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Tropical Cyclone Overview THE DVORAK TECHNIQUE

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Simplified Visible Technique given here (See Technical Report for full details) ... poor intensity estimates of very small storms 'midgets' at night ... – PowerPoint PPT presentation

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Title: Tropical Cyclone Overview THE DVORAK TECHNIQUE


1
Tropical Cyclone OverviewTHE DVORAK TECHNIQUE
  • Introduction
  • Visible Technique
  • IR Technique
  • Strengths and Weaknesses
  • Lab Exercise Visible Pattern Classification

2
Measurements of Tropical Cyclones
3
The Dvorak Technique uses Satellite Measurements
4
Most Tropical Cyclone Basins Do Not Have Aircraft
Reconnaissance Data
5
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)

6
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.

7
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.

8
Four Basic Patterns
  • Curved Band Pattern
  • Shear Pattern
  • Central Dense Overcast (CDO) Pattern
  • Eye Pattern

9
Four Basic Patterns
  • Pattern is not always obvious
  • System may move from one pattern to another

10
Patterns and associated T Numbers
11
Empirical relationship between T number and wind
speed
12
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

13
Curved Band Pattern
14
Curved Band Pattern
  • DT number determined by curvature of band around
    10? log spiral

15
Curved Band Pattern Contd
  • 1.0 to 2.0 2.5 3.0 3.5
    4.0 4.5
  • DT Number

16
Example Tropical Storm Ivan 1115 UTC 23
September 1998
17
Example Curved Band
18
Curved Band Pattern
  • Tropical Storm Ivan curves 0.7 around log 10
    spiral. This corresponds to DT3

19
Shear Pattern
20
Shear Pattern DT Numbers
1 latitude 60 nautical miles (nmi) 111 km
21
Example Hurricane Bertha 2015 UTC 11 July 1996
22
Hurricane Bertha Contd
23
Example Shear Pattern
  • Distance of low level rotation less than 1/2 lat
    (30 nmi) from dense cloud (-31 C or colder)
  • DT3.0

24
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.

25
Central Dense Overcast (CDO)
26
CDO
  • No eye
  • DT number determined by CFBFDT
  • CFCENTRAL FEATURE
  • BFBANDING FEATURE
  • DTDATA T NUMBER

27
Example Hurricane Georges 1545 UTC 21 September
1998
28
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

29
Example CDO Central Feature (CF) Contd
30
Example CDO - Banding Feature (BF)

31
Example CDO - Banding Feature (BF) Contd
32
Example CDO Data T Number
  • CF BF DT
  • CF 5
  • BF 0.5
  • DT 5.5

33
Eye Pattern
34
Eye Pattern
  • DT number determined by CFBFDT
  • CFCENTRAL FEATURE
  • BFBANDING FEATURE
  • DTDATA T NUMBER

35
Example Hurricane Georges 1945 UTC 18 September
1998
36
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

37
Eye Number
38
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

39
Eye Adjustment
40
Example Eye - Banding Feature (BF)

( Same as with CDO)
41
Banding Feature (BF)
42
Data T Number
  • CF BF DT
  • CF 6 - 1 5
  • BF 0.5
  • DT 5.5

43
Banding Eye Pattern
44
Banding Eye Pattern
  • DT number determined by CFBFDT
  • CFCENTRAL FEATURE
  • BFBANDING FEATURE
  • DTDATA T NUMBER

45
Example Banding Eye Hurricane Bonnie 2131 UTC
25 August 1998
46
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

47
Banding Width
48
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.

49
Eye Adjustment
50
Example Banding Eye - Banding Feature (BF)

( Same as with CDO)
51
Banding Feature (BF)
52
Data T Number
  • CF BF DT
  • CF 5 - 1 4
  • BF 2.0
  • DT 6.0

53
Infrared (IR) Technique
  • Can be used during night as well as during day
  • At times more objective than visible technique

54
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55
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

56
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57
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58
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.

59
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)

60
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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