Training Session: Satellite Applications on Tropical Cyclones:
Dvorak Technique
NOAA/NESDIS
STAR/CORP/RAMM
CIRA / Fort Collins, CO
Dvorak Technique: Background Information
• “The Dvorak tropical cyclone (TC) intensity estimation technique has been employed by global tropical analysis centers as the primary method for monitoring tropical systems for the last three decades. Over this time the technique has likely saved tens-of-thousands of lives in regions where over one billion people are directly affected by TCs (commonly called hurricanes, typhoons or cyclones). In fact, it is difficult to think of any other single meteorological technique that has withstood the test of time and had the life-saving impact.”
• Velden et al BAMS 2006
Dvorak Technique: Overview
• The Dvorak Technique estimates tropical cyclone intensity by analyzing satellite image patterns and IR cloud top temperatures.
Dvorak Technique: Overview
• Intensity is assigned with intensity units (called T-numbers ranging from 1 to 8, in 0.5 increments), where one T-number represents one day’s intensity change at an average rate.
• The T-number can be given as a maximum surface wind speed or a minimum sea-level pressure.
Dvorak Technique: Overview
Dvorak Technique: Procedure
• Original Dvorak (1984) 10 Steps:– 1. Locate center– 2 and 3. Select pattern and assign DT (Data T-No.) – 4. 24-h trend– 5. Assign MET (Model expected T-No.)– 6. Assign PT (Pattern T-No.)– 7. Use DT, MET, and PT to get final T-No.– 8. Apply constraints to T-No.– 9. Adjust T-No. to CI- Number (current intensity)– 10. Forecast 24-h CI-No.
Dvorak Technique: Procedure
• Simplified Approach:– 1. Locate Center– 2. Assign Pattern– 3. Make measurements (Visible or EIR) – 4. Assign T-Number– 5. Assign CI (Current Intensity)
Dvorak Technique: Procedure
1. The center location is used in making measurements.
2. The pattern must be determined before measurements can be made.
3. The measurements are meant to be as objective as possible, but still involve qualitative evaluations.
4. The T-number and CI number are the same for intensifying or steady TC’s. With weakening TC’s the CI number may be higher than the T-number.
Center location (fixing)
– Important component of the Dvorak Technique’s intensity assignment procedure
– Additional uses of center locations:
• Track the motion of the tropical cyclone
• Numerical model initialization
Center location (fixing) Overview
• Center Location = surface center– Center of circulation– Lowest sea-level pressure
• Visible and IR methods – Dvorak– Eye– Distinct and inferred center with shear pattern and low-level clouds– Spiral bands and curved cloud lines– Wedge method
• Using animation– Low-level cloud motions– Deep layer cloud motions– Ignore cirrus layer cloud motions– Mid-level centers tilted from surface center
• Using microwave images– Thick cirrus clouds in visible and IR images obscure features below, used for center location– Thick cirrus clouds in microwave images are more transparent, and the microwave images
may often provide better views of features, for improved center locations• Using 3.9-micrometer images at night
Center Location
• Center Location = surface center– Center of circulation– Lowest sea-level pressure
Center Location
• Methods for locating the center in single images – Eye– Spiral bands and curved cloud lines– Low-level cloud lines, with shear pattern– Wedge method
Where’s the center ?
