Therapy for the cyclone with an
identity crisis…
“Subtropical
Gustav”
9 Sep 2002
Quiz: Separate the 5 tropical cyclones from the 5 extratropical.
Images
courtesy
NCDC
Some relevant questions…
• What makes a cyclone warm or cold-core?
• If all low pressure areas result from a column of air that
is on average warmer than its environment, how can
there be cold-core cyclones?
• What are the hydrostatic consequences of this
thermodynamic structure & the resulting profile of
cyclone “strength”?
• What about existence of mixed phase cyclones?
• Why the fuss?
60 knots is 60 knots!
Classic warm-core cyclone: TC
Hurricane Bonnie (1998) Temperature Anomaly
-
12km
+
6km
1km
Image courtesy Mark DeMaria, CIRA/CSU
www.cira.colostate.edu/ramm/tropic/amsustrm.asp
Low pressure results
from column of air on
average warmer than
environment, with the
anomalous warmth in
the troposphere
Source:
Advanced Microwave
Sounder (AMSU)
Temperature Anomaly
Classic warm-core cyclone: TC
TC Height Field (m)
from hydrostatic
balance
Warm: expansion of
surfaces
Cold: contraction of
height surface
Classic warm-core cyclone: TC
Height anomaly from
zonal mean shaded
Height perturbation
increases with
altitude in
troposphere
Classic warm-core cyclone: TC
• Intensifies through: sustained convection, surface fluxes.
• Cyclone strength greatest near the top of the PBL
 Gradient wind balance in a convective environ.
-
+
Cold
Stratosphere
Z
Troposphere
W
a
r
m
L
Height perturbation
Classic cold-core cyclone: Extratropical
Cleveland Superbomb Temperature Anomaly
Low pressure results
from column of air
on average
warmer than
environment, with
the anomalous
warmth in the
stratosphere
L
2.5 NCAR/NCEP reanalysis
Classic cold-core cyclone: Extratropical
Height anomaly from
zonal mean shaded
Height perturbation
decreases with
altitude in
troposphere
Classic cold-core cyclone: Extratropical
• Intensifies through: baroclinic development, tropopause
lowering.
• Cyclone strength greatest near tropopause
 QG balance in a minimally convective environ
Cold
-
+
Warm
Stratosphere
Z
Troposphere
Cold
Warm
L
Height perturbation
Hybrid (non-conventional) cyclone
What if an occluded extratropical cyclone moves over warm water?
Characteristics of tropical and extratropical cyclones.
Stratosphere
Z
+
Warmer
Troposphere
Colder
Warmer
L
Height perturbation
Examples of nonconventional cyclones: Past research
Tannehill (1938):
1938 New England Hurricane
Pierce (1939):
1938 New England Hurricane
Knox (1955):
Hurricane Hazel
Palmén (1958):
Hurricane Hazel
Simpson (1972):
“Neutercanes”
Hebert & Poteat (1975):
Subtropical cyclones
Kornegay & Vincent (1976):
T.C. Candy
Bosart (1981):
President’s Day Snowstorm
DiMego & Bosart (1982):
Hurricane Agnes
Gyakum (1983):
QE2 Storm
Shapiro & Keyser (1990):
Warm seclusion extratropical
Bosart & Lackmann (1995):
Hurricane David
Beven (1997):
Cyclone diagram, Hybrid cyclones, Mediterranean
Miner et al. (2000):
Hurricane “Huron”
Thorncroft & Jones (2000):
Hurricanes Iris & Felix
Non-conventional cyclones: Examples
1938 New England Hurricane
?
940hPa
Pierce 1939
•
Began as intense tropical cyclone
•
Rapid transformation into an intense
hybrid cyclone over New England
(left)
•
Enormous damage ($3.5 billion adjusted
to 1990). 10% of trees downed in New
England. 600+ lives lost.
