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Tropical Cyclone Overview
THE DVORAK TECHNIQUE
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Introduction
Visible Technique
IR Technique
Strengths and Weaknesses
Lab Exercise: Visible Pattern Classification
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Measurements of Tropical Cyclones
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The Dvorak Technique uses Satellite
Measurements
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Most Tropical Cyclone Basins Do Not
Have Aircraft Reconnaissance Data
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Technique Reference
NOAA Technical Report NESDIS 11
Tropical Cyclone Intensity Analysis Using Satellite
Data
Vernon F. Dvorak
Satellite Applications Laboratory
Washington, D.C.
September 1984 (Reprinted October 1985)
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Overview of the Dvorak Technique
• Visible and Infrared Technique
• Simplified Visible Technique given here (See
Technical Report for full details)
• Uses patterns and measurements as seen on
satellite imagery to assign a number (T number)
representative of the cyclone’s strength.
• The T number scale runs from 0 to 8 in
increments of 0.5.
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Overview of the Dvorak Technique
Cont’d
• In the following examples, only the Data T
Number (DT) will be calculated, the final
(official) T number assigned to a tropical cyclone
includes further considerations.
• DT computations familiarize one to various
tropical cyclone patterns.
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Four Basic Patterns
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Curved Band Pattern
Shear Pattern
Central Dense Overcast (CDO) Pattern
Eye Pattern
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Four Basic Patterns
• Pattern is not always obvious
• System may move from one pattern to another
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Patterns and associated T Numbers
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Empirical relationship between T
number and wind speed
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Finding the Cloud System Center (CSC)
• First step in the Dvorak technique
• From Dvorak (1985):
“The cloud system center is defined as the focal
point of all the curved lines or bands of the cloud
system. It can also be thought of as the point
toward which the curved lines merge or spiral.”
• Several situations
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Curved Band Pattern
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Curved Band Pattern
• DT number determined
by curvature of band
around 10 log spiral
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Curved Band Pattern Cont’d
1.0 to 2.0
2.5
3.0
3.5
DT Number
4.0
4.5
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Example: Tropical Storm Ivan
1115 UTC 23 September 1998
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Example: Curved Band
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Curved Band Pattern
• Tropical Storm Ivan curves 0.7 around log 10
spiral. This corresponds to DT=3
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Shear Pattern
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Shear Pattern DT Numbers
1° latitude = 60 nautical miles (nmi) = 111 km
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Example: Hurricane Bertha 2015 UTC
11 July 1996
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Hurricane Bertha Cont’d
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Example: Shear Pattern
Distance of low level rotation less than 1/2° lat
(30 nmi) from dense cloud (-31° C or colder):
DT=3.0
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T Numbers for Weakening Systems
• T numbers decrease before cyclone’s winds
• Current intensity (CI) number represents strength
of weakening system and is larger than T
number.
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Central Dense Overcast (CDO)
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CDO
• No eye
• DT number determined by CF+BF=DT
– CF=CENTRAL FEATURE
– BF=BANDING FEATURE
– DT=DATA T NUMBER
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Example: Hurricane Georges 1545 UTC
21 September 1998
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Example: CDO Central Feature (CF)
• Measure Diameter of CDO in degrees latitude
• For a well defined CDO
– 3/4 °
CF=2
– 1 1/4 °
CF=3
– 1 3/4 °
CF=4
– >2 1/4 °
CF=5
• For an irregular CDO
– 1° to 1 1/2 °
CF=2
– >1 1/2 °
CF=3
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Example: CDO Central Feature (CF)
Cont’d
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Example: CDO - Banding Feature (BF)
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Example CDO - Banding Feature (BF)
Cont’d
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Example: CDO Data T Number
CF + BF = DT
CF = 5
BF = 0.5
DT = 5.5
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Eye Pattern
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Eye Pattern
• DT number determined by CF+BF=DT
– CF=CENTRAL FEATURE
– BF=BANDING FEATURE
– DT=DATA T NUMBER
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Example: Hurricane Georges 1945 UTC
18 September 1998
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Example: Eye - Central Feature (CF)
• CF=E-number+Eye Adjustment
• E-number a measure of the hurricane’s radius in
degrees latitude
– 1/4°
E-no.=3
– 1/2°
E-no.=4
– 3/4°
E-no.=5
– 1°
E-no.=6
– >1°
E-no.=7
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Eye Number
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Eye - Central Feature Cont’d
• Eye adjustment
1. Poorly defined or ragged eyes: Subtract 0.5 for
E-no.  4.5 and 1 for E-no. 5.
