principles of the dvorak method

Principles of the Dvorak Method

Andrew Burton, Severe Weather WA

APSATS 2002 Melbourne, Australia

•Origins

•Overview

•Measurements

•Expectations

•Pattern matching

•Origins

•Overview

•Measurements•Expectations

•Pattern matching

Some Terminology The (only) Boring slide

T Numbers for every occasion

T = Tropical

DT = Data T Number

MET = Model Expected T No

PT = Pattern T No

FT = Final T No

Developed with North West Pacific and Atlantic data.

Probably the single most important tool in tropical cyclone analysis to date.

Operational use since 1973

Ongoing development over 15 years.

Enhanced infrared technique 1978/1984

Origins

Locating the centre

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.”

Centre not always obvious, especially at night.

•Low level centre

•Use all available data – passive microwave to the rescue!

•Maintain track continuity.

•Centre location can influence intensity measurement

Locating the centre

Pattern Types

•Eye

•Curved Band

•Shear

•Covered Centre

Principles of the Dvorak Method Intensity schematic

Eye patterns

Physical principle: strength of the thermal contrast between the eye and the surrounding convection indicates strength of the system

Method: Measure the warmest brightness temperature in the eye and the coldest surrounding temperature in the deep convection.

Hurricane Erika 1515 UTC 8 September 1997

• Warmest eye pixel 16 °C

• Coldest pixel 30 nmi from

center -71 °C

• Nomogram gives Eye no. =7

Eye pattern using digital IR

Curved bands

Physical principle: the “wrap-aroundness” of the convective bands indicates the vorticity associated with the system.

Method: Measure the curvature of the band

Curved Bands

APSATS 2002 Melbourne, AustraliaTS Ivan 23/9/98 11:15 UTC

Measuring the arc length:• Follow the convection, not

cirrus blow-off• Easier to do with VIS than

Enhanced IR.• You may have small breaks

in convection and draw through

Principles of the Dvorak MethodCurved Bands

• Log10 spiral overlay.• Spiral should lie along the

axis of the of the band, and roughly parallel the inside edge of the band.

• Measure the arc length.• “Tightest inner curvature”• “Cloud minimum wedge”

LOG10Spiral

Curved Bands

Measuring the arc length:• Can be very subjective.• Inexperienced analysts

tend to go too high (fooled by cirrus or outer bands).

• This storm is somewhere between 0.70 and 0.85.

Curved Bands

Step 2A, Curved Band

Note: Southern Hemisphere Example

0.100.20

0.300.40

0.700.80

0.100.20

0.300.40

0.700.80

A wrap of 0.80would equal a Data T of T3.5

0.100.20

0.300.40

0.700.80

We could have addedWe could have addedan additional 0.05an additional 0.05for this portion of for this portion of wrap, giving a totalwrap, giving a totalwrap of 0.85wrap of 0.85

Physical principle: greater involvement of the low level centre with the deep convection indicates a stronger system.

Method: Measure the distance form the low level centre to the edge of the “dense overcast”

Shear patterns

Shear pattern

70nm = T1.570nm = T1.5

Shear pattern

VIS=Central Dense Overcast (CDO)

IR=Embedded Centre (EC)

Covered Centre Patterns

Physical principle?: The weakest link?

Method: Appearance and size of the overcast + degree of banding.

Central Dense Overcast

“It is the pattern formed by the clouds of a tropical cyclone that is related to the cyclone’s intensity and not the amount of clouds in the pattern” - Dvorak 1984

Intensity measurement not dependent on centre location

Does size matter?

A quick diversion on size

Central Dense Overcast

Physical principle: Greater involvement of low level centre with deep convection indicates a stronger system.

Method: Measure coldest surrounding temperature.

Embedded Centre

•Restrictions on use.

•Sensitive to centre location.

•Methodology similar to eye patterns

•Best used after disappearance of an eye.

Just plain ugly!

Embedded Centre

Embedded Centre pattern – problematic

Dependent on a temperature measurement related to a centre you can’t see.

Dvorak temperature enhancement calibrated to North West Pacific.

Cyclones occur at higher latitudes in NH.

What might that mean for “southerners”?

Reality check! TCs Elaine & Vance, March Reality check! TCs Elaine & Vance, March 19991999

• Compare current image to image 24 hours ago.• Determine if the cloud features in the current

image look better defined, the same or worse.• If better, the trend is Developed (D)• If the same, the trend is Same (S)• If worse, the trend is Weakened (W)

MET = Model Expected T number

• For systems with a 24-Hour Trend of D or W, determine pattern evolution and apply appropriate adjustment to Final-T from 24 hours ago.

• - Slow (+ .5)• Normal (+ 1.0) • + Rapid (+ 1.5)

• Assumes you are routinely doing Dvorak intensity estimates - can’t do a “one-timer”!

MET = Model Expected T number

Select the pattern in the diagram that best matches your storm picture – within one column of the MET - SUBJECTIVE

EIR Patterns

Pattern T number – an adjustment to the MET.

Choosing the best estimate – the Final T-no

•Use DT when cloud features are “clear cut”

•Otherwise use MET (possibly adjusted by “Pattern T-no)

More objective

RULES, RULES, RULES

And then…..

Acknowledgements:

Paul J. McCrone

Chief Forecaster

HQ Air Force Weather Agency (AFWA)

Meteorological Satellite Applications

Mark DeMaria

Regional and Mesoscale Meteorology Team

NESDIS/CIRA

Colorado State University, Ft. Collins CO

principles of the dvorak method

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