•
Basic theories do not explain a frontal
hurricane
Non-conventional cyclones: Examples
Christmas 1994
Hybrid New England Storm
NCDC
•
Gulf of Mexico extratropical cyclone that
acquired partial tropical characteristics
•
A partial eye was observed when the
cyclone was just east of Long Island
•
Wind gusts of 50-100mph observed across
southern New England
•
Largest U.S. power outage (350,000) since
Andrew in 1992
•
Forecast 6hr earlier: chance of light rain,
winds of 5-15mph.
Model interpretation: What type of development?
PMIN=1009hPa
PMIN=1001hPa
PMIN=1003hPa
PMIN=1005hPa
Why is the phase of a cyclone important?
• Predictability is a function of cyclone phase
• Model interpretation/trust is a function of phase
• It is often not at first apparent what the model is
forecasting, or the nature of cyclone development
• MPI is a function of cyclone phase
Impact is a function of structural evolution and interaction
Fran (1996):No transition
Analysis courtesy
NOAA/NWS/NHC
Floyd (1999):
Transition from
pos. tilt trough
Hazel (1954):
Transition from a
neg. tilt trough
Analysis courtesy
NCAR/NCEP
Reanalysis-2
Analysis courtesy
Jim Abraham, CHC
Significance:
• Classification
• Better understanding of the current state
• Applying conceptual models
• The type/extent of expected impact/damage
• Quantifying potential for intensity change and its uncertainty
– Scales of motion dependence
– Maximum intensity
• How can intensity change be forecast if there is great structural uncertainty?
• Amount of intrinsic (mis)trust of numerical model forecasts
 Need a diagnosis of basic cyclone structure that is more
flexible than only tropical or extratropical
Goal:
A more flexible approach to cyclone characterization
To describe the basic structure of tropical,
extratropical, and hybrid cyclones simultaneously
using a cyclone phase space.
Phase Space
Parameter A
Cyclone Parameter 1: Vertical structure
-VT: Thermal Wind [Warm vs. Cold Core]
Warm core
Hybrid
Cold Core
300mb
-
+
-
+
-
+
600mb
900mb
Height perturbation
Height perturbation
Height perturbation
Cyclone Parameter -VT: Thermal Wind
Warm-core example:
Hurricane Floyd 14 Sep 1999
Z
Z
Z
Z
Z
Z
Vertical profile of
ZMAX-ZMIN is proportional
to thermal wind (VT).
 ( Z MAX  Z MIN )
  | VT |
 ln p
Two layers of interest
300hPa
 ( Z MAX  Z MIN )
  | VTU |
 ln p
600hPa
Z
Z
Z
600hPa
 ( Z MAX  Z MIN )
  | VTL |
 ln p
900hPa
Cyclone Parameter -VT: Thermal Wind
Cold-core example:
Cleveland Superbomb 26 Jan 1978
300hPa
 ( Z MAX  Z MIN )
  | VTU |
 ln p
600hPa
600hPa
 ( Z MAX  Z MIN )
  | VTL |
 ln p
900hPa
Cyclone Parameter 2: Horizontal structure
B: Thermal Asymmetry
Symmetric
Hybrid
Asymmetric
Cyclone Parameter B: Thermal Asymmetry
• Defined using storm-relative 900-600hPa mean
thickness field (shaded) asymmetry within 500km
radius:
B  Z 600hPa  Z 900hPa  Z 600hPa  Z 900hPa
R
L
L
Cold
B >> 0:
Frontal
Warm
B0: Nonfrontal
Cyclone Parameter B: Thermal Asymmetry
Conventional Tropical cyclone: B  0
Forming
L
Mature
Decay
L
L
Conventional Extratropical cyclone: B varies
Developing
L
B >> 0
Mature
L
B>0
Occlusion
L
B0
Constructing Phase Space
Constructing 3-D phase space from cyclone
parameters: B, -VTL, -VTU
A trajectory within 3-D generally too
complex to visualize in an
operational setting
 Take two cross sections (slices) :
B
-VTU
-VTL
-VTL
Hurricane Mitch (1998)
Case of symmetric, warm-core development and decay
Classic tropical cyclone
Symmetric warm-core evolution: Hurricane Mitch (1998)
Slice 1: B Vs. -VTL
Symmetric warm-core evolution: Hurricane Mitch (1998)
Slice 1: B Vs. -VTL
Symmetric warm-core evolution: Hurricane Mitch (1998)
Slice 2: -VTL Vs. -VTU
Upward
warm core
development
maturity, and
decay.
