The Dvorak Technique (developed in 1974 by Vernon Dvorak) is a widely used system to 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) and the Air Force Weather Agency (AFWA).^[1] Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station on March 26, 2004. ... A weather satellite is a type of artificial satellite that is primarily used to monitor the weather and climate of the Earth. ... The U.S. National Hurricane Center is the division of National Weather Services Tropical Prediction Center responsible for tracking and predicting the likely behavior of tropical depressions, tropical storms and hurricanes. ... The National Oceanic and Atmospheric Administration (NOAA) is a scientific agency of the United States Department of Commerce focused on the conditions of the oceans and the atmosphere. ... Shield of the Air Force Weather Agency The Air Force Weather Agency (AFWA) is the lead strategic weather center of the United States Air Force. ...

Details of technique

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, tend to change in appearance in a predictable manner. The structure and organization of the tropical cyclone are compared to what it was previously to determine if the storm has weakened, remained the same, or strengthened. Various central cloud features and banding features are compared with an analysis of many previous storms to arrive at an intensity estimate.^[2] 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 Current Intensity (CI) value between 1 (minimum intensity) and 8 (maximum intensity) are assigned to the storm.^[3] The T-number and CI value are the same except for weakening storms, in which case the CI is higher.^[4][5] The table at right shows the approximate surface wind speed (MWS) and sea level pressure (MSLP) that corresponds to a given T-number.^[1] Eye of Category 4 Hurricane Isabel seen from the International Space Station on September 15, 2003 The eye is a region of mostly calm weather found at the center of strong tropical cyclones. ... diurnal (daily) rhythm of air pressure in northern Germany (black curve is air pressure) Atmospheric pressure is the pressure above any area in the Earths atmosphere caused by the weight of air. ...

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: The U.S. National Hurricane Center is the division of National Weather Services Tropical Prediction Center responsible for tracking and predicting the likely behavior of tropical depressions, tropical storms and hurricanes. ... Lowest pressure 882 mbar (hPa) (Lowest pressure ever recorded in an Atlantic hurricane) Damages $16-20 billion (2005 USD) Fatalities 22 direct, 40 indirect Areas affected Jamaica, Haiti, Cayman Islands, Cuba, Honduras, Nicaragua, Belize, Yucatán Peninsula, Florida, Bahamas, Atlantic Canada Part of the 2005 Atlantic hurricane season Hurricane Wilma... The 2005 Atlantic hurricane season was the most active Atlantic hurricane season in recorded history, shattering previous records on repeated occasions. ...

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. ^[6]

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] The University of Wisconsin–Madison is a public university located in Madison, Wisconsin. ...

Satellite Images of Selected Tropical Storms and Associated T-Number

Tropical Storm Katrina at T2.5	Tropical Storm Wilma at T3.0	Tropical Storm Rita at T3.5

Lowest pressure 902 mbar (hPa) Damages $75 billion (2005 USD) (costliest Atlantic hurricane in history) Fatalities ≥1,836 total Areas affected Bahamas, South Florida, Cuba, Louisiana (especially Greater New Orleans), Mississippi, Alabama, Florida Panhandle, most of eastern North America Part of the 2005 Atlantic hurricane season Hurricane Katrina was the... Lowest pressure 882 mbar (hPa) (Lowest pressure ever recorded in an Atlantic hurricane) Damages $16-20 billion (2005 USD) Fatalities 22 direct, 40 indirect Areas affected Jamaica, Haiti, Cayman Islands, Cuba, Honduras, Nicaragua, Belize, Yucatán Peninsula, Florida, Bahamas, Atlantic Canada Part of the 2005 Atlantic hurricane season Hurricane Wilma... Lowest pressure 895 mbar (hPa)[1] Damages $10 billion (2005 USD)[1] Fatalities 7 direct, 55–112 indirect[citation needed] Areas affected Bahamas, Florida, Cuba, Yucatán Peninsula, Louisiana, Texas, Mississippi, Arkansas Part of the 2005 Atlantic hurricane season Hurricane Rita is the fourth-most intense Atlantic hurricane ever recorded...

See also

Other tools used to determine tropical cyclone intensity:

• Quikscat
• TRMM

The QuikSCAT (Quick Scatterometer) is an earth-observing satellite that provides wind speed and direction information over oceans to NOAA.It is a quick recovery mission to fill the gap created by the loss of data from the NASA Scatterometer (NSCAT) that was lost in June 1997. ... Artist conception of the TRMM satellite. ...

References

1. ^ ^a b c "Objective Dvorak Technique."
2. ^ ""Tropical Cyclone Forecasters Reference Guide"."
3. ^ "NOAA HRD FAQ."
4. ^ Leffler, J.W. "T-Number Curve Comparison between JTWC and JMA."
5. ^ "The Dvorak Technique Explained."
6. ^ "NHC Tropical Depression 24 Discussion Number 3."

External Links

Agencies issuing Dvorak intensity estimates:

• Air Force Weather Agency
• NOAA/NESDIS Satellite Analysis Branch
• About the TAFB

Other:

• Dvorak, 1974 Abstract