Assignment
Topic – Dvorak Technique
The Dvorak technique is a method using enhanced
Infrared and/or visible satellite imagery to
quantitatively estimate the intensity of a tropical
system. Cloud patterns in satellite imagery normally
show an indication of cyclogenesis before the storm
reaches tropical storm intensity.
In other words
The Dvorak technique is a method using enhanced
Infrared and/or visible satellite imagery to
quantitatively estimate the intensity of a tropical
system. Cloud patterns in satellite imagery normally
show an indication of cyclogenesis before the storm
reaches tropical storm intensity. Indications of
continued development and/or weakening can also be
found in the cloud features. Using these features, the
pattern formed by the clouds of a tropical cyclone,
expected systematic development, and a series of rules,
an intensity analysis and forecast can be made. This
information is then standardized into an intensity code.
HISTORY OF THE DVORAK TECHNIQUE
1973 Dvorak Technique was invented
1975 for visible imagery
2 satellites with 9km resolution
Used at night
Limited the sampling of max sustained surface winds
1984 for infrared imagery (method)
8 satellites with 4 km resolution
More trustworthy to forecasters
More accurate intensity estimates
• The Dvorak Technique quantifies Tropical Cyclone
(TC) intensity on a 1-8 scale (at 0.5 intervals) called T-
Numbers, which are used in a variety of ways.
• The final output of the technique is the Current
Intensity (CI) number.
• The T-numbers are used to fit the cyclone to a curve
of
a model or show removal from a curve.
• C.I. numbers also range from 1 to 8.
• C.I. numbers directly related to the intensity in terms
of the max wind speed (MSP) but not directly related to
cloud features.
• Two factors affecting the C.I. number:
(i). The surveyed delay in the reduction of MSP after
cloud features show weakening.
(ii). The climax of the cyclones winds between satellite
Observations.
• The CI number is driven by the Final-T (FT) Number.
• In turn, the FT is driven by the Data-T (DT) number,
the Model Expected-T (MET) number, and the Pattern-
T (PT or PAT) number.
• The DT is often created from other sub-numbers.
Dvorak Technique Terms
• CSC - Cloud System Centre – The centre of the
disturbance or cyclone. It is usually defined by an eye, a
low-level circulation centre, or by other cloud features.
• CDO - Central Dense Overcast – A dense solid-
looking mass of clouds covering the CSC, often lying
within the curve of the cyclone’s curved cloud band.
• CCC - Central Cold Cover – A large cold or dense
overcast covering the CSC that lacks structure and
obscures the cyclone centre.
• CI Number - Current Intensity number - The final
output of the Dvorak technique and the estimated
intensity of the cyclone.
• DT Number - Data-T number - The estimated
intensity of the cyclone based on the convective cloud
pattern.
• PT or PAT Number - Pattern-T number - The
intensity estimates from comparing the cyclone cloud
pattern to predetermined patterns.
• MET Number - Model Expected-T number - The
intensity estimates from the 24-hr old FT number and
a determined intensity trend.
• FT Number - Final-T number - The intensity
estimates for a given time selected from the DT, PT, or
MET numbers. It is used with a set of rules to determine
the CI number for this time and the MET number 24
hours later.
• FI Number - Forecast Intensity number - 24 hr
intensity forecast based on the CI and observed signals
in the cyclone cloud pattern and the environment.
• Eye Number - Part 1 of the intensity estimate for an
eye pattern based on surrounding cloud top
temperatures (IR) or embedded distance in a CDO
(VIS).
• Eye Adjustment - Part 2 of the intensity estimate for
an eye pattern based on eye temperature (IR) or eye size
and clarity (VIS).
• CF Number - Central Feature number - The part of
the intensity estimate based on the central features of a
cyclone. This number is produced by the eye, CDO, and
embedded centre patterns.
• BF Number - Banding Feature number - The part of
the intensity estimate based on the banding surrounding
central features of a cyclone. This number can be used
with the eye, CDO, and embedded centre patterns.
A major component of the dvorak technique wings on
forecasters classifying the shape of pattern of clouds
observe on visible and infrared satellite imaginary into
four basic categories:
• Curved band pattern
• Swear pattern
• Central dense overcast pattern
• Eye pattern
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.
REFERENCES
B.A. Harper, K. Hoarau, J. A. Knaff, C. W. Landsea,
Science. 313,452 (AAAS, 2006).
V. F. Dvorak, Mon. Weather Rev. 103, 420 (1975).
V.F. Dvorak, Satellite Applications Laboratory,
NOAA. 18 (1984).
https://www.ssd.noaa.gov/PS/TROP/dvorak.html
