a study of several aspects of the dvorak technique · a study of several aspects of the dvorak...

A Study of Several Aspects of the

Dvorak Technique

A Study of Several Aspects of the

Dvorak Technique

Jack Beven, John Cangialosi, and Todd Kimberlain

National Hurricane Center

February 2016

The Dvorak TechniqueThe Dvorak Technique• A method for estimating the

intensity of tropical cyclones

(TCs) from VIS/IR imagery

based on “measurements” of

the TC cloud pattern and a set

of rules.

• Based on correlations between

the TC cloud patterns and the

intensity

• Comes in manual and

automated versions.

• Technique works best on

strong TCs but less so with

weaker ones; skill of the

analyst is extremely

important.

• Related technique for

subtropical cyclones.

• A method for estimating the

intensity of tropical cyclones

(TCs) from VIS/IR imagery

based on “measurements” of

the TC cloud pattern and a set

of rules.

• Based on correlations between

the TC cloud patterns and the

intensity

• Comes in manual and

automated versions.

• Technique works best on

strong TCs but less so with

weaker ones; skill of the

analyst is extremely

important.

• Related technique for

subtropical cyclones.

EYE TYPES

Dvorak Technique Cloud PatternsDvorak Technique Cloud Patterns

Advanced Dvorak TechniqueAdvanced Dvorak Technique

Dean (2007) Ike (2008)

Controversial Issues of the

Dvorak Technique

Controversial Issues of the

Dvorak Technique• Calibration of the winds/pressures

• The Dvorak Technique conceptual model

for development/weakening

• Potential regional variations in the cloud

patterns

• Possible shortcomings of the infrared eye

cloud pattern measurements

• Adjustment for cold-topped infrared

curved band cloud patterns

• Use of outer banding for infrared cloud

patterns

• Adjustment for eye clarity for visible eye

cloud patterns

• What to do when a system is over land?

• The constraints on development/

weakening

• Calibration of the winds/pressures

• The Dvorak Technique conceptual model

for development/weakening

• Potential regional variations in the cloud

patterns

• Possible shortcomings of the infrared eye

cloud pattern measurements

• Adjustment for cold-topped infrared

curved band cloud patterns

• Use of outer banding for infrared cloud

patterns

• Adjustment for eye clarity for visible eye

cloud patterns

• What to do when a system is over land?

• The constraints on development/

weakening

For all the issues, Dvorak Technique

intensity estimates compare pretty

well to aircraft data

For all the issues, Dvorak Technique

intensity estimates compare pretty

well to aircraft data

• Intensity Constraints – Reality from aircraft-based

best track data

• Intensity Constraints – Data-T (DT) Number

• Technique Validation – CI Number, Final-T (FT)

Number, and DT Number

• Intensity Constraints – Reality from aircraft-based

best track data

• Intensity Constraints – Data-T (DT) Number

• Technique Validation – CI Number, Final-T (FT)

Number, and DT Number

What We’ve Looked AtWhat We’ve Looked At

Why are there constraints in the

Dvorak Technique?

Why are there constraints in the

Dvorak Technique?

• Weak systems sometimes

lose all convection during the

diurnal minimum

• Cloud patterns for weak

systems sometimes look

unrealistically strong

• Strong systems sometimes

don’t intensify as quickly as

the cloud pattern suggests

• In weakening systems, the

decay of winds and pressures

usually somewhat lags

behind that of the cloud

pattern

• Weak systems sometimes

lose all convection during the

diurnal minimum

• Cloud patterns for weak

systems sometimes look

unrealistically strong

• Strong systems sometimes

don’t intensify as quickly as

the cloud pattern suggests

• In weakening systems, the

decay of winds and pressures

usually somewhat lags

behind that of the cloud

pattern Air recon intensity lagged behind Dvorak

estimates during Mitch’s intensification

Air recon intensity lagged behind Dvorak

estimates during Mitch’s intensification

0

20

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10/

22/1

998

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998

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998

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25/1

998

10/

26/1

998

10/

27/1

998

10/

28/1

998

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998

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998

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998

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1/19

98

11/

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98

11/

3/19

98

11/

4/19

98

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6/19

98

Win

d (k

t)

Date

1998 Mitch BT vs. Fix Intensity

Best Track

TAFB

SAB

Hurricane Wilma (2005)Hurricane Wilma (2005)

75 kt/975 mb75 kt/975 mb (Images ~12 hr apart)(Images ~12 hr apart) 150 kt/892 mb150 kt/892 mb

Some tropical cyclones clearly violate the DT development constraints.

Wilma deepened from 970 mb to 882 mb in ~12 hr.

How often does this happen? Does it happen every time it is observed

in the cloud pattern? Do the constraints need to be changed?

Some tropical cyclones clearly violate the DT development constraints.

Wilma deepened from 970 mb to 882 mb in ~12 hr.

How often does this happen? Does it happen every time it is observed

in the cloud pattern? Do the constraints need to be changed?

Testing the Dvorak ConstraintsTesting the Dvorak Constraints

• Used the Atlantic best track database

from 1998-2012 (the GPS dropsonde era).

• Used only those cases when the TC

stayed over water and reconnaissance

aircraft data were available within 3 h.

• Converted the observed intensity

changes at 6, 12, 18, and 24 h intervals to

the equivalent Dvorak T-number

changes.

• The results only use the best track

intensity changes. They do not examine

the corresponding Dvorak Technique

estimates.

• Changes of less than 0.25 T-numbers in

the specified time period were treated as

no change – a steady-state TC

• Used the Atlantic best track database

from 1998-2012 (the GPS dropsonde era).

• Used only those cases when the TC

stayed over water and reconnaissance

aircraft data were available within 3 h.

• Converted the observed intensity

changes at 6, 12, 18, and 24 h intervals to

the equivalent Dvorak T-number

changes.

