Transcript Principles of the Dvorak Method

Principles of the Dvorak Method
Andrew Burton, Severe Weather WA
Principles of the Dvorak Method
•Overview
•Origins
•Measurements
•Expectations
•Pattern matching
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
•Overview
•Origins
•Measurements
•Expectations
•Pattern matching
APSATS 2002 Melbourne, Australia
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
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
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Origins
Operational use since 1973
Developed with North West Pacific and Atlantic data.
Enhanced infrared technique 1978/1984
Ongoing development over 15 years.
Probably the single most important tool in
tropical cyclone analysis to date.
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
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.
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Locating the centre
•Low level centre
•Use all available data – passive microwave to the rescue!
•Maintain track continuity.
•Centre location can influence intensity measurement
APSATS 2002 Melbourne, Australia
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Pattern Types
•Eye
•Curved Band
•Shear
•Covered Centre
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
APSATS 2002 Melbourne, Australia
Intensity schematic
Principles of the Dvorak Method
Eye patterns
Method: Measure the warmest brightness temperature
in the eye and the coldest surrounding temperature in
the deep convection.
Physical principle: strength of the thermal contrast
between the eye and the surrounding convection
indicates strength of the system
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Eye pattern using digital IR
•
Warmest eye pixel 16 °C
•
Coldest pixel 30 nmi from
center -71 °C
•
Nomogram gives Eye no. =7
Hurricane Erika 1515 UTC 8 September 1997
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
APSATS 2002 Melbourne, Australia
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Curved bands
Method: Measure the curvature of the band
Physical principle: the “wrap-aroundness” of the
convective bands indicates the vorticity associated
with the system.
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Curved Bands
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
TS Ivan 23/9/98
11:15 UTC
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Curved Bands
Measuring the arc length:
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•
•
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
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Curved 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”
APSATS 2002 Melbourne, Australia
LOG10
Spiral
Principles of the Dvorak Method
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.
APSATS 2002 Melbourne, Australia
Step 2A, Curved Band
Note: Southern
Hemisphere
Example
APSATS 2002 Melbourne, Australia
Step 2A, Curved Band
APSATS 2002 Melbourne, Australia
Step 2A, Curved Band
APSATS 2002 Melbourne, Australia
Step 2A, Curved Band
0.70
0.80
0.60
0.50
0.40
0.30
0.10
0.20
APSATS 2002 Melbourne, Australia
A wrap of 0.80
would equal a
Data T of T3.5
Step 2A, Curved Band
0.70
0.80
0.60
0.50
0.40
0.30
0.10
0.20
APSATS 2002 Melbourne, Australia
We could have added
an additional 0.05
for this portion of
wrap, giving a total
wrap of 0.85
0.70
0.80
0.60
0.50
0.40
0.30
0.10
0.20
APSATS 2002 Melbourne, Australia
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Shear patterns
Method: Measure the distance form the low level
centre to the edge of the “dense overcast”
Physical principle: greater involvement of the low level
centre with the deep convection indicates a stronger
system.
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Shear patterns
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Shear pattern
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Shear pattern
70nm = T1.5
APSATS 2002 Melbourne, Australia
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Covered Centre Patterns
IR=Embedded Centre (EC)
VIS=Central Dense Overcast (CDO)
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Central Dense Overcast
Method: Appearance and size of the overcast +
degree of banding.
Physical principle?: The weakest link?
Does size matter?
“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
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
A quick diversion on size
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Central Dense Overcast
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Embedded Centre
Method: Measure coldest surrounding temperature.
Physical principle: Greater involvement of low level
centre with deep convection indicates a stronger
system.
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Embedded Centre
•Restrictions on use.
•Sensitive to centre location.
•Methodology similar to eye patterns
•Best used after disappearance of an eye.
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Embedded Centre
Just plain ugly!
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
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”?
APSATS 2002 Melbourne, Australia
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Reality check!
TCs Elaine & Vance, March 1999
APSATS 2002 Melbourne, Australia
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
MET = Model Expected T number
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•
Compare current image to image 24 hours ago.
Determine if the cloud features in the current
image look better defined, the same or worse.
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•
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If better, the trend is Developed (D)
If the same, the trend is Same (S)
If worse, the trend is Weakened (W)
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
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”!
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Pattern T number – an adjustment to the MET.
Select the pattern in the diagram that best
matches your storm picture – within one column of
the MET - SUBJECTIVE
A
EIR Patterns
B
C
APSATS 2002 Melbourne, Australia
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
Choosing the best estimate – the Final T-no
More objective
•Use DT when cloud features are “clear cut”
•Otherwise use MET (possibly adjusted by
And then…..
RULES, RULES, RULES
APSATS 2002 Melbourne, Australia
“Pattern T-no)
APSATS 2002 Melbourne, Australia
Principles of the Dvorak Method
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
APSATS 2002 Melbourne, Australia