RAMMT/CIRA Tropical Cyclone Overview THE DVORAK TECHNIQUE Introduction Visible Technique IR Technique Strengths and Weaknesses Lab Exercise: Visible Pattern Classification

RAMMT/CIRA Measurements of Tropical Cyclones

RAMMT/CIRA The Dvorak Technique uses Satellite Measurements

RAMMT/CIRA Most Tropical Cyclone Basins Do Not Have Aircraft Reconnaissance Data

RAMMT/CIRA 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)

RAMMT/CIRA 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.

RAMMT/CIRA 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.

RAMMT/CIRA Four Basic Patterns Curved Band Pattern Shear Pattern Central Dense Overcast (CDO) Pattern Eye Pattern

RAMMT/CIRA Four Basic Patterns Pattern is not always obvious System may move from one pattern to another

RAMMT/CIRA Patterns and associated T Numbers

RAMMT/CIRA Empirical relationship between T number and wind speed

RAMMT/CIRA 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

RAMMT/CIRA Curved Band Pattern

RAMMT/CIRA Curved Band Pattern DT number determined by curvature of band around 10 log spiral

RAMMT/CIRA Curved Band Pattern Contd 1.0 to DT Number

RAMMT/CIRA Example: Tropical Storm Ivan 1115 UTC 23 September 1998

RAMMT/CIRA Example: Curved Band

RAMMT/CIRA Curved Band Pattern Tropical Storm Ivan curves 0.7 around log 10 spiral. This corresponds to DT=3

RAMMT/CIRA Shear Pattern

RAMMT/CIRA Shear Pattern DT Numbers 1° latitude = 60 nautical miles (nmi) = 111 km

RAMMT/CIRA Example: Hurricane Bertha 2015 UTC 11 July 1996

RAMMT/CIRA Hurricane Bertha Contd

RAMMT/CIRA 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

RAMMT/CIRA 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.

RAMMT/CIRA Central Dense Overcast (CDO)

RAMMT/CIRA CDO No eye DT number determined by CF+BF=DT –CF=CENTRAL FEATURE –BF=BANDING FEATURE –DT=DATA T NUMBER

RAMMT/CIRA Example: Hurricane Georges 1545 UTC 21 September 1998

RAMMT/CIRA 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

RAMMT/CIRA Example: CDO Central Feature (CF) Contd

RAMMT/CIRA Example: CDO - Banding Feature (BF)

RAMMT/CIRA Example CDO - Banding Feature (BF) Contd

RAMMT/CIRA Example: CDO Data T Number CF + BF = DT CF = 5 BF = 0.5 DT = 5.5

RAMMT/CIRA Eye Pattern

RAMMT/CIRA Eye Pattern DT number determined by CF+BF=DT –CF=CENTRAL FEATURE –BF=BANDING FEATURE –DT=DATA T NUMBER

RAMMT/CIRA Example: Hurricane Georges 1945 UTC 18 September 1998

RAMMT/CIRA Example: Eye - Central Feature (CF) CF=E-number+Eye 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 –>1°E-no.=7

RAMMT/CIRA Eye Number

RAMMT/CIRA 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 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

RAMMT/CIRA Eye Adjustment

RAMMT/CIRA Example: Eye - Banding Feature (BF) ( Same as with CDO)

RAMMT/CIRA Banding Feature (BF)

RAMMT/CIRA Data T Number CF + BF = DT CF = = 5 BF = 0.5 DT = 5.5

RAMMT/CIRA Banding Eye Pattern

RAMMT/CIRA Banding Eye Pattern DT number determined by CF+BF=DT –CF=CENTRAL FEATURE –BF=BANDING FEATURE –DT=DATA T NUMBER

RAMMT/CIRA Example Banding Eye: Hurricane Bonnie 2131 UTC 25 August 1998

RAMMT/CIRA 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

RAMMT/CIRA Banding Width

RAMMT/CIRA 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 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.

RAMMT/CIRA Eye Adjustment

RAMMT/CIRA Example: Banding Eye - Banding Feature (BF) ( Same as with CDO)

RAMMT/CIRA Banding Feature (BF)

RAMMT/CIRA Data T Number CF + BF = DT CF = = 4 BF = 2.0 DT = 6.0

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

RAMMT/CIRA

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

RAMMT/CIRA

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.

RAMMT/CIRA 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)

RAMMT/CIRA 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