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 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 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 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 - 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 < MET. Note: MET is Model-Estimated T, which is extrapolated from previous Dvorak estimate
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. 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.
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