An Evaluation of Dvorak Technique–Based Tropical Cyclone Intensity Estimates

Abstract

The satellite-based Dvorak technique (DVKT) is the most widely available and readily used tool for operationally estimating the maximum wind speeds associated with tropical cyclones. The DVKT itself produces internally consistent results, is reproducible, and has shown practical accuracy given the high cost of in situ or airborne observations. For these reasons, the DVKT has been used in a reasonably uniform manner globally for approximately 20 years. Despite the nearly universal use of this technique, relatively few systematic verifications of the DVKT have been conducted. This study, which makes use of 20 yr of subjectively determined DVKT-based intensity estimates and best-track intensity estimates influenced by aircraft observations (i.e., 62 h) in the Atlantic basin, seeks to 1) identify the factors (intensity, intensity trends, radius of outer closed isobar, storm speed, and latitude) that bias the DVKT-based intensity estimates, 2) quantify those biases as well as the general error characteristics associated with this technique, and 3) provide guidance for better use of the operational DVKT intensity estimates. Results show that the biases associated with the DVKT-based intensity estimates are a function of intensity (i.e., maximum sustained wind speed), 12-h intensity trend, latitude, and translation speed and size measured by the radius of the outer closed isobar. Root-mean-square errors (RMSE), however, are shown to be primarily a function of intensity, with the best signal-to-noise (intensity to RMSE) ratio occurring in an intensity range of 90-125 kt (46-64 m s 21 ). The knowledge of how these factors affect intensity estimates, which is quantified in this paper, can be used to better calibrate Dvorak intensity estimates for tropical cyclone forecast operations, postseason best-track analysis, and climatological reanalysis efforts. As a demonstration of this capability, the bias corrections developed in the Atlantic basin are also tested using a limited east Pacific basin sample, showing that biases and errors can be significantly reduced.

Cyclone Center: Can Citizen Scientists Improve Tropical Cyclone Intensity Records?

“…Such a project could improve the CC consensus by including corrections for biases in TC intensity estimates that originate from the Dvorak technique, as demonstrated in Knaff et al (2010). The information presented here is just a hint of what is possible with the data that have been (and are still continuously being) collected.…”

“…In general, the technique involves the interpretation of an IR image of a TC and the application of a number of constraints and set procedures. Dvorak has been used consistently at all global TC forecast agencies for at least the last 20 years, and validation studies (e.g., Gaby et al 1980;Knaff et al 2010) have shown that average differences between Dvorak intensity estimates and aircraft reconnaissance-based best-track data are quite low, ranging from 1.5 to 9 knots (kt; 1 kt = 0.51 m s -1 ).…”

Recent intense hurricane response to global climate change

“…Even at the most basic level, the definition of wind speed varies considerably around the globe, with 1, 10, 3 and even 2-min averaging periods in use (Harper et al 2008). Some homogenization is provided by the universal use of the Dvorak satellite interpretation technique (Velden et al 2006;Knaff et al 2010) but there remain issues with the approaches used to convert from the satellite pattern code to a surface wind speed.…”

“…There may have been a consistent change in analysis practice and/or satellite data that led to increasing analysis of intense systems over the time period (Knaff et al 2010). We check this potential impact by using the objective satellite-based reanalysis undertaken by Kossin et al (2007).…”

Are Special Processes at Work in the Rapid Intensification of Tropical Cyclones?

“…Most contemporary satellite-based intensity estimates are based on the Dvorak technique (Velden et al 2006), in which intensity is estimated from image patterns and infraredderived cloud-top temperatures. Satellite-based storm intensities are usually within roughly 10 kt of aircraft reconnaissance-based estimates, at least since 1989 (Knaff et al 2010). Some discussion of the effect of Dvorak-type techniques on tropical cyclone intensity estimation can be found in Schreck et al (2014).…”