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 (Penv) – calculated from

numerical analyses

*Issues

Wind from MSLP

63.0max 5.1738.9518.0755.0960.14633.18 cPPSV

MSLP from Wind

envsrm

srm PSV

VMSLP

*483.0*587.12

254.24483.0286.23

2

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.

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)

envsrm

srm PSV

V

2

11 26.18

MSLP

63.0max 5.1984.8515.0238.5446.6 cPPSV

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 V500c is an Atlantic climatological V500 based on max wind, latitude.

39/34500 rV

cVVS 500/500

Environmental Pressure

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

hPaPOCIPenv 2

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 VerificationName Date Observed Calculated Comments

LatitudeGale

radius MotionMax Wind Pe Press. Press.

Max wind

S nm knots knots hPa hPa hPa knots

Agnes 1 6 Mar 1956 19.2 300 10 60 (52) 1002 961 963 (968) 62 Max wind estimate.2 25 Dec 1974 12.5 30 3 95 (81) 1004 950 949 (962) 94 Max wind estimate.

Kathy 23 Mar 1984 15.6 40 5 100 1010 940 946 106Max wind before instrument failure.

Orson 22 Apr 1989 19.2 200 12 105 1008 905 935 131

Max wind derived from measurement at 36m.

Ian 2 Mar 1992 20.8 45 6 80 1010 960 964 84Max wind sampled at 5 minutes

Oliver 7 Feb 1993 17.1 100 3 90 1010 950 957 97Limited sampling of max wind.

Olivia 10 April 1996 20.3 200 12 105 1008 927 926 105Varanus Is pressure; Barrow Is Max wind

Rachel 7 Jan 1997 20.3 100 6 70 1010 971 973 72

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 MovieOf Tropical CycloneOrson 1989

Courtesy of NCDC

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 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

Locations

HurricanesNon-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

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 lagIntensity 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 Stratifications12-h Intensity Trend [kt]

Weakening Steady/Intensifying Rapid

< -2.5 ≥ -2.5 and < 7.5 ≥7.5

Latitude[o]

< 20 20 to 30 > 30

<20 ≥ 20 and < 30 ≥30

Translation Speed [kt]

Slow Average Fast

<6.0 ≥6.0 and < 14.0 ≥ 14.0

Radius of Outer Closed Isobar [nmi]

Small Average Large

< 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 65-120 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.

3

max

2

maxmaxmax 364.32930.10

776.0484.11)(

VVVVRMSE

max63.0

4

max

3

max

2

maxmax

305.0018.05.1884.0sin144.25

577.32247.14723.4255.2545.49

VROCIc

VVVVBias

Sensitivities

Factors Predicted 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 = 1.4ktRMSE = 8.9 kt

Bias = -0.5ktRMSE = 9.5kt

Bias = 2.1ktRMSE=10.2 kt

Bias=0.0 ktRMSE=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?