With
landfall,
warm-core
weakens
more rapidly
in lower
troposphere
than upper.
Symmetric warm-core evolution: Hurricane Mitch (1998)
Slice 2: -VTL Vs. -VTU
Upward
warm core
development
maturity, and
decay.
With
landfall,
warm-core
weakens
more rapidly
in lower
troposphere
than upper.
December 1987 Extratropical Cyclone
Case of asymmetric, cold-core development and decay
Classic occlusion of an extratropical cyclone
Asymmetric cold-core evolution: Extratropical Cyclone
Slice 1: B Vs. -VTL
Asymmetric cold-core evolution: Extratropical Cyclone
Slice 1: B Vs. -VTL
Asymmetric cold-core evolution: Extratropical Cyclone
Slice 2: -VTL Vs. –VTU
Asymmetric cold-core evolution: Extratropical Cyclone
Slice 2: -VTL Vs. –VTU
Hurricane Floyd (1999)
Multiple phase evolution:
Case of extratropical transition of a tropical cyclone
5
Warm-to-cold core transition:
Extratropical Transition of Hurricane
Floyd (1999): B Vs. -VTL
4
3
B
5
2
4
3
1
-VTL
2
1
Warm-to-cold core transition:
Extratropical Transition of Hurricane Floyd (1999)
B Vs. -VTL
B
Provides
for objective
indicators of
extratropical
transition
lifecycle.
Extratropical transition
ends when –VTL < 0
Extratropical transition
begins when B=10m
-VTL
Spaghetti Plot of 34 Cyclone Phase Trajectories based
Navy NOGAPS operational analyses
960hPa 970hPa 980hPa 990hPa 1000hPa 1010hPa
Asymmetric
Cold-Core
Asymmetric Warmcore
B
Symmetric Cold-core
Symmetric Warm Core
-VTL
upon
Composite Mean ET Structural Evolution Summary
34-Cyclone Composite Mean Phase NOGAPS-analysis based
Trajectory with key milestones labeled
TE+24h
TE
TMID
TE+48h
TB
TE+72h
TB-24h
TB-72h
TB-48h
Variability About the Composite Mean
Boxes represent the calculated one standard deviation spread about the
34-cyclone consensus mean trajectory for each time
Considerable
variability
about mean
once transition
completed=>
posttropical
phase can take
many forms….
Floyd (1999): Non-intensifying
cold-core development (40%)
Hugo (1989): Explosive cold-core
development (20%)
Dennis (1999): “ET-Interruptus”. ( 5%)
Charley (1986): Schizophrenia
(20%)
Keith (1988): Explosive warmseclusion development (15%)
Hurricane Olga (2001)
Multiple phase evolution:
Case of tropical transition of a cold-core cyclone
Cold-to-warm core transition:
Tropical Transition of Hurricane Olga (2001)
-VTU Vs. -VTL
-VTU
Tropical transition
begins when –VTL > 0
(subtropical status)
-VTL
Tropical
transition
completes when
–VTU > 0
(tropical status)
Summary of cyclone types within the phase space
Summary of cyclone types within the phase space
?Polar lows?
Phase space limitations
• Cyclone phase diagrams are dependent on the quality of the
analyses upon which they are based.
• Three dimensions (B, -VTL, -VTU) are not expected to explain all
aspects of cyclone development
• Other potential dimensions: static stability, long-wave pattern, jet
streak configuration, binary cyclone interaction, tropopause
height/folds, surface moisture availability, surface roughness...
