A Study of the Constraints of the Dvorak Technique, Part 2: Examining Changes in the Cloud Patterns

The Dvorak Technique for estimating the intensity of tropical cyclones (TCs) has been used world-wide for more than 40 years. It includes a methodology for quantifying the cloud patterns associated with TCs, along with a set of constraints (rules) that govern how quickly the intensity estimates can change over periods of 6, 12, 18, and 24 hours. Previous work by the authors determined that observed TC intensity changes (based on best tracks from aircraft reconnaissance data) violated the technique constraints in about 1% of the available cases. This paper evaluates changes in the Dvorak Technique intensity estimates based on TC cloud patterns (the Data-T number) to see how often these measurements suggest the technique constraints should be broken. The data set for this study is twelve years of Data-T numbers analyzed by the NHC Tropical Analysis and Forecast Branch for Atlantic TCs. Preliminary results show that the intensity changes derived from the Data-T numbers violate the technique constraints in 1% (for 24 h changes) to 8 % (for 6 h changes) of the available cases, which is greater than the rate seen in the best track data. Additional work to assess the significance of this result is ongoing.