Dvorak technique

The Dvorak technique (developed in 1973 by Vernon Dvorak^[1]) is a widely used system to subjectively estimate tropical cyclone intensity based solely on visible and infrared satellite images. Several agencies issue Dvorak intensity 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), and the Joint Typhoon Warning Center at the Naval Pacific Meteorology and Oceanography Center in Pearl Harbor, Hawaii.^[2]

Details of technique

Dvorak T-Number and Corresponding Intensity^[3]

T-Number	Winds			Category (SSHS)	Min. Pressure (millibars)
	(knots)	(mph)	(km/h)		Atlantic	NW Pacific
1.0 - 1.5	25	29	46	TD	----	----
2.0	30	35	56	TD	1009	1000
2.5	35	40	65	TS	1005	997
3.0	45	52	83	TS	1000	991
3.5	55	63	102	TS	994	984
4.0	65	75	120	Cat 1	987	976
4.5	77	89	143	Cat 1–2	979	966
5.0	90	104	167	Cat 2–3	970	954
5.5	102	117	189	Cat 3	960	941
6.0	115	132	213	Cat 4	948	927
6.5	127	146	235	Cat 4	935	914
7.0	140	161	260	Cat 5	921	898
7.5	155	178	287	Cat 5	906	879
8.0	170	196	315	Cat 5	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 tracked over 24 hours 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.^[4] 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).^[5] The T-number and CI value are the same except for weakening storms, in which case the CI is higher.^[6][7] The table at right shows the approximate surface wind speed and sea level pressure that corresponds to a given T-number.^[2]

Pattern types

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.^[8]

Usage

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. ^[9]

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–Madison 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.^[2]

Satellite Images of Selected Tropical Storms and Associated T-Number

Tropical Storm Wilma at T3.0	Tropical Storm Dennis at T4.0	Hurricane Jeanne at T5.0	Hurricane Emily at T6.0

See also

Other tools used to determine tropical cyclone intensity:

• Quikscat
• TRMM

Other methods used for determining intensity from satellite imagery:

• Extratropical transition technique
• Hebert-Poteat technique

References

1. ^ Norcross, Bryan (2007). Hurricane Almanac: The Essential Guide to Storms Past, Present, and Future. Macmillan. p. 125. ISBN 0312371527. 
2. ^ ^{a b c} "Objective Dvorak Technique". University of Wisconsin. http://cimss.ssec.wisc.edu/tropic/research/products/dvorak/odt.html. Retrieved 2006-05-29. 
3. ^ Satellite and Information Service Division (April 17, 2005). "Dvorak Current Intensity Chart". National Oceanic and Atmospheric Administration. http://www.ssd.noaa.gov/PS/TROP/CI-chart.html. Retrieved 2006-06-12. 
4. ^ "Tropical Cyclone Forecasters Reference Guide". Naval Research Laboratory. http://www.nrlmry.navy.mil/~chu/chap6/ch6apb.htm. Retrieved 2006-05-29. 
5. ^ "NOAA HRD FAQ". NOAA. http://www.aoml.noaa.gov/hrd/tcfaq/H1.html. 
6. ^ Leffler, J.W. "T-Number Curve Comparison between JTWC and JMA". http://metoc.npmoc.navy.mil/jtwc/tnumcurves.jpg. 
7. ^ "The Dvorak Technique Explained". NOAA. http://www.ssd.noaa.gov/PS/TROP/dvorak.html. Retrieved 2006-05-29. 
8. ^ De Maria, Mark. "Satellite Application is Tropical Weather Forecasting". http://www.comet.ucar.edu/class/satmet/11_Apr19_1999/html/demariamn/sld030.htm. Retrieved 2006-05-29. 
9. ^ "NHC Tropical Depression 24 Discussion Number 3". NOAA. http://www.nhc.noaa.gov/archive/2005/dis/al242005.discus.003.shtml?. Retrieved 2006-05-29. 

External links

Agencies issuing Dvorak intensity estimates

• Joint Typhoon Warning Center
• UW–CIMSS (Advanced Dvorak Technique)
• NOAA/NESDIS Satellite Analysis Branch
• About the TAFB

Other

• Tropical Cyclone Intensity Analysis and Forecasting from Satellite Imagery Dvorak, 1974. (PDF, 1.3 MB)
• Dvorak Tropical Cyclone Wind Speed Biases Determined from Reconnaissance-based "Best Track" Data (1997-2003) Franklin and Brown
• The Dvorak Technique Through Time Dr. Jack Beven. (WRF File. Requires WebEx player)