Dvorak technique

The Dvorak technique (developed in 1974 by Vernon Dvorak) is a common system for subjectively determining the intensity of tropical cyclones using satellite images and infrared images .

Technical details

**Dvorak T numbers with corresponding intensity**
T number	Wind speed		minimum central pressure (hPa)
T number	( Knot )	(km / h)	Atlantic	NW Pacific
1.0 - 1.5	25th	46	----	----
2.0	30th	56	1009	1000
2.5	35	65	1005	997
3.0	45	83	1000	991
3.5	55	102	994	984
4.0	65	120	987	976
4.5	77	143	979	966
5.0	90	167	970	954
5.5	102	189	960	941
6.0	115	213	948	927
6.5	127	235	935	914
7.0	140	260	921	898
7.5	155	287	906	879
8.0	170	315	890	858
Note: The central pressures in the NW Pacific are set lower because the air pressure in the Pacific is generally lower than in the Atlantic.

The Dvorak technique exploits the fact that developing cyclones of the same intensity mostly have similar properties, and that the appearance of these cyclones usually changes characteristically with increasing intensity. The structure of a tropical cyclone is analyzed every 24 hours in order to identify whether the cyclone has intensified, weakened or maintained its intensity. Several templates of typical storm structures are available with which the core and rain band structure of the storm can be compared in order to be able to draw conclusions about the intensity. If an eye can be made out on infrared images of the storm , the intensity can also be deduced from the temperature differences of the warm eye and the surrounding cloud structure. ( 'Where the warmer the eye and the colder the cloud peaks of the eyewall are, the stronger the hurricane.) Upon completion of the analysis for each system a so-called T-number and a CI number ( Current Intensity , "current intensity" ). The T numbers range from 1 (lowest intensity) to 8 (highest intensity). In most storms, T-numbers are identical to CI-numbers, except in rapidly weakening systems where the CI-number is higher than the T-number.

Structure types

A distinction is made between different structures that can be observed during a storm and that allow an upper and lower limit of intensity.

curved rain band structure (T1.0 – T4.5)
Shear structure (T1.5 – T3.5)
Central Dense Overcast (CDO) (T2.5 – T5.0)
Initial eye stage (T4.0 – T4.5)
Eye structure (T4.5 – T8.0)

If the structure has been recognized, a T number can then be deduced by analyzing the features found (e.g. length and curvature of the rain bands).

use

The Dvorak technique is mostly only used in the Atlantic with stronger systems for comparison with other observations (reconnaissance flights, QuikSCAT data as well as ship and buoy observations ). Only for storms in the East Atlantic, where reconnaissance flights would be too expensive due to the great distance to the base, the Dvorak technology is used specifically to determine the storm intensity. However, the Dvorak technique is the most important basis for determining the storm intensity for storms in all other development areas. (Reconnaissance flights are only carried out in the Atlantic and the East Pacific )