rammt/cira tropical cyclone overview the dvorak technique introduction visible technique ir...
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RAMMT/CIRA
Tropical Cyclone OverviewTHE DVORAK TECHNIQUE
• Introduction• Visible Technique• IR Technique• Strengths and Weaknesses• Lab Exercise: Visible Pattern Classification
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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
• 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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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
• Tropical Storm Ivan curves 0.7 around log 10 spiral. This corresponds to DT=3
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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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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: 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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Eye Pattern
• DT number determined by CF+BF=DT– CF=CENTRAL FEATURE– BF=BANDING FEATURE– DT=DATA T NUMBER
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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 - Central Feature Cont’d
• Eye adjustment1. 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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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 - 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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Eye - Central Feature Cont’d
• Eye adjustment1. 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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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– 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– 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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Summaryof 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