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 motions Rotate 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 = 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’02934 hPa –Lili’ hPa Maximum Surface Wind Speed –Lili’02125 kt –Isidore’ 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 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

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 = = 3.5 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.=3 1/2° E-no.=4 3/4°E-no.=5 1°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 for E-No.  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 = = 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 –DGDT 4.0 –MGDT 4.0 –LGDT 4.5 –BDT 5.0 –WDT 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) –OW4.0 –DG4.5 –MG4.5 –LG5.0 –B5.5 –W6.0 –CDG6.5 Eye Temperature Gray Shade (E-adj) –+ or – 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 = = 5.0 –(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= = 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 C-5.2C Mitch C10.3C Rita C20.3C Katrina C15.8C Isabel C18.3C Floyd C18.8C Georges C13.8C Iris C-25.2C Opal C-60.2C Erin C17.3C

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?

PATTERNIMAGEINTENSITY GIVEN BY: CURVED BANDVIS, EIRSPIRAL ARC DISTANCE OF BAND SURROUNDING CENTER T-No. 180 o 360 o 540 o SHEARVIS, EIRDISTANCE OF CENTER FROM EDGE OF DEEP CB CLOUDS AND CENTER DEFINITION T-No. (degrees latitude) Embedded Beneath

PATTERNIMAGEINTENSITY GIVEN BY: CDO (Central Dense Overcast) VISSIZE OF CDO (CF) AND BANDING (BF) CF-No. CDO Diameter (degrees latitude) ¾2¼ + BF-No. (0 to +2.0) CDO (Embedded Center) EIRSURROUNDING TEMPERATURE OWLGW CF-No. SURROUNDING BD SHADE + BF ~ BDGMG

PATTERNIMAGEINTENSITY GIVEN BY: EYEVISDISTANCE OF EYE FROM CDO EDGE (CF) AND BANDING (BF) E-No. EYE Embedded Distance (degrees lat.) BF-No. (0 to +2.0) LGW E-No. SURROUNDING BD SHADE + Eadj (± 1.0) + BF ~ B DGMG 76 EYEEIR SURROUNDING TEMP. (E-No.) AND EYE TEMPERATURE (Eadj) OW 4.0 CDG