Dvorak technique

Subjective technique to estimate tropical cyclone intensity

The Dvorak technique (developed between 1969 and 1984 by Vernon Dvorak) is a widely used system to estimate tropical cyclone intensity (which includes tropical depression, tropical storm, and hurricane/typhoon/intense tropical cyclone intensities) based solely on visible and infrared satellite images. Within the Dvorak satellite strength estimate for tropical cyclones, there are several visual patterns that a cyclone may take on which define the 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), central cold cover (CCC) pattern, banding eye pattern (T4.0–T4.5), and eye pattern (T4.5–T8.0). Both the central dense overcast and embedded eye pattern use the size of the CDO. The CDO pattern intensities start at T2.5, equivalent to minimal tropical storm intensity (40 mph, 65 km/h). The shape of the central dense overcast is also considered. The eye pattern utilizes the coldness of the cloud tops within the surrounding mass of thunderstorms and contrasts it with the temperature within the eye itself. The larger the temperature difference is, the stronger the tropical cyclone. 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. The CCC pattern indicates little development is occurring, despite the cold cloud tops associated with the quickly evolving feature. Several agencies issue Dvorak intensity numbers for tropical cyclones and their precursors, including 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 Meteorology and Oceanography Command in Pearl Harbor, Hawaii.

The initial development of this technique occurred in 1969 by Vernon Dvorak, using satellite pictures of tropical cyclones within the northwest Pacific Ocean. The system as it was initially conceived involved pattern matching of cloud features with a development and decay model. As the technique matured through the 1970s and 1980s, measurement of cloud features became dominant in defining tropical cyclone intensity and central pressure of the tropical cyclone's low pressure area. Use of infrared satellite imagery led to a more objective assessment of the strength of tropical cyclones with eyes, using the cloud top temperatures within the eyewall and contrasting them with the warm temperatures within the eye itself. Constraints on short term intensity change are used less frequently than they were back in the 1970s and 1980s. The central pressures assigned to tropical cyclones have required modification, as the original estimates were 5–10 hPa (0.15–0.29 inHg) too low in the Atlantic and up to 20 hPa (0.59 inHg) too high in the northwest Pacific. This led to the development of a separate wind pressure relationship for the northwest Pacific, devised by Atkinson and Holliday in 1975, then modified in 1977.

+Dvorak T Number and Corresponding Intensity T Number 1 min Winds Category (SSHWS) Min. Pressure (millibars) (knots) (mph) (km/h) Atlantic NW Pacific1.0 – 1.5 25 29 45 below TD 2.0 30 35 55 TD 1009 10002.5 35 40 65 TS 1005 9983.0 45 52 83 TS 1000 9913.5 55 63 102 TS Cat 1 994 9844.0 65 75 120 Cat 1 987 9764.5 77 89 143Cat 1–Cat 2 979 9665.0 90 104 167 Cat 2–Cat 3 970 9545.5 102 117 189 Cat 3 960 9416.0 115 132 213 Cat 4 948 9276.5 127 146 235 Cat 4 935 9157.0 140 161 260 Cat 5 921 8987.5 155 178 287 Cat 5 906 8798.0 170 196 315 Cat 5 890 8588.5 185 213 343 Cat 5 873 841Note: The pressures shown for the NW Pacific basin are lower as the pressure of the entire basin are relatively lower than that of the Atlantic basin. 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. 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" (an abbreviation for Tropical Number) and a Current Intensity (CI) value are assigned to the storm. These measurements range between 1 (minimum intensity) and 8 (maximum intensity). The T number and CI value are the same except for weakening storms, in which case the CI is higher. For weakening systems, the CI is held as the tropical cyclone intensity for 12 hours, though research from the National Hurricane Center indicates that six hours is more reasonable. Both the central dense overcast and embedded eye pattern utilize the size of the CDO. The CDO pattern intensities start at T2.5, equivalent to minimal tropical storm intensity The shape of the central dense overcast is also considered. The farther the center is tucked into the CDO, the stronger it is deemed. The CCC pattern, with its large and quickly developing mass of thick cirrus clouds spreading out from an area of convection near a tropical cyclone center within a short time frame, indicates little development. When it develops, rainbands and cloud lines around the tropical cyclone weaken and the thick cloud shield obscures the circulation center. While it resembles a CDO pattern, it is rarely seen. Winds within tropical cyclones can also be estimated by tracking features within the CDO using rapid scan geostationary satellite imagery, whose pictures are taken minutes apart rather than every half hour. 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.

Several agencies issue Dvorak intensity numbers for tropical cyclones and their precursors. These include the National Hurricane Center's Tropical Analysis and Forecast Branch (TAFB), the National Oceanic and Atmospheric Administration's Satellite Analysis Branch (SAB), and the Joint Typhoon Warning Center at the Naval Pacific Meteorology and Oceanography Center in Pearl Harbor, Hawaii. 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. Note that in this case the Dvorak T number (in this case T2.5) was simply used as a guide but other factors determined how the NHC decided to set the system's intensity. 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. Citizen science site Cyclone Center uses a modified version of the Dvorak technique to categorize post 1970 tropical weather.

The most significant benefit of the use of the technique is that it has provided a more complete history of tropical cyclone intensity in areas where aircraft reconnaissance is neither possible nor routinely available. Intensity estimates of maximum sustained wind are currently within of what aircraft are able to measure half of the time, though the assignment of intensity of systems with strengths between moderate tropical storm force and weak hurricane or typhoon force is the least certain. Its overall precision has not always been true, as refinements in the technique led to intensity changes between 1972 and 1977 of up to The method is internally consistent in that it constrains rapid increases or decreases in tropical cyclone intensity. Some tropical cyclones fluctuate in strength more than the 2.5 T numbers per day limit allowed by the rule, which can work to the technique's disadvantage and has led to occasional abandonment of the constraints since the 1980s. Systems with small eyes near the limb, or edge, of a satellite image can be biased too weakly using the technique, which can be resolved through use of polar orbiting satellite imagery. Subtropical cyclone intensity cannot be determined using Dvorak, which led to the development of the Hebert Poteat technique in 1975. Cyclones undergoing extratropical transition, losing their thunderstorm activity, see their intensities underestimated using the Dvorak technique. This led to the development of the Miller and Lander extratropical transition technique which can be used under these circumstances.