Tropical Cyclone Overview THE DVORAK TECHNIQUE

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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 cyclones strength.
• The T number scale runs from 0 to 8 in increments
  of 0.5.

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Overview of the Dvorak Technique Contd

• 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 Contd

• 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 DT3

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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 Contd
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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)
• DT3.0

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T Numbers for Weakening Systems

• T numbers decrease before cyclones 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 CFBFDT
• CFCENTRAL FEATURE
• BFBANDING FEATURE
• DTDATA 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 CF2
• 1 1/4 CF3
• 1 3/4 CF4
• gt2 1/4 CF5
• For an irregular CDO
• 1 to 1 1/2 CF2
• gt1 1/2 CF3

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Example CDO Central Feature (CF) Contd
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Example CDO - Banding Feature (BF)

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Example CDO - Banding Feature (BF) Contd
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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 CFBFDT
• CFCENTRAL FEATURE
• BFBANDING FEATURE
• DTDATA T NUMBER

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Example Hurricane Georges 1945 UTC 18 September
1998
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Example Eye - Central Feature (CF)

• CFE-numberEye Adjustment
• E-number a measure of the hurricanes radius in
  degrees latitude
• 1/4 E-no.3
• 1/2 E-no.4
• 3/4 E-no.5
• 1 E-no.6
• gt1 E-no.7

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Eye Number
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Eye - Central Feature Contd

• 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 lt 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 CFBFDT
• CFCENTRAL FEATURE
• BFBANDING FEATURE
• DTDATA 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)

• CFE-numberEye 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 Contd

• 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 lt 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 lt 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