Satellites are the primary observing systems in the tropics. The most widely used technique to estimate tropical cyclone intensity is called the Dvorak Technique developed by Vernon Dvorak in the early 1970s while he was working for the National Oceanic and Atmospheric Administration. The technique uses enhanced infrared and/or visible satellite imagery from geostationary or polar orbiting satellites to estimate intensity.
Originally, the technique stepped an analyst through a flow chart to determine the cloud pattern associated with a given tropical cyclone. Curved band, shear, eye and central dense overcast (CDO) are the most common patterns. Basically, the more banding, the closer the low-level center is to the overcast, the better defined and more centrally embedded within the overcast the eye is, or the larger and rounder the CDO appears, the higher the intensity estimate for the various patterns.
A paper on the Dvorak Technique by Velden et al. was published in the Bulletin of the American Meteorological Society in September 2006. The paper states that “the Dvorak technique’s practical appeal and demonstrated skill in the face of tremendous dynamic complexity place it among the great meteorological innovations of our time. It is difficult to think of any other meteorological technique that has withstood the test of time and had the same life-saving impact.” The entire paper can be found online at http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2FBAMS-87-9-1195.
The Dvorak Technique has been improved over time and a more objective version has been in use at the National Hurricane Center for the past few years. Microwave imagery from satellites has also helped tremendously in estimating intensity for weaker tropical cyclones. I’ll get to that eventually.
For now, just know that the Dvorak Technique is used to provide the maximum sustained wind speed estimate in tropical cyclones most of the time. Reference to the Dvorak estimates is made in the majority of Tropical Cyclone Discussions from the NHC. But also note that these are “estimates” of the maximum sustained wind speed and not true measurements of the 1-minute average wind at an elevation of 10 meters discussed in my previous blog. It is far easier to define what the maximum sustained wind speed is than to measure it.