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

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)

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

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 (P env ) – calculated from numerical analyses *Issues

Wind from MSLP

MSLP from Wind

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 P env (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.

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).

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

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 V500 c is an Atlantic climatological V500 based on max wind, latitude.

Environmental Pressure Environmental pressure (P env ) is estimated from the Pressure of Outer Closed Isobar (POCI)

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

Australian Region Verification NameDateObservedCalculatedComments Latitude Gale radiusMotion Max WindPePress. Max wind Snmknots hPa knots Agnes 1 6 Mar (52) (968)62Max wind estimate Dec (81) (962)94Max wind estimate. Kathy23 Mar Max wind before instrument failure. Orson22 Apr Max wind derived from measurement at 36m. Ian2 Mar Max wind sampled at 5 minutes Oliver7 Feb Limited sampling of max wind. Olivia10 April Varanus Is pressure; Barrow Is Max wind Rachel7 Jan Port Hedland pressure; Bedout Is Max wind 1 Maximum wind value is an open water estimate, lower value in parenthesis is based on the conversion of observed gusts to mean winds. 2 Maximum wind is an estimate based upon reanalysis (Courtney&Shepherd 2008), the lower value in parenthesis is from BoM (1977). Another small eye case?

HurSAT Movie Of Tropical Cyclone Orson 1989 Courtesy of NCDC

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

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

Validation (BoM & La Reuion)

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?

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

Locations Hurricanes Non-Hurricanes

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.

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

Errors in terms of T-number Sweet spot TAFB Biases

Differences Between Agencies Timing differences – Coordination Calibration issues Center location – CDO and Embedded eye

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

Calibration 10-bit, 8-bit, 7-bit image resolution – NMAP – MCIDAS BD curve differences

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

Further Stratifications 12-h Intensity Trend [kt] WeakeningSteady/IntensifyingRapid < -2.5≥ -2.5 and < 7.5≥7.5 Latitude[ o ] < 2020 to 30> 30 <20≥ 20 and < 30≥30 Translation Speed [kt] SlowAverageFast <6.0≥6.0 and < 14.0≥ 14.0 Radius of Outer Closed Isobar [nmi] SmallAverageLarge < 165≥ 165 and < 270≥ 270

100 nmi 200 nmi 300 nmi ROCI

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 kts Size introduces biases at the higher intensities (>100kt)

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

Sensitivities FactorsPredicted Bias Sensitivity Latitude (mean=23 degrees)≈-0.32kt per degree Sin Latitude (mean=0.39)-28.9 kt per sin unit Translation Speed ( mean=9.9 kt)≈-0.33 kt per kt Speed Factor (mean=6.3 kt)-0.88 kt per kt ROCI (mean=195 n mi)0.018 kt per n mi Intensity trend ( mean= 3.4)-0.35 kt per kt

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

MSLP from bias corrected Dvorak

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.

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?

Questions?