Operational Use of the Dvorak Technique at the

NHC

Operational Use of the Dvorak Technique at the

NHC

NATIONAL HURRICANE CENTER

NATIONAL HURRICANE CENTER

JACK BEVENJACK BEVEN

WHERE AMERICA’S CLIMATE AND WEATHER SERVICES BEGINWHERE AMERICA’S CLIMATE AND WEATHER SERVICES BEGIN

Operational BackgroundOperational Background• The Tropical Analysis and Forecast Branch

(TAFB) of the NHC performs manual subjective Dvorak location and intensity estimates.

• The TAFB also performs Hebert-Poteat subtropical cyclone technique intensity estimates.

• NHC philosophy is that these estimates are supposed to be as independent of other data sources as possible (not 100% possible!).

• Intensity estimates from the Advanced Dvorak Technique (ADT) are used mainly to supplement to the manual estimates.

• The Tropical Analysis and Forecast Branch (TAFB) of the NHC performs manual subjective Dvorak location and intensity estimates.

• The TAFB also performs Hebert-Poteat subtropical cyclone technique intensity estimates.

• NHC philosophy is that these estimates are supposed to be as independent of other data sources as possible (not 100% possible!).

• Intensity estimates from the Advanced Dvorak Technique (ADT) are used mainly to supplement to the manual estimates.

Operational BackgroundOperational Background• The Hurricane Specialists Unit (HSU, the TC

forecasters) of the NHC integrates the Dvorak analyses with other data sources (when available).

• Even when they are the only data source, the HSU forecasters are free to employ the Dvorak analyses in whatever way they feel is appropriate. This applies to both real-time forecasting and post-analysis.

• The Hurricane Specialists Unit (HSU, the TC forecasters) of the NHC integrates the Dvorak analyses with other data sources (when available).

• Even when they are the only data source, the HSU forecasters are free to employ the Dvorak analyses in whatever way they feel is appropriate. This applies to both real-time forecasting and post-analysis.

Operational BackgroundOperational Background• Full Dvorak analyses are normally made every

six hours at the synoptic times, with locations provided at intermediate synoptic times.

• Full Dvorak analyses can be made at non-synoptic times if necessary.

• The TAFB uses the 1984 version of the Dvorak Technique.

• No changes have been made to the wind and pressure basic calibration of the technique.

• No changes have been made to the basic Dvorak procedures on the technique flow charts.

• Full Dvorak analyses are normally made every six hours at the synoptic times, with locations provided at intermediate synoptic times.

• Full Dvorak analyses can be made at non-synoptic times if necessary.

• The TAFB uses the 1984 version of the Dvorak Technique.

• No changes have been made to the wind and pressure basic calibration of the technique.

• No changes have been made to the basic Dvorak procedures on the technique flow charts.

Departures from Dvorak (1984)Departures from Dvorak (1984)

• Cloud systems centers that do not meet the convective criteria can be tracked by providing a location and an intensity estimate of “too weak to classify”.

• When measuring infrared eye patterns in step 2C, occasionally a BD enhancement color surrounds the eye that is too narrow to use for the eye number. In these cases, the color shade is not used to determine the eye number, but is used to determine the eye adjustment.

• Cloud systems centers that do not meet the convective criteria can be tracked by providing a location and an intensity estimate of “too weak to classify”.

• When measuring infrared eye patterns in step 2C, occasionally a BD enhancement color surrounds the eye that is too narrow to use for the eye number. In these cases, the color shade is not used to determine the eye number, but is used to determine the eye adjustment.

Step 2C - BD Color Used For Eye Adjustment Can Differ From Color Used For Eye Number

Step 2C - BD Color Used For Eye Adjustment Can Differ From Color Used For Eye Number

Jeanne (2004)

Black (B) completely surrounds the eye. However, the B ring is less than 0.5 degrees thick. So, it cannot be used for the eye number. The eye number uses Light Gray (LG) for a 5.0, while the eye adjustment is determined by a Warm Medium Gray (WMG) embedded in B (+1.0) - CF=6.0

Black (B) completely surrounds the eye. However, the B ring is less than 0.5 degrees thick. So, it cannot be used for the eye number. The eye number uses Light Gray (LG) for a 5.0, while the eye adjustment is determined by a Warm Medium Gray (WMG) embedded in B (+1.0) - CF=6.0

5.0LG 1.0 6.0 6.00.0WMG eye embdd in LG/B

Departures from Dvorak (1984)Departures from Dvorak (1984)• In step 2C/page 36, there is a rule for the eye

adjustment for large or elongated eyes in infrared imagery. The TAFB does not use this rule.