• Examples with :– Eye – Central Dense Overcast
Parallax
Center Location
• Increasing difficulty and uncertainty…going down the following list:
– Well-defined eye– Large, ragged eye– Cloud covered, poorly defined eye– Central Dense Overcast (i.e. eye completely
obscured by thick cirrus cloud)
Center Location
– Partly “exposed” center due to vertical shear
– Spiral bands with deep convective clouds
– Curved cloud lines with small low-level clouds
Center Location
• Wedge method (IR)
Center Location
• Using animation:• Both:
– Low-level cloud motions– Deep convective cloud motionsRotate cyclonically and spiral in toward the center
Also,– Upper and mid-level cloud motions may indicate circulation
centers (but they may be displaced from the surface center, by vertical wind shear)
– Upper level cirrus cloud motions, flow outward turning anticyclonically moving away from the center
– Close to the center, with low vertical wind shear, the upper level cirrus motions may show cyclonic outflow motion
Loop1
\LOOP1 dir
Short loop to 1845
Loop 2
Same thing at RSO high res to 1845
\LOOP2 dir
Loop 3
Use all images in \LOOP1 dir to 2245Z
Center Location
• Using 3.9-micrometer images at night
• 3.9 micrometer has less water vapor attenuation than the standard IR
• Enhancement curves can be applied to the 3.9 micrometer images to accentuate the low-level clouds, and then be used as a replacement for visible images at night
Loop4 Ch2 Vis
From \LOOP3 dir
Loop 5 Ch2-vis
From \LOOP4 dir
Center location• Microwave images available from polar
orbiting satellites ( and other low orbit satellites, i.e. TRMM)
• Allows views of cloud and rain patterns, that are obscured by thick cirrus clouds in the IR and visible images.
Satellite products used for TC Intensity Estimates
• Dvorak Technique
• Objective Dvorak Technique (IR pixel data) and Advanced Dvorak Technique
• Advanced Microwave Sounding Unit (AMSU) algorithms
• Scatterometer winds
• Low-level cloud motion vectors
Dvorak Technique
• The Dvorak technique uses patterns and measurements from satellite imagery to estimate the strength of a tropical cyclone.
• Four basic pattern types– Curved band pattern– Shear pattern– CDO (Central Dense Overcast) pattern– Eye pattern
Curved Band Pattern
Curved Band -- TS Jose 2005
Shear Pattern
Shear – TS Lee 2005
Central Dense Overcast (CDO)
CDO – TS Alpha 2005
Eye Pattern – KATRINA 2005
2005 Atlantic Hurricanes: 10 of 15 have well defined Eye Pattern
Dvorak Technique
• Uses patterns and measurements as seen on satellite imagery to assign a number (T number) as an estimate of the tropical cyclone intensity.
• The T number scale runs from 0 to 8 in increments of 0.5.– Minimal Tropical Storm intensity(35 kt) is T2.5– Minimal Hurricane intensity(65 kt) is T4.0
Empirical relationship between T number and wind speed
Tropical Cyclone Intensity
• Intensity: highest surface wind speed at any location within the TC
• U.S. policy: 10-m, 1-min wind to nearest 5-knots (knot = n.mi./h, 60 n.mi. = 70 mi. = 111 km = 1 deg lat, 1 m/s = 1.946 knots)
• Alternate indicator of intensity is the central pressure, or minimum sea-level pressure (MSLP) in hPa (mb)
Tropical Cyclone Pressure Wind Relationship
• Pressure : Wind = MSLP : Vmax
– MSLP = minimum sea-level pressure– Vmax = maximum surface wind (10-m, 1-min wind)
Pressure-wind relationship
• Central pressure, i.e. minimum sea-level pressure (MSLP) is well correlated with maximum surface wind speed (Vmax)
• An average pressure-wind relationship is used to assign intensity as MSLP and Vmax, in the absence of additional observations, such as aircraft data.
Pressure-wind relationship
• Aircraft observations reveal deviations from the average pressure-wind relationship
• Environmental and structure characteristics influence the pressure wind relationship– Environmental pressure– Latitude– Size– Intensity trend– TC Motion– Radius of Maximum Wind– Landfall
Isidore’02 vs Lili’02
• Lowest MSLP – Isidore’02 934 hPa– Lili’02 938 hPa
• Maximum Surface Wind Speed – Lili’02 125 kt– Isidore’02 110 kt
Dvorak Technique
Dvorak Intensity Assignment
• Simple Procedure:–Find the Center.–Identify the Pattern.–Make a Measurement.
