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Tropical Cyclone Overview THE DVORAK TECHNIQUE

Tropical Cyclone Overview THE DVORAK TECHNIQUE. Introduction Visible Technique IR Technique Strengths and Weaknesses Lab Exercise: Visible Pattern Classification . Measurements of Tropical Cyclones. The Dvorak Technique uses Satellite Measurements.

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Tropical Cyclone Overview THE DVORAK TECHNIQUE

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  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 cyclone’s strength. • The T number scale runs from 0 to 8 in increments of 0.5.

  7. 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.

  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 Cont’d 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 DT=3

  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 Cont’d

  23. 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

  24. 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.

  25. Central Dense Overcast (CDO)

  26. CDO • No eye • DT number determined by CF+BF=DT • CF=CENTRAL FEATURE • BF=BANDING FEATURE • DT=DATA 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 ° 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

  29. Example: CDO Central Feature (CF) Cont’d

  30. Example: CDO - Banding Feature (BF)

  31. Example CDO - Banding Feature (BF) Cont’d

  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 CF+BF=DT • CF=CENTRAL FEATURE • BF=BANDING FEATURE • DT=DATA T NUMBER

  35. Example: Hurricane Georges 1945 UTC 18 September 1998

  36. 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

  37. Eye Number

  38. 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

  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 CF+BF=DT • CF=CENTRAL FEATURE • BF=BANDING FEATURE • DT=DATA T NUMBER

  45. Example Banding Eye: Hurricane Bonnie 2131 UTC 25 August 1998

  46. 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

  47. Banding Width

  48. 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.

  49. Eye Adjustment

  50. Example: Banding Eye - Banding Feature (BF) ( Same as with CDO)

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