Dvorak technique

From Wikipedia, the free encyclopedia

The Dvorak technique (developed in 1974 by Vernon Dvorak) is a widely used system to subjectively estimate tropical cyclone intensity based solely on visible and infrared satellite images. Several agencies issue Dvorak numbers for cyclones of sufficient intensity. These include the National Hurricane Center's Tropical Analysis and Forecast Branch (TAFB), the NOAA/NESDIS Satellite Analysis Branch (SAB), the Joint Typhoon Warning Center at the Naval Pacific Meteorology and Oceanography Center in Pearl Harbor HI, and the Air Force Weather Agency (AFWA).^[1]

1 Details of technique
- 1.1 Pattern types
2 Usage
3 See also
4 References
5 External links

**Dvorak T-Number and Corresponding Intensity**^[2]
T-Number	Winds (knots)	Minimum Pressure (millibars)
T-Number	Winds (knots)	Atlantic	NW Pacific
1.0 - 1.5	25	----	----
2.0	30	1009	1000
2.5	35	1005	997
3.0	45	1000	991
3.5	55	994	984
4.0	65	987	976
4.5	77	979	966
5.0	90	970	954
5.5	102	960	941
6.0	115	948	927
6.5	127	935	914
7.0	140	921	898
7.5	155	906	879
8.0	170	890	858
Note: The pressures shown for the NW Pacific are lower as the pressure of that whole environment is lower as well.

In a developing cyclone, the technique takes advantage of the fact that cyclones of similar intensity tend to have certain characteristic features, and as they strengthen, they tend to change in appearance in a predictable manner. The structure and organization of the tropical cyclone are compared to what they were 24 hours previously to determine if the storm has weakened, maintained its intensity, or strengthened. Various central cloud and banding features are compared with templates that show typical storm patterns and their associated intensity.^[3] If infrared satellite imagery is available for a cyclone with a visible eye pattern, then the technique utilizes the difference between the temperature of the warm eye and the surrounding cold cloud tops to determine intensity (colder cloud tops generally indicate a more intense storm). In each case a "T-number" and a Current Intensity (CI) value are assigned to the storm. These measurements range between 1 (minimum intensity) and 8 (maximum intensity).^[4] The T-number and CI value are the same except for weakening storms, in which case the CI is higher.^[5]^[6] The table at right shows the approximate surface wind speed and sea level pressure that corresponds to a given T-number.^[1]

There are several visual "patterns" that a cyclone may take on which put upper and lower bounds on its intensity. The primary patterns used are:

curved band pattern (T1.0-T4.5)
shear pattern (T1.5-T3.5)
central dense overcast (CDO) pattern (T2.5-T5.0)
banding eye pattern (T4.0-T4.5)
eye pattern (T4.5 - T8.0)

Once a pattern is identified, the storm features (such as length and curvature of banding features) are further analyzed to arrive at a particular T-number. ^[7]

The National Hurricane Center will often quote Dvorak T-numbers in their tropical cyclone products. The following example is from discussion number 3 of Tropical Depression 24 (eventually Hurricane Wilma) of the 2005 Atlantic hurricane season:

BOTH TAFB AND SAB CAME IN WITH A DVORAK SATELLITE INTENSITY ESTIMATE OF T2.5/35 KT. HOWEVER ...OFTENTIMES THE SURFACE WIND FIELD OF LARGE DEVELOPING LOW PRESSURE SYSTEMS LIKE THIS ONE WILL LAG ABOUT 12 HOURS BEHIND THE SATELLITE SIGNATURE. THEREFORE... THE INITIAL INTENSITY HAS ONLY BEEN INCREASED TO 30 KT. ^[8]

Note that in this case the Dvorak T-number (in this case T2.5) was simply used as a guide but other factors determined what the NHC decided to set the discussion intensity at.

The Cooperative Institute for Meteorological Satellite Studies (CIMSS) at the University of Wisconsin has developed the Objective Dvorak Technique (ODT). This is a modified version of the Dvorak technique which uses computer algorithms rather than subjective human interpretation to arrive at a CI number. This is generally not implemented for tropical depressions or weak tropical storms.^[1]