–WHAT DO YOU MEASURE IN THE IMAGE?
Curved Band Pattern– What do You Measure?
• Spiral Arc Distance of Curved Band surrounding the Center– Curved Band – Continuous deep convective
spiral band– Same measurement used for both Visible and
IR images
Curved Band Pattern
• DT number determined by curvature of band around 10 log spiral
Curved Band Pattern
1.0 to 2.0 2.5 3.0 3.5 4.0 4.5
DT Number
Example: Tropical Storm Ivan 1115 UTC 23 September 1998
Example: Curved Band
Curved Band Pattern
• Tropical Storm Ivan curves 0.7 around log 10 spiral. This corresponds to DT=3.0
• T3.0 = Vmax of 45 kt
• National Hurricane Center’s 09UTC and 15 UTC Advisories had Vmax of 45 kt
Shear Pattern – What do You Measure?
• Distance of Center from Edge of Deep Convective Clouds
– Additional indicator: How well defined is the low level center?
– Same measurement used for both Visible and IR images
Shear Pattern DT Numbers
Distance: 1° latitude = 60 nautical miles (n.mi.) = 111 km)
Shear Pattern
Shear Pattern – TS Danielle – 18 Aug 2004
Shear Pattern Measurement
Shear Pattern Measurement
• Distance of Center from Edge of Deep Convective Clouds
• For: 1715 UTC 18 Aug 2004
• Tropical Storm Danielle
• = 45 km = 0.4 deg lat = DT 3.0
• T3.0 = 45 kt
CDO = Central Dense Overcast
• Defined as ….dense overcast cloud mass that over the center….
• It covers the most tightly curved inner coils of the curved band pattern
• There is no eye in the satellite images with the CDO pattern, however an eye may exist beneath, obscured by the clouds
• The eye first appears within the CDO and exists within the CDO, defining the eye pattern
CDO Pattern (Visible Image) – What do You Measure?
• Size of CDO (Central Feature, CF)
• Extent of Surrounding Deep Convective Cloud Band (Banding Feature, BF)
CDO
• No eye
• DT number determined by CF+BF=DT– CF=CENTRAL FEATURE– BF=BANDING FEATURE– DT=DATA T NUMBER
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
CDO - Banding Feature (BF)
CDO Pattern Measurement
• Example: Tropical Storm RITA, north of Cuba
• 1515 UTC 19 Sep 05 Visible Image
CDO Pattern Measurement
• 140 km well-defined CDO diameter
• 140 km = 1.25 deg lat = CF 3.0
• Banding feature, narrow band more than ½ around center, wide band less than ½ around center = BF 0.5
• DT = 3.0 +0.5 = 3.5
• T3.5 = 55 kt intensity
Eye Pattern (Visible Image) – What do You Measure?
• Distance of Eye from CDO Edge is defined as the “Embedded Distance” (CF)
• Extent of Surrounding Deep Convective Cloud Band (Banding Feature, BF)
Example: Hurricane Georges 1945 UTC 18 September 1998
Eye Number
Eye Pattern
• DT number determined by CF+BF=DT– CF=CENTRAL FEATURE
• CF = E-No + E-adj
– BF=BANDING FEATURE– DT=DATA T NUMBER = E-No + E-adj + BF
Eye - Central Feature (CF)