• The results only use the best track

intensity changes. They do not examine

the corresponding Dvorak Technique

estimates.

• Changes of less than 0.25 T-numbers in

the specified time period were treated as

no change – a steady-state TC20

30

40

50

60

70

80

90

100

110

120

130

10/21 10/23 10/25 10/27 10/29 10/31

BEST TRACKSat (TAFB)Sat (SAB)ADTAC (sfc)AC (flt->sfc)AC (DVK P->W)ScatterometerSurfaceDrop (sfc)Drop (LLM xtrp)AMSUAnalysis

Win

d S

pee

d (

kt)

Date (Month/Day)

Hurricane SandyOctober 2012

2030405060708090

100110120130140150160170180190200

10/20 10/21 10/22 10/23 10/24 10/25

BEST TRACKSat (TAFB)Sat (SAB)ADTAC (sfc)AC (flt->sfc)AC (DVK P->W)ScatterometerDrop (sfc)Drop (LLM xtrp)AMSU

Win

d S

pee

d (k

t)

Date (Month/Day)

Hurricane Patricia20 - 24 October 2015

Results for the 24-h changesResults for the 24-h changes

• Out of 559 cases for intensifying

systems, 1.1% of the best track intensity

changes broke the 24-h constraints.

• Out of 273 cases for weakening systems,

none of the best track intensity changes

broke the 24-h constraints.

• Out of 559 cases for intensifying

systems, 1.1% of the best track intensity

changes broke the 24-h constraints.





• Out of 557 cases for intensifying systems,

0.7% of the best track intensity changes











How often do the changes in the

Data-T numbers break constraints?

How often do the changes in the

Data-T numbers break constraints?

DT numbers, while

subjective, are not

constrained in the

Dvorak Technique.

They violated the FT

constraints about 1%

of the cases for 24-h

changes up to about

8% of the cases for 6-h

changes – higher

percentages than for

the best track data.

Note: The DT data set

includes all available

DT regardless of the

presence of aircraft.

The two data sets are

not homogeneous!

DT DT

BT BT

• Dvorak calibration and validation studies were

made in the 1970’s using the aircraft data of that

era.

• Current aircraft-based best track allows for a

more rigorous assessment of the accuracy and

calibration of the Dvorak Technique than was

possible prior to 1998.

• Dvorak calibration and validation studies were

made in the 1970’s using the aircraft data of that

era.

• Current aircraft-based best track allows for a

more rigorous assessment of the accuracy and

calibration of the Dvorak Technique than was

possible prior to 1998.

Dvorak Technique Errors

A Closer Examination

Dvorak Technique Errors

A Closer Examination

R2=0.928

R2=0.899

R2=0.931

R2=0.935

R2=0.934

• In the Atlantic basin, the vast majority of the observed 6,

12, 18, and 24-h hour intensity changes (based on

aircraft-based best track data) fall within the constraints

of the Dvorak Technique.

• The technique constraints appear to need little or no

change in the Atlantic basin.

• The constraints for the CIMSS Automated Dvorak

Technique (which are looser than those of the subjective

Dvorak Technique) may need adjustment.

• However, there is a fundamental question: Should the

constraints be large enough to cover every case, or just a

statistically significant majority of the cases?

• In the Atlantic basin, the vast majority of the observed 6,

12, 18, and 24-h hour intensity changes (based on

aircraft-based best track data) fall within the constraints

of the Dvorak Technique.

• The technique constraints appear to need little or no

change in the Atlantic basin.

• The constraints for the CIMSS Automated Dvorak

Technique (which are looser than those of the subjective

Dvorak Technique) may need adjustment.

• However, there is a fundamental question: Should the

constraints be large enough to cover every case, or just a

statistically significant majority of the cases?

Summary and ConclusionsSummary and Conclusions

• Data-T number changes fall outside of the constraints of

the technique more often than the best track data.

• For the TAFB satellite fixes, the various Dvorak numbers

show a low intensity bias for weaker system and a high

intensity bias for stronger ones.

• The Data-T numbers by themselves produce slightly less

accurate results, suggesting that the other components

of the technique are adding value to the analyses.

• Data-T number changes fall outside of the constraints of

the technique more often than the best track data.

• For the TAFB satellite fixes, the various Dvorak numbers

show a low intensity bias for weaker system and a high

intensity bias for stronger ones.

• The Data-T numbers by themselves produce slightly less

accurate results, suggesting that the other components

of the technique are adding value to the analyses.

Summary and ConclusionsSummary and Conclusions

• Put the current results through more rigorous

statistical significance tests.

• Stratify the various analyses by intensity.

• Investigate the constraints issue in other basins,

especially the western North Pacific (need ground

truth).

• Experiment with modifications to some of the

rules in the Dvorak Technique to see if they can

be improved.

• Put the current results through more rigorous

statistical significance tests.

• Stratify the various analyses by intensity.

• Investigate the constraints issue in other basins,

especially the western North Pacific (need ground

truth).

• Experiment with modifications to some of the

rules in the Dvorak Technique to see if they can

be improved.

Future WorkFuture Work

D-24 D-18 D-12 D-6 W-6 W-12

Keith (2000) Wilma (2005) Bret (1999) Bret (1999) Henri (2003) Lili (2002)

Wilma (2005) Dean (2007) Keith (2000)Humberto

(2001)Paloma (2008)

Felix (2007) Felix (2007)Humberto

(2001)Iris (2001)

Iris (2001) Felix (2007)

Katrina (2005)

Wilma (2005)

Felix (2007)

Lorenzo (2007)

Gustav (2008)

The Rule Breaking TCsThe Rule Breaking TCs

a study of several aspects of the dvorak technique · a study of several aspects of the dvorak...

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