OPERATIONAL IMPLEMENTATION OF AN OBJECTIVE ANNULAR HURRICANE INDEX

OPERATIONAL IMPLEMENTATION OF AN OBJECTIVE ANNULAR HURRICANE INDEX ANDREA B. SCHUMACHER 1, JOHN A. KNAFF 2, THOMAS A. CRAM 1, MARK DEMARIA 2, JAMES P. KOSSIN 3 1 CIRA, Colorado State University, Fort Collins, CO 3 CIMSS, University of Wisconsin-Madison, WI 2 NOAA/NESDIS,

Overview Annular hurricanes: structural characteristics General environmental conditions Intensity characteristics: Motivation for objective prediction scheme Objectively determining annular structure: Annular Hurricane Index Step 1: Screening Step 2: Linear discriminant analysis Verification Operational example: Daniel 2006 (EP 05)

Annular Hurricane Structure q Distinctly axisymmetric q Large circular eyes q Greatly reduced rainband activity q Lasts at least 3 hours q Rare, occur ~4% of the time Isabel (2003)

Environmental Conditions Weak Easterly/Southeasterly Wind Shear Weak Relative Eddy Flux Convergence 200 h. Pa Easterlies SSTs in a range 25. 4 to 28. 6 °C steady or decreasing. OR Weak Easterly shear, under an upper ridge, over SST <28. 6 °C ALSO Intensity > 85 kt (Knaff et al. 2003)

Annular Hurricane Intensity Characteristics Do not weaken rapidly after max intensity Intensity is very close to 85% MPI wrt SST Have large intensity biases & larger than normal intensity errors

Determining Annular Structure Subjective Diagnostic Objective Parameter Source Large eyewall radius Rc IR Imagery Warm Eye ΔTeye IR Imagery Vertical shear SHRD NCEP analysis 200 h. Pa winds U 200 NCEP analysis Eddy flux convergence REFC NCEP analysis SST Reynolds SST Intensity NHC Best Track Yellow = Structure Blue = Environment

Annular Hurricane Index (AHI)

AHI Step 1: Screening Parameter Prescreening Criterion Intensity < 84 kt Rc <50 km ΔTeye < 15 o. C SHRD >11. 3 ms-1 U 200 < -11. 8 or > 1. 5 ms-1 REFC < -9 or > 11 ms-1 day-1 SST <24. 3 or > 29. 1 o. C +/- 3 standard deviations from means of AH’s (1995 -2003) 976 (54 AH) cases (6 h) > 84 kt intensity 241 cases after screening (53 AH) Hit Rate = 100%, False Alarm Rate = 19%

AHI Step 2: Linear Discriminant Analysis (Overview) Graphical Interpretation of LDA for Case With 2 Predictors (x, y) and 2 Groups DF=0 DF = c 0 + c 1 x + c 2 y Coordinate transformation that provides maximum separation of groups (From www. doe-mbi. ucla. edu) Refs: Wilks (2006), Hennon & Hobgood (MWR, 2003)

AHI Step 2: Linear Discriminant Analysis (cont…) Only predictors with significant annular vs. nonannular differences in means were used** SST U 200 – 200 h. Pa zonal winds σ c – azimuthal standard deviation of BTs at Rc VAR – variance of azimuthally-averaged BTs from TC center to 600 km ΔT eye - max difference between Rc and any azimuthally-averaged BT at smaller radius ** exceeds 95% confidence level using Student’s T test

Verification Dependent Years (1995 -2003) STEP 1: Screening Reduced 941 (54) cases to 241 (53) FAR = 19% STEP 2: LDA “Y” LDA “N” AH “Y” 46 7 AH “N” 56 132 Hit Rate ~ 87 % FA Rate ~ 6 % Independent Years (2004 -2006) STEP 1: Screening Reduced 387 (7) cases to 82 (7) FAR = 19% STEP 2: LDA “Y” LDA “N” AH “Y” 7 0 AH “N” 14 61 Hit Rate ~ 100 % FA Rate ~ 4 %

AHI Output & Interpretation If case doesn’t pass screening, AHI set to 0. If screening is passed, LDA function value is linearly scaled to obtain the Annular Hurricane Index, which ranges between 1 & 100. AHI is displayed at the end of the SHIPS model output file. AHI = 0 No annular structure AHI = 1 Worst match to annular structure AHI = 100 Best match to annular structure

Example: Daniel 2006 7/20/06 0 Z, vmax=95 kt AHI = 0 7/22/06 0 Z vmax = 130 kt AHI = 100 7/21/06 0 Z, vmax=120 kt AHI = 50

AHI Output Displayed at End of SHIPS Model Output File * * SCREENING STORM INTENSITY SCREENING SST SCREENING VERTICAL SHEAR SCREENING 200 h. Pa ZONAL WIND SCREENING 200 h. Pa MOM FLUX CONV SCREENING GOES RAD COLD BR TEMP SCREENING GOES EYE-RING BR TEMP ---> ---> * * PASSED PASSED > -9 m/s-day ? ---> PASSED -0. 1 m/s-day < 11 m/s-day ? ---> PASSED = 106 km > 50 km ? ---> PASSED = 66. 2 C > 15 °C ? ---> PASSED ANNULAR HURRICANE INDEX EP EP 052006 07/22/2006 00 UTC = = = 130 27. 3 5. 7 -13. 2 kt C C kt kt kt > > < < > < = -0. 1 m/s-day = 84 24. 3 29. 1 21. 97 -22. 94 2. 92 kt ? °C ? kt ? STORM MAY BE ANNULAR, PASSED SCREENING CALCULATE AHI FROM DISCRIMINANT ANALYSIS ******************************** * * ANNULAR HURRICANE INDEX (AHI) VALUE = 100. * (AHI = 100. IS BEST MATCH TO ANNULAR * STRUCTURE) * (AHI = 1. IS WORST MATCH TO ANNULAR

Summary AHI is an objective algorithm that determines the likelihood of annular structure in an existing hurricane using SHIPS environmental predictors and 6 -hr stormcentered GOES IR imagery For the period 1995 -2006, the AHI algorithm had a hit rate of 96% and a false alarm rate of 4% The AHI will be tested in a real-time operational setting, running concurrently with the SHIPS model, at the National Hurricane Center during the 2007 hurricane season. After the 2007 season, if evaluation of the algorithm is favorable the transition to an operational product will be pursued