• CF= E-number + Eye Adjustment• E-number a measure of the embedded
distance of the hurricane eye in degrees latitude
1/4° E-no.=31/2° E-no.=43/4° E-no.=51° E-no.=6>1° E-no.=7
E-adj
• Eye adjustment
1. Poorly defined or ragged eyes: Subtract 0.5 for E-no. 4.5 and 1 for E-no. 5.
2. Well-defined circular eyes: Add 0.5 -1.0 for E-No. 6.
3. Large eyes: Limit T-no. to T6 for round, well defined eyes, and to T5 for large ragged eyes.
Eye Adjustment
Example: Eye - Banding Feature (BF)
( Same as with CDO)
Banding Feature (BF)
Data T Number
CF + BF = DT
CF = E-No + E-adj
= 6 - 1 = 5
BF = 0.5
DT = 5.5
Enhanced IR (EIR) Dvorak Technique
• Measurements are made according to discrete IR temperature ranges using a standard enhancement on IR images
• The EIR Technique provides intensity estimates independent of the Visible for the CDO and Eye patterns, but is unchanged for the Curved Band and Shear Patterns
• The EIR Technique when applied to the CDO pattern is called the “Embedded Center Pattern”
IR Enhancement Curve Temperatures and Associated Gray Shades / Colors
• NESDIS/RAMM
– Black to White– Cyan to Gray– Blue shades– Green shades– Red shades– Yellow to Black– White to Black
• IR temperature (deg C)
– >-31– -31 to -49– -50 to -59– -60 to -69– -70 to -79– -80 to -89– -90 and colder
IR Enhancement Curve Temperatures and Associated Gray Shades / Colors
• BD Curve Gray Shade
– WMG (warm medium gray)– OW (off white)– DG (dark gray)– MG (medium gray)– LG (light gray)– B (black)– W (white)– CMG (cold medium gray)– CDG (cold dark gray)
• IR temperature (deg C)
– > 9– 9 to -30– -31 to -41– -42 to -53– -54 to -63– -64 to -69– -70 to -75– -76 to -80– -81 and colder
CDO Pattern (Enhanced IR Image using Dvorak BD curve) –
What do You Measure?
Surrounding BD curve Gray Shade – OW DT 3.5– DG DT 4.0– MG DT 4.0– LG DT 4.5– B DT 5.0– W DT 5.0
– (also called “Embedded Center Pattern”)
The Center must lie within the “Surrounding Gray Shade” by at least
the minimum “Embedded Distance”
• Diagram:
• DT = CF (BF=0)
Eye Pattern (Enhanced IR Image using Dvorak BD curve) –
What do You Measure?Surrounding BD Curve Gray Shade (E-No)
– OW 4.0– DG 4.5– MG 4.5– LG 5.0– B 5.5– W 6.0– CDG 6.5
• Eye Temperature Gray Shade (E-adj) – + or – 0.5-1.0 or 0
The Eye must be encircled by the “Surrounding Gray Shade” of at least
the minimum “Narrowest Width”
• Diagram
• DT = CF (BF=0) = E-No + E-adj
E-adj
Dvorak EIR Measurement
• Dennis, 1815 UTC 6 July 05
• Surrounding BD Curve Gray Shade = OW
• DT=CF= 3.5
– (blue circle is R=55 km = 1deg lat diameter)– (BF=0)
Dvorak EIR Measurement
• Dennis, 0015 UTC 7 July 05
• Surrounding BD Curve Gray Shade = LG
• DT=CF=4.5
– (blue circle is R=55 km = 1deg lat diameter)– (BF=0)
Dvorak EIR Measurement
• Dennis, 0615 UTC 7 July 05
• Surrounding BD Curve Gray Shade = B
• DT=CF=5.0
– (blue circle is R=55 km = 1deg lat diameter)– (BF=0)
Dvorak EIR Measurement
• Dennis, 0615 UTC 7 July 05
• Eye pattern• Surrounding BD Curve Gray Shade = B• E5.5• E-adj = -0.5 with Eye T = B and Surrounding T = B
• DT= CF = 5.5 - 0.5 = 5.0
– (blue circle is R=55 km = 1deg lat diameter)– (BF=0)
Dvorak EIR Measurement
• Dennis, 1215 UTC 7 July 05
• Surrounding BD Curve Gray Shade = W
• DT=5.0
– (blue circle is R=55 km = 1deg lat diameter)– (BF=0)
Dvorak EIR Measurement
• Dennis, 1815 UTC 7 July 05
• Eye pattern• Surrounding BD Curve Gray Shade = W• E6• E-adj = 0 with Eye T = LG and Surrounding T = W
• DT= 6.0 – 0.0 = 6.0
– (BF=0)
Dvorak EIR Measurement
• Dennis, 0315 UTC 10 July 05
• Eye pattern• Surrounding BD Curve Gray Shade = W• E6.0• E-adj = +1.0 with Eye T = WMG and Surrounding T = W
• DT= CF= 6.0 + 1.0 = 7.0
– (BF=0)
Objective Dvorak Technique
• Original version – Dvorak (1984) – “analysis using digital IR data”
• Velden, Olander, Zehr (1998) – ODT
• Computation used for hurricane intensities remains essentially unchanged
• What is it? – Two IR temperature measurements, given a center location
Two IR temperature measurements
• 1) Surrounding temperature – Warmest pixel from those located on r=55 km circle
• 2) Eye temperature – Warmest pixel within the eye
• Table assigns intensity to nearest 0.1 T-No.