2. Large eyes: Limit T-no. to T6 for round, well
defined eyes, and to T5 for large ragged eyes.
3. For MET  6, 0.5 or 1 may be added to DT for
well defined eye in smooth CDO when DT <
MET.
Note: MET is Model-Estimated T,
which is extrapolated from previous
Dvorak estimate
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Eye Adjustment
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Example: Eye - Banding Feature (BF)
( Same as with CDO)
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Banding Feature (BF)
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Data T Number
CF + BF = DT
CF = 6 - 1 = 5
BF = 0.5
DT = 5.5
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Banding Eye Pattern
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Banding Eye Pattern
• DT number determined by CF+BF=DT
– CF=CENTRAL FEATURE
– BF=BANDING FEATURE
– DT=DATA T NUMBER
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Example Banding Eye: Hurricane
Bonnie 2131 UTC 25 August 1998
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Example: Banding Eye - Central Feature
(CF)
• CF=E-number+Eye Adjustment
• E-number a measure of the width of the band in
degrees latitude
– 1/4°
E-no.=3
– 3/4°
E-no.=4
– 11/4°
E-no.=5
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Banding Width
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Eye - Central Feature Cont’d
• Eye adjustment
1. Poorly defined or ragged eyes: Subtract 0.5 for
E-no.  4.5 and 1 for E-no.  5.
2. Large eyes: Limit T-no. to T6 for round, well
defined eyes, and to T5 for large ragged eyes.
3. For MET  6, 0.5 or 1 may be added to DT for
well defined eye in smooth CDO when DT <
MET.
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Eye Adjustment
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Example: Banding Eye - Banding
Feature (BF)
( Same as with CDO)
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Banding Feature (BF)
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Data T Number
CF + BF = DT
CF = 5 - 1 = 4
BF = 2.0
DT = 6.0
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Infrared (IR) Technique
• Can be used during night as well as during day
• At times more objective than visible technique
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Example Digital IR: Hurricane Erika
1515 UTC 8 September 1997
• Warmest eye pixel 16 °C
• Warmest pixel 30 nmi
(55 km) from center -57
°C
• Nomogram gives Eye no.
=5.8 or close to 6
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Dvorak Analysis of TC Intensity
• Strengths
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Consistent, relatively simple approach to a difficult task
Time proven, the primary technique for more than 15 year
Valid for all geographic regions
Patterns based on cloud response to vorticity
Highly reproducible
Better validation and confidence for the more intense storms
• Weaknesses
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Some aspects are too subjective
Subceptible to large errors in weaker systems T-number < 4
“spin down” times are too uniform
poor intensity estimates of very small storms “midgets” at night
Does not account for subtropical or extratropical transition
Does not compensate for large translation speeds (left to the forecaster)
Training and experience are very important because of the subjective nature of
the method.
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Improvements to the Dvorak Technique
• Make the method more objective by using computer
resources and digital data.
– Objective version of IR technique developed by Chris
Velden, U. Wisconsin
• Formalize methods to compensate for known weaknesses
• Improvement of the CI rules, using observed decay rates
from aircraft.
• Incorporation of other routinely available satellite
products (SSMI, AMSU, POES)
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Summary of Lesson 2
• The Dvorak technique uses patterns and measurements
from satellite imagery to estimate the strength of a
tropical cyclone.
• Four basic types
– Curved band pattern
– Shear pattern
– CDO pattern
– Eye pattern
• Banded eye
• IR and visible techniques
• Objective version of IR technique developed by U.
Wisconsin
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