• For step2C/visible eye patterns and step 2D/visible CDO pattern, the TAFB interpolates between the distances/sizes in the tables to produce Central Feature numbers at 0.5 T-number resolution.

• For step 6/Pattern T-Numbers, the TAFB does not strictly follow the rule that the Pattern T-Number must be within one column (0.5 T-Numbers) of the Model Expected T-number.

• In step 2C/page 36, there is a rule for the eye adjustment for large or elongated eyes in infrared imagery. The TAFB does not use this rule.

• For step2C/visible eye patterns and step 2D/visible CDO pattern, the TAFB interpolates between the distances/sizes in the tables to produce Central Feature numbers at 0.5 T-number resolution.

• For step 6/Pattern T-Numbers, the TAFB does not strictly follow the rule that the Pattern T-Number must be within one column (0.5 T-Numbers) of the Model Expected T-number.

Departures from Dvorak (1984)Departures from Dvorak (1984)

Original FT Constraints for Original FT Constraints for storms with Tstorms with T≥4.0 (Dvorak)≥4.0 (Dvorak)::

Modified FT Constraints now in Modified FT Constraints now in use for developing storms above use for developing storms above T1.5 (24 hr or more after the initial T1.5 (24 hr or more after the initial T1) (Lushine 1977):T1) (Lushine 1977):

1.0 T-numbers over 6 hr1.0 T-numbers over 6 hr

1.5 T-numbers over 12 hr1.5 T-numbers over 12 hr

2.0 T-numbers over 18 hr2.0 T-numbers over 18 hr

2.5 T-numbers over 24 hr2.5 T-numbers over 24 hr

• The TAFB uses a modified set of the Dvorak (1984) constraints on the allowed Final T-Number changes (step 7/8) based on Lushine (1977). This deals with when the analyst can allow a looser constraint.

• The TAFB uses a modified set of the Dvorak (1984) constraints on the allowed Final T-Number changes (step 7/8) based on Lushine (1977). This deals with when the analyst can allow a looser constraint.

Departures from Dvorak (1984)Departures from Dvorak (1984)• For step 10/Forecast Intensity, the TAFB uses a

combination of Dvorak (1984) and rules from the version of the technique published in 1995.

• For systems that spend a significant amount of time over land and then re-emerge over water, the TAFB re-starts Dvorak analyses using the observed Data T-Number and Pattern T-Number.

• For step 10/Forecast Intensity, the TAFB uses a combination of Dvorak (1984) and rules from the version of the technique published in 1995.

• For systems that spend a significant amount of time over land and then re-emerge over water, the TAFB re-starts Dvorak analyses using the observed Data T-Number and Pattern T-Number.

Dvorak Error DistributionDvorak Error Distribution

These errors are from comparison to aircraft-based best track data. Images courtesy of Brown and Franklin

These errors are from comparison to aircraft-based best track data. Images courtesy of Brown and Franklin

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0 130.0 140.0 150.0 160.0 170.0

CI N

umbe

r

Best Track Wind (kt)

ADT 8.1.1 CI vs. Best Track Wind

Alex

Bonnie

Colin

TD-5

Danielle

Earl

Fiona

Hermine

Igor

Karl

Matthew

Paula

Richard

Tomas

2010 ADT validation results

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0 130.0 140.0 150.0 160.0 170.0

Fina

l T-n

umbe

r

Best Track Wind (kt)

ADT 8.1.1 Final T-Number vs. Best Track Wind

Alex

Bonnie

Colin

TD-5

Danielle

Earl

Fiona

Hermine

Igor

Karl

Matthew

Paula

Richard

Tomas

2010 ADT validation results

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

910.0920.0930.0940.0950.0960.0970.0980.0990.01000.01010.01020.0

CI N

umbe

r

Best Track MSLP (mb)

ADT 8.1.1 CI vs. Best Track MSLP

Alex

Bonnie

Colin

TD-5

Danielle

Earl

Fiona

Hermine

Igor

Karl

Matthew

Paula

Richard

Tomas

2010 ADT validation results

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

910.0920.0930.0940.0950.0960.0970.0980.0990.01000.01010.01020.0

Fina

l T-N

umbe

r

Best Track MSLP (mb)

ADT 8.1.1 Final T-Number vs. Best Track MSLP

Alex

Bonnie

Colin

TD-5

Danielle

Earl

Fiona

Hermine

Igor

Karl

Matthew

Paula

Richard

Tomas

2010 ADT validation results