• Intensity increases as Surrounding T gets colder and as the Eye T gets warmer.
ODT and IR Temperatures with Single Image Maxima of some Atlantic Intense Hurricanes
• Wilma 7.8 -81.2C -5.2C• Mitch 7.5 -75.2C 10.3C• Rita 7.5 -75.2C 20.3C• Katrina 7.4 -74.2C 15.8C• Isabel 7.3 -73.2C 18.3C• Floyd 7.1 -72.2C 18.8C• Georges 6.9 -70.2C 13.8C• Iris 6.9 -75.2C -25.2C• Opal 6.5 -79.2C -60.2C• Erin 6.0 -58.2C 17.3C
ODT - Improvements
• Multi-radius computations of “surrounding temperature”
• Time averaging (6-h running mean) of frequent (30-min) interval computations
• Limits on rate of weakening
• New computations for weak (pre-hurricane) intensities
Advanced Dvorak Technique (ADT)
• CIMSS, University of Wisconsin
• Tim Olander and Chris Velden
• ADT Improvements:– Automated Center Location– Applicable to the weaker “tropical storm”
intensity range (T2 to T4)
Dvorak Technique to estimate Tropical Cyclone Intensity
What do you measure?
PATTERN IMAGE INTENSITY GIVEN BY:
CURVED BAND VIS, EIR SPIRAL ARC DISTANCE OF BAND SURROUNDING CENTER
0.5 1.0 1.5
3.52.5 4.54.03.01.5 T-No.
180o 360o 540o
SHEAR VIS, EIR DISTANCE OF CENTER FROM EDGE OF DEEP CB CLOUDS AND CENTER DEFINITION
1.0 0.5 0
3.02.0 3.52.51.5 T-No.
(degrees latitude)
Embedded Beneath
PATTERN IMAGE INTENSITY GIVEN BY:
CDO (Central Dense Overcast)
VIS SIZE OF CDO (CF) AND BANDING (BF)
1.0 1.5 2.0
432 5CF-No.
CDO Diameter (degrees latitude)
¾ 2¼
+ BF-No. (0 to +2.0)
CDO (Embedded Center)
EIR SURROUNDING TEMPERATURE
OW LG W
5.04.03.5CF-No.
SURROUNDING BD SHADE+ BF ~ 0
4.0 4.5 5.0
BDG MG
PATTERN IMAGE INTENSITY GIVEN BY:
EYE VIS DISTANCE OF EYE FROM CDO EDGE (CF) AND BANDING (BF)
0.5 0.75 1.0
543 E-No.
EYE Embedded Distance (degrees lat.)
0.25
+ BF-No. (0 to +2.0)
LG W
6.05.04.5E-No.
SURROUNDING BD SHADE
+ Eadj (± 1.0) + BF ~ 0
4.5 5.5 6.5
B DG MG
76
EYE EIR SURROUNDING TEMP. (E-No.) AND EYE TEMPERATURE (Eadj)
OW
4.0
CDG