Operational Use of the Dvorak Technique at the NHC

1. NATIONAL CENTERS FOR ENVIRONMENTAL PREDICTION Operational Use of the Dvorak Technique at the NHC JACK BEVEN NATIONAL HURRICANE CENTER WHERE AMERICA’S CLIMATE AND WEATHER SERVICES BEGIN

2. Operational Background • The Tropical Analysis and Forecast Branch (TAFB) of the NHC performs manual subjective Dvorak location and intensity estimates. • The TAFB also performs Hebert-Poteat subtropical cyclone technique intensity estimates. • NHC philosophy is that these estimates are supposed to be as independent of other data sources as possible (not 100% possible!). • Intensity estimates from the Advanced Dvorak Technique (ADT) are used mainly to supplement to the manual estimates.

3. Operational Background • The Hurricane Specialists Unit (HSU, the TC forecasters) of the NHC integrates the Dvorak analyses with other data sources (when available). • Even when they are the only data source, the HSU forecasters are free to employ the Dvorak analyses in whatever way they feel is appropriate. This applies to both real-time forecasting and post-analysis.

4. Operational Background • Full Dvorak analyses are normally made every six hours at the synoptic times, with locations provided at intermediate synoptic times. • Full Dvorak analyses can be made at non-synoptic times if necessary. • The TAFB uses the 1984 version of the Dvorak Technique. • No changes have been made to the wind and pressure basic calibration of the technique. • No changes have been made to the basic Dvorak procedures on the technique flow charts.

5. Departures from Dvorak (1984) • Cloud systems centers that do not meet the convective criteria can be tracked by providing a location and an intensity estimate of “too weak to classify”. • When measuring infrared eye patterns in step 2C, occasionally a BD enhancement color surrounds the eye that is too narrow to use for the eye number. In these cases, the color shade is not used to determine the eye number, but is used to determine the eye adjustment.

6. Black (B) completely surrounds the eye. However, the B ring is less than 0.5 degrees thick. So, it cannot be used for the eye number. The eye number uses Light Gray (LG) for a 5.0, while the eye adjustment is determined by a Warm Medium Gray (WMG) embedded in B (+1.0) - CF=6.0 LG 5.0 1.0 6.0 0.0 6.0 Step 2C - BD Color Used For Eye Adjustment Can Differ From Color Used For Eye Number Jeanne (2004) WMG eye embdd in LG/B

7. Departures from Dvorak (1984) • In step 2C/page 36, there is a rule for the eye adjustment for large or elongated eyes in infrared imagery. The TAFB does not use this rule. • For step2C/visible eye patterns and step 2D/visible CDO pattern, the TAFB interpolates between the distances/sizes in the tables to produce Central Feature numbers at 0.5 T-number resolution. • For step 6/Pattern T-Numbers, the TAFB does not strictly follow the rule that the Pattern T-Number must be within one column (0.5 T-Numbers) of the Model Expected T-number.

8. Departures from Dvorak (1984) • The TAFB uses a modified set of the Dvorak (1984) constraints on the allowed Final T-Number changes (step 7/8) based on Lushine (1977). This deals with when the analyst can allow a looser constraint.

9. Departures from Dvorak (1984) • For step 10/Forecast Intensity, the TAFB uses a combination of Dvorak (1984) and rules from the version of the technique published in 1995. • For systems that spend a significant amount of time over land and then re-emerge over water, the TAFB re-starts Dvorak analyses using the observed Data T-Number and Pattern T-Number.

10. Dvorak Error Distribution These errors are from comparison to aircraft-based best track data. Images courtesy of Brown and Franklin

11. 2010 ADT validation results

12. 2010 ADT validation results

13. 2010 ADT validation results

14. 2010 ADT validation results