48 What is the Dvorak technique and how is it used?
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This article is from the Storms FAQ, by Chris Landsea landsea@aoml.noaa.gov with numerous contributions by others.
48 What is the Dvorak technique and how is it used?
The Dvorak technique is a methodology to get estimates of tropical cyclone
intensity from satellite pictures. Vern Dvorak developed the scheme using
a pattern recognition decision tree in the early 1970s (Dvorak 1975, 1984).
Utilizing the current satellite picture of a tropical cyclone, one matches
the image versus a number of possible pattern types: Curved band Pattern,
Shear Pattern, Eye Pattern, Central Dense Overcast (CDO) Pattern, Embedded
Center Pattern or Central Cold Cover Pattern. If infrared satellite
imagery is available for Eye Patterns (generally the pattern seen for
hurricanes, severe tropical cyclones and typhoons), then the scheme
utilizes the difference between the temperature of the warm eye and the
surrounding cold cloud tops. The larger the difference, the more intense
the tropical cyclone is estimated to be. From this one gets a data
"T-number" and a "Current Intensity (CI) Number". CI numbers have been
calibrated against aircraft measurements of tropical cyclones in the
Northwest Pacific and Atlantic basins. On average, the CI numbers
correspond to the following intensities:
CI Maximum Sustained Central Pressure
Number One Minute Winds (mb)
(kt) (Atlantic) (NW Pacific)
0.0 <25 ---- ----
0.5 25 ---- ----
1.0 25 ---- ----
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 that this estimation of both maximum winds and central pressure
assumes that the winds and pressures are always consistent. However,
since the winds are really determined by the pressure gradient, small
tropical cyclones (like the Atlantic's Andrew in 1992, for example)
can have stronger winds for a given central pressure than a larger
tropical cyclone with the same central pressure. Thus caution is urged
in not blindly forcing tropical cyclones to "fit" the above pressure-
wind relationships. (The reason that lower pressures are given to
the Northwest Pacific tropical cyclones in comparison to the higher
pressures of the Atlantic basin tropical cyclones is because of the
difference in the background climatology. The Northwest Pacific basin
has a lower background sea level pressure field. Thus to sustain a
given pressure gradient and thus the winds, the central pressure must
accordingly be smaller in this basin.)
The errors for using the above Dvorak technique in comparison to
aircraft measurements taken in the Northwest Pacific average 10 mb with
a standard deviation of 9 mb (Martin and Gray 1993). Atlantic tropical
cyclone estimates likely have similar errors. Thus an Atlantic hurricane
that is given a CI number of 4.5 (winds of 77 kt and pressure of 979 mb)
could in reality be anywhere from winds of 60 to 90 kt and pressures of
989 to 969 mb. These would be typical ranges to be expected; errors
could be worse. However, in the absence of other observations, the
Dvorak technique does at least provide a consistent estimate of what the
true intensity is.
While the Dvorak technique was calibrated for the Atlantic and
Northwest Pacific basin because of the aircraft reconnaissance data
ground truth, the technique has also been quite useful in other
basins that have limited observational platforms. However, at some
point it would be preferable to re-derive the Dvorak technique to
calibrate tropical cyclones with available data in the other basins.
Lastly, while the Dvorak technique is primarily designed to provide
estimates of the current intensity of the storm, a 24 h forecast of the
intensity can be obtained also by extrapolating the trend of the
CI number. Whether this methodology provides skillful forecasts is
unknown.
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