Operational Use of the Dvorak Technique and Associated Issues at the National Hurricane

Operational Use of the Dvorak Technique and associated issues at the National Hurricane Center, Miami, Florida, USA

1. The Tropical Analysis and Forecast Branch (TAFB) of the National Hurricane Center (NHC) performs manual subjective Dvorak location and intensity estimates.The TAFB also performs similar estimates for subtropical cyclones using the Hebert-Poteat subtropical cyclone technique.

2. It is the NHC philosophy that these estimates are supposed to be as independent of other data sources as possible (which is not 100% possible).

3. Full Dvorak analyses are made every six hours at the synoptic times, with locations provided at the intermediate synoptic times. Full Dvorak analyses can be made at non-synoptic times if necessary.

4. The TAFB uses the 1984 version of the Dvorak Technique. No changes have been made to the basic calibration, and the winds and pressures reported to the HSU use the 1984 tables. There are also no departures from the technique flow charts.

5. Departures from Dvorak (1984) in TAFB operations:

a. 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”.

b. When measuring infrared eye patterns in step 2C, occasionally a BD enhancement color completely 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.The following image is an example from Atlantic Hurricane Jeanne in September 2004.

c. On page 36 of Dvorak (1984)there is a rule concerning the eye adjustment for large or elongated eyes in infrared imagery as part of step 2C. The TAFB does not use this rule.

d. For the visible eye patterns in step 2C and the visible CDO pattern in step 2D, the TAFB interpolates between the distances/sizes given in the tables to produce Central Feature numbers at 0.5 T-number resolution.

e. For Pattern T-Numbers in step 6, 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.

f. The TAFB uses a modified set of the Dvorak (1984) constraints on the allowed Final T-Number changes in steps 7 and 8 based on Lushine (1977). This deals with when the analyst can allow a looser constraint on changes in the Final T-number. The differences are in the following table.

g. For the forecast intensity in step 10, the TAFB uses a combination of Dvorak (1984) and rules from the version of the technique published in 1995.

h. 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.

6. The Hurricane Specialist Unit (HSU) of the NHC creates the tropical cyclone forecasts. This unit uses the Dvorak estimates from TAFB, as well as one provided by the Satellite Analysis Branch, in its operations. It is the responsibility of the HSU to integrate the Dvorak analyses with other data sources (when available). Even when Dvorak estimates 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.

7. The Advanced Dvorak Technique (ADT) developed by the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin is not yet considered operational by the NHC. The ADT data are used as a supplement to the manual estimates.

References:

Dvorak, V. E., 1984: Tropical cyclone intensity analysis using satellite data. NOAA Tech. Rep. NESDIS 11, National Oceanic and Atmospheric Administration, Washington, DC, 47 pp.

Dvorak, V. E., 1995: Tropical clouds and cloud systems observed in satellite imagery: Tropical cyclones. Workbook Vol. 2., 359 pp. [Available from NOAA/NESDIS, 5200 Auth Rd., Washington, DC, 20333.]

Hebert, P. J., and K. O. Poteat, 1975: A satellite classification technique for subtropical cyclones. NOAA Technical Memorandum NWS SR-83, U.S. Dept. of Commerce, National Weather Service, Ft. Worth TX, 25 pp.

Lushine, J.B., 1977: A Relationship Between Weakening of Tropical Cyclone Cloud Patterns and Lessening of Wind Speed. NOAA Tech. Mem. NESS 85, 12 pp.