1. Can Dvorak Intensity Estimates be Calibrated? John A. Knaff NOAA/NESDIS Fort Collins, CO

2. Dvorak Technique: Overview The Dvorak Technique estimates tropical cyclone intensity by analyzing satellite image patterns and IR cloud top temperatures. Intensity is assigned with intensity units (called T- numbers ranging from 1 to 8, in 0.5 increments), where one T-number represents one day’s intensity change at an average rate. The T-number can be given as a maximum surface wind speed or a minimum sea-level pressure.

5. Dvorak Technique: Procedure Simplified Approach: –1. Locate Center –2. Assign Pattern –3. Make measurements (Visible or EIR) –4. Assign T-Number –5. Assign CI (Current Intensity)

6. Why Calibrate? Through a concerted WMO effort the Dvorak Technique (circa 1984) was taught to and adopted at all WMO RMSCs and Tropical Cyclone Warning Centers by the late 1980’s The method has been found to be relatively stable with respect to satellite sensor resolution (Zehr, Beven, DeMaria) Historical records exist for these estimates, thus these represent a quality climate record of global tropical cyclone intensity. A systematic validation vs. Aircraft estimated winds (i.e., best track) has not been done and is needed. HOW ACCURATE/PERSISE ?

7. Questions I Want to Answer 1.Does the Dvorak Technique have systematic biases wrt intensity, intensity trend, size, translational speed, and latitude? 2.What are the error characteristics of such estimates as a function of …. ? 3.Can CI estimates be improved for? –Operationally (for advisories) –Post-season (i.e., best tracking) –Reanalysis of historical TCs

8. Approach TC fix, advisories and best track data, 1989-2007 For each fix –Interpolate intensity from the best track (truth) –Get Radius of Outer Closed Isobar from advisories and best tracks, interpolate to fix time –Compare intensity (fix vs. truth) Homogeneous fix record from 1) Satellite Analysis Branch (SAB, Washington) and 2) Tropical Analysis and Forecast Branch (TAFB, Miami) Stratify by factors (composite) –Intensity –Intensity & Intensity trend (12-h) –Intensity & Latitude –Intensity & Size (ROCI) –Intensity & Speed of translation Regression using the composite datasets that span the range of factors.

10. Stratified by Intensity (~T-number)

11. Stratified by Intensity Trend & Intensity TAFBSAB

12. Partial Correlations: Remaining Factors TAFB FactorPartial Correlation Intensity Trend0.52 Storm Speed0.27 Sin (latitude)0.38 ROCI-0.24 SAB FactorPartial Correlation Intensity Trend0.54 Storm Speed0.39 Sin (latitude)0.39 ROCI-0.34

13. Multiple Regression – Based Bias Correction Valid for Vmax 20-140kt

14. A Universal Relationship Assume a function for biases as a fixed function of intensity Recalculate the regression coefficients. Valid for Vmax 20-140kt

15. Conclusions Shown –A bias correction for the Dvorak intensity estimates has been developed Biases are a function of –Intensity –Intensity change –Latitude –Size (ROCI) –Storm speed –Can be applied in real-time, and for reanalyses Not shown –RMSE appear to be solely a function of intensity