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Tropical Cyclone OverviewTHE DVORAK TECHNIQUE

• 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

• 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 4.0 4.5

DT Number

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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 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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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 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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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– 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