Dvorak Analysis Challenges during Rapid Intensification

Accurate analysis of tropical cyclones is an essential part of the tropical cyclone (TC) forecast process. It not only sets forecaster expectations of TC evolution in the near term, but also affects the initialization of aids and models on which the forecast is based. Since its inception a half century ago, the Dvorak technique has been used operationally across the globe to subjectively estimate TC intensity from visible and infrared satellite imagery. The Joint Typhoon Warning Center (JTWC) relies heavily on the 1984 version of the method, as much of its area of responsibility lacks in-situ observations and aerial reconnaissance, and requires the analyst and forecaster to understand the limitations and biases of Dvorak analysis.

Tasked with providing detailed evaluations of storm position, structure, and intensity, JTWC’s Satellite Operations department identified the rapid intensification (RI) phase of a TC as a frequent contributing factor to flawed estimates. RI is defined by an increase in maximum sustained winds of 30 knots in a 24-hour period. Though an RI event by itself does not necessarily yield any new biases in the Dvorak technique, it can magnify existing ones. Convective features, system size, latitude, and rate of intensification are characteristics that can contribute to faulty assessments of intensity. We present TC’s that underwent particularly extreme RI over the Western North Pacific, South Pacific, and Indian Ocean basins in conjunction with the Dvorak technique to highlight these challenges and propose solutions to mitigate their impacts.