• However, the chosen three parameters represent a large
percentage of the variance & explain the crucial structural
changes.
Often model analysis representation is poor
Warm-Seclusion Dilemma: Hurricane Fabian
(2003): NOGAPS Phase Verification
Hurricane Fabian (2003): Temperature anomaly
(°C) from zonal mean
At what
point has
extratropical
transition
occurred?
Cyclone Phase Forecasting
Real-time Cyclone Phase Analysis & Forecasting
• Phase diagrams produced in real-time for
various operational and research models.
• Provides insight into cyclone evolution that
may not be apparent from conventional
analyses
• Web site: http://moe.met.fsu.edu/cyclonephase
• Also available a historical archive of CPS
diagrams for nearly 100 cyclones
Example real-time cyclone availability for AVN
Example AVN forecast oceanic extratropical cyclone
Forecast phase analysis
Zoomed
Example AVN forecast oceanic extratropical cyclone
Forecast phase analysis
Zoomed
Example use of cyclone phase
space for structural forecast
 Subtropical to tropical Karen (2001)
Example: (Sub)tropical cyclogenesis or tropical transition
• Conventional forecast fields: [MSLP, T, PV, E] do not always
provide sufficient insight into nature of cyclone development.
•
This type of development is critical to both threat and expected
predictability (model fallibility).
• Need to understand structural future before we can begin to
anticipate intensity evolution
• Derived phase may be more valuable than the explicit numerical
intensity forecast.
• Example: Hurricane Karen(2001)
Karen (2001)
1800 UTC 10 October
1800 UTC 12 October
1800 UTC 11 October
1800 UTC 13 October
Forecast MSLP
PMIN=1009hPa
PMIN=1001hPa
PMIN=1003hPa
PMIN=1005hPa
400hPa Temperature
Increasing thermal symmetry & lower tropospheric warm core
Warm core development at surface then building upward
-VTU
-VTL
Multiple model solutions:
Measure of structural forecast uncertainty
NGP
AVN
UKM
Cyclone phase trajectory ensembling
Other Past/Current CPS Uses
•
•
•
•
•
•
Tropical cyclone genesis diagnosis/forecasting
Subtropical cyclone genesis diagnosis/forecasting
Timing of extratropical transition
Timing of tropical transition
Diagnosis of structural predictability
Diagnosis of when to switch NEXRAD radars to
tropical mode
1999-2004 Model performance for phase forecasting
– Nam:
Does not analyze or generate shallow/deep warm-core cyclones
– CMC:
Does not analyze or generate deep warm-core cyclones
– MM5:
Rapid warm-core intensification, although highly CPS-dependent
– GFDL:
Best initial warm-core representation, although exaggerates warmcore depth & symmetry for hybrid systems. Shallow warm-core is
unstable: rapid conversion of shallow-warm core to deep warm
core; poorly handles extratropical transition
– NGP:
Best initial warm-core representation, although exaggerates warmcore depth & symmetry for hybrid systems. Synthetic bogus
appears to delay onset of extratropical transition & produce
overintensification thereafter [too frequent high latitude warm
seclusions]
– GFS, UKM:
Both underestimate initial warm-core intensity
& depth
A closing thought from “The Simpsons”…
1996, Episode “Hurricane Neddy”
Homer: “Oh Lisa! There's no record of a hurricane
ever hitting Springfield.“
Lisa :
“Yes, but the records only go back to 1978 when
the Hall of Records was mysteriously blown away!”
Quiz: Separate the 5 tropical cyclones from the 5 extratropical.
Images
courtesy
NCDC
Separate the 5 tropical cyclones from the 5 extratropical.
Images
courtesy
NCDC
Noel (2001)
Unnamed TC
(1991)
Michael (2000)
“Perfect” Storm
(1991)
Extratropical
Low
President’s Day
Blizzard (1979)
Floyd (1999)
Gloria (1985)
Superstorm
of 1993