The objective of this section is to provide an overview of the Dvorak technique (Dvorak, 1973, and 1984). This technique supplies the bulk of TC fixes for the Pacific and Indian Oceans. Although mean absolute errors inherent in intensities obtained through the Dvorak technique are large (approximately 15 hPa), these fixes are frequently treated as ground truth in many TC intensity forecast verification studies, because these are the only data available. The following provides an outline of the steps required to develop an intensity estimate (CI) and forecast when using the Dvorak technique.
1. How has the Disturbance Changed Since the Previous Observations?
The current satellite imagery is compared with the picture from the previous observation (approximately 24 hours long term and 6 hours short term). Has it developed, remained the same, or weakened?
2. Current T-Number is Assigned.
The central cloud features and banding features are compared using various satellite enhancement curves and templates. A current T-number is assigned based on these comparisons. A T-number which may range from T1 to T8 is defined as a description of a TC in terms of cloud characteristics visible in satellite imagery. The current intensity (i.e., CI Number) is then computed using the T-number and a few rules for re-developing or weakening storms. The CI number is considered the best estimate of the current maximum winds and sea-level pressure of the TC. Table B-10 lists the CI numbers and their approximate intensity.
3. Indications of Ongoing Change.
A disturbance will normally exhibit indications of intensification or weakening in the satellite imagery as it develops or weakens. A Plus or Minus sign may be added to the code when the dominant signs of ongoing change conflict with the trend indicated in the observation.
4. 24 Hour Forecast Intensity.
The normal forecast intensity change using the Dvorak model is one T-number increase or decrease during a 24 hour period. A plus or minus sign will be added to the code to indicate a slower or faster trend.
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Appendix A Appendix C