 hPa

14. Australian Region Verification Another small eye case?

15. HurSAT Movie Of Tropical Cyclone Orson 1989 Courtesy of NCDC

16. Preliminary Results from La Reunion Courtesy of Sebastien Langlade Tropical cyclone forecaster - RSMC La Reunion

17. Validation (BoM & La Reuion)

18. Observations from C&K Concerning the Dvorak Technique • K&Z and C&K produced high MSLP biases for Dvorak-based intensities less than or equal to 55 kt (CI=3.5), suggesting that the accounting for translation speed was causing an error. • However, when we reexamined the aircraft based intensities in this range, this was not the case. • Was there a bias in Dvorak-based intensities causing this issue?

19. Re-examining Dvorak Intensity estimates • The last systematic examinations completed 2003 and 1988 • 1989-2007 • All Dvorak fixes within 2-h of an aircraft fix • Two agencies (TAFB, SAB) • Stratify by • Intensity • + latitude • + intensity trend • + TC size (ROCI) • + translation

20. Locations Hurricanes Non-Hurricanes

21. Time Series • general bias between TAFB and SAB that has diminished since 2002 • upward trend in TAFB errors • No visually detectable change points related to technological changes • **An average of the fixes from SAB and those from TAFB reduced the errors and biases.

22. Statistics WRT Intensity • low bias between 35 and 65 kt, and above 120 kt • High bias between 75 and 100 kt • There is a “sweet spot” between 100 and 120 kt. Sweet spot Sweet spot

23. Errors in terms of T-number TAFB Biases Sweet spot

24. Differences Between Agencies • Timing differences • Coordination • Calibration issues • Center location • CDO and Embedded eye

25. Timing Differences Time lag Intensity differences (First Homogenous fix) TAFB Leads

26. Calibration • 10-bit, 8-bit, 7-bit image resolution • NMAP • MCIDAS • BD curve differences

27. Center Location • CDO • Shear (lower) • Embedded Eye (higher) • Classification is subjective to some degree • Dependent on center position

28. Further Stratifications

29. 100 nmi ROCI 200 nmi 300 nmi

30. Results • Summary • Intensity trends are most important and effect all intensity ranges • Latitude is important for more intense storms at high latitude. • Translation effects intensities estimates of Hurricanes 65-120 kts • Size introduces biases at the higher intensities (>100kt)

31. Summary of Findings • Errors are a function of Intensity • Biases are a function of intensity, intensity trends, latitude, TC size and translation speed.

32. Sensitivities

33. Bias Correction/Error Estimation Bias = 1.4kt RMSE = 8.9 kt Bias = -0.5kt RMSE = 9.5kt Bias = 2.1kt RMSE=10.2 kt Bias=0.0 kt RMSE=10.7 kt

34. MSLP from bias corrected Dvorak

35. Summary • The Knaff & Zehr WPR has been modified for easy use in most operational settings. • Used at BoM in operations; standardizing estimates between Perth, Darwin, PNG, Brisbane. • Has been favorably evaluated by RSMC La Reunion • This work lead to a re-examination of the Dvorak technique. • Biases and errors have been documented • a method for bias correction and error estimation has been developed • Combining the WPR and the Dvorak bias correction objective analysis can create unbiased wind estimates and corresponding MSLP estimates.

36. Remaining Issues / Future Topics • Continue to validate the WPRs vs. NHC best track and aircraft-based MSLP. • RMW/eye size correction for WPRs • TC size (S) for this application estimated directly from IR imagery • Same sort of bias, RMSE analysis for the AMSU intensity and size estimates • Do better bogus estimates make better forecasts… HFIP?

37. Questions?