1. TC Lifecycle and Intensity Changes Part III: Dissipation / Transition
Hurricane Katrina (2005)
August 24-29
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2. Outline Dissipation Contributing Factors Extratropical Transition
Definition and Statistics
Common Large-Scale Factors
Cyclone Phase Space Diagrams
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3. TC Dissipation: Contributing Factors
TC moves over a significant landmass:
Loss of heat and moisture fluxes allow
adiabatic cooling to dominate along
inflow trajectories
Increased surface friction drives
additional inflow
Net result is a decrease in deep convection
required to maintain the warm core
(considerable shallow convection continues)
Warm core weakens and system fills
over hours
At landfall, convection can temporarily
increase due to enhanced friction
and heat and moisture fluxes still
occurring offshore
Katrina (2005)
12h prior to
landfall
Katrina (2005)
at landfall
Katrina (2005)
12h after to
landfall
GOES IR imagery
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4. Strong northerly shear
TC Dissipation: Contributing Factors
An increase in vertical wind shear:
Moderate vertical shear can tilt the vortex,
spreading the warm core column over
a larger area, which increases the
minimum surface pressure
Strong vertical shear can “decouple” the
low-level circulation from the upper-level
warm core and its convection
TS Chris (2006)
Minimal Shear
Strong northerly shear
(12 hrs later)
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5. Black arrow denotes the shear vector
TC Dissipation: Contributing Factors
An increase in vertical wind shear:
Moderate vertical shear can significantly affect the convective structure
Convection is often located down-shear-left (with respect to the shear vector)
Hurricane Bonnie (1998)
Shear
Vector
Black arrow denotes the shear vector
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6. Saharan Air Layer (SAL) From Dunion and Velden (2004)
TC Dissipation: Contributing Factors
Joyce (2000)
TC moves into or draws in dry air:
The ingestion of dry mid-level air will
induce local evaporative cooling,
negative buoyancy, and cold/dry
convective downdrafts with low θe.
If a critical mass of such downdrafts
reach low levels, the inflow layer θe
will be considerably reduced and
unable to recover before reaching the
base of the eyewall cloud, preventing
deep eyewall convection.
27 Sep
Dry
60 kts
Moist
28 Sep
27 Sep
Dry
80 kts
Saharan Air Layer (SAL)
29 Sep
Moist
28 September
SAL
28 Sep
Joyce
60 kts
GOES IR
SSMI Water Vapor
From Dunion and Velden (2004)
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7. Ocean Mixed Layer Depth Intensity at Marked Locations
TC Dissipation: Contributing Factors
TC moves over cool SSTs or a Shallow Oceanic Mixed Layer:
Strong surface winds in a TC boundary layer
generate upwelling beneath the storm
If the warm oceanic mixed layer is shallow,
cold water will be mixed to the surface
Colder water will reduce the sensible and
latent fluxes, which will limit any increases
in inflow θe and the potential for deep
eyewall convection
Hurricane Opal (1995)
SST Pre-Storm C B A
Ocean Mixed Layer Depth C B
Intensity at Marked Locations
A = 100 knots
B = 130 knots
C = 80 knots A
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8. TC Dissipation: Contributing Factors
Slow TC motion induces upwelling:
Slow moving TCs generate a lot of
upwelling in one location
In some cases the warm ocean mixed
layer can be completely eroded leaving
SSTs < 26°C beneath the storm
Note: Opal was not a slow moving TC
Hurricane Opal (1995)
SST Post-Storm
Eye
Warm Mixed Layer
SSTs 26-30ºC
Colder Deep Ocean
SSTs < 26ºC
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9. Extratropical Transition
What is Extratropical Transition?
A warm-core tropical cyclone moves north over colder water and structurally
changes to a cold-core cyclone with distinct cross-storm asymmetries
Statistics of Extratropical Transition:
Hart and Evans (2001)
46% of all Atlantic TCs transitioned ( )
50% of landfalling TCs transition
Transition is most common in October but occurs in all months
Transition most often occurs between 35ºN and 45ºN
Transition always involves interaction with a baroclinic cold-core trough
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10. Extratropical Transition
Indicators of Extratropical Transition:
Acceleration of the TC into the Mid-latitudes
Movement over cold SSTs
Loss of organized convection in the inner core
Loss of circulation in the upper-level outflow
Acquisition of front-like characteristics
Redistribution of precipitation to poleward or western side
Spreading out of the surface wind field
Asymmetry in temperature and moisture fields
Intrusion of dry air into mid-levels of the storm
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11. Extratropical Transition
Hurricane Michael (2000)
Extratropical
Northward
acceleration
of Michael
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12. Extratropical Transition
Hurricane Michael (2000)
A front begins
to form
Michael
Outflow only
to the north
Evidence of
a dry slot
From Abraham et al. (2004)
Tropical
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13. Extratropical Transition
Hurricane Michael (2000)
Precipitation
primarily north
of the center
Michael
becomes
extratropical
From Abraham et al. (2004)
Tropical
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14. Extratropical Transition
Hurricane Michael (2000)
Estimated
Center
From Abraham et al. (2004)
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15. Extratropical Transition
Cyclone Phase Space:
Developed by Bob Hart in 2003
Used to distinguish between a symmetric warm-core system (a TC)
and an asymmetric cold core system (an extratropical cyclone)
mb Thickness Symmetry
Checks for large synoptic-scale temperature gradients and fronts
across the system’s track
TCs are symmetric (low values near zero)
Extratropical cyclones are asymmetric (large positive values)
mb Thermal Wind
Checks for increasing or decreasing thickness with height to infer
a warm core (increasing) or cold-core (decreasing) system
TCs are warm-core (positive values)
Extratropical cyclones are cold-core (negative values)
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16. Extratropical Transition
Hurricane Michael (2000)
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17. TC Lifecycle and Intensity Changes
Part III: Dissipation and Transition
Summary
Contributing Factors to Dissipation
Movement over a significant land mass (physical processes)
Moderate to strong vertical shear (physical processes)
Dry Environment (physical processes)
Cool SSTs and/or shallow oceanic mixed layer (physical processes)
Extratropical Transition
Definition
Statistics of occurrence
Indicators of Transition
Cyclone Phase Space and assessing transition
Tropical
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18. References
Abraham, J., J. W. Strapp, C. Fogerty, and M. Wolde, 2004: Extratropical transition of Hurricane
Michael: An aircraft investigation Bull. Amer. Met. Soc., 85, 1323–1339
Black, M. L., J. F. Gamache, F. D. Marks Jr., C. E. Samsury, and H. E. Willoughby, 2002: Eastern Pacific
Hurricanes Jimena of 1991 and Olivia of 1994: The effect of vertical shear on structure
and intensity. Mon. Wea. Rev., 130, 2291–2312.
Bosart, L. A., C. S. Velden, W. E. Bracken, J. Molinari, and P. G. Black, 2000: Environmental influences
on the rapid intensification of Hurricane Opal (1995) over the Gulf of Mexico. Mon. Wea. Rev.,
128,
Dunion, J. P., and C. S. Velden, 2004: The impact of the Saharan air layer on Atlantic tropical cyclone
activity. Bull. Amer. Met. Soc., 75,
Hart, R. E., 2003: A cyclone phase space derived from thermal wind and thermal asymmetry.
Mon. Wea. Rev. , 131, 585–616
Hart, R. E., and J. L. Evans, 2001: A climatology of the extratropical transition of Atlantic tropical
cyclones. J. Climate, 14, 546–564.
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