Transcript Lecture #5: TC Evolution #1

TC Lifecycle and Intensity Changes
Part I: Genesis
Hurricane Katrina (2005)
August 24-29
Tropical
M. D. Eastin
Outline
Tropical Cyclone Genesis
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Tropical
Large-Scale Factors
Easterly Waves and MCVs
CISK Mechanism
WISHE Mechanism
VHT Mechanism
MP Mechanism
M. D. Eastin
TC Genesis
Genesis:
The transformation of a “disorganized” cold-core convective
system into a self-sustaining synoptic-scale warm-core vortex
with a cyclonic circulation at the surface
Necessary (but not sufficient) Conditions:
• Pre-existing convection
• Significant planetary vorticity
• Favorable wind shear pattern
• Moist Mid-troposphere
• Warm ocean with deep mixed layer
• Conditionally unstable atmosphere
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TC Genesis
Pre-existing Convection:
• Source of latent heating
• Persistent heating in one area will
lower the local surface pressure
and begin to converge air toward
the low pressure
(recall the hypsometric equation)
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TC Genesis
Significant Planetary Vorticity:
• Convection near the equator results in
little if any rotation in the low-level inflow
• Convection off the equator will contain rotation
in the low level inflow due to appreciable
Coriolis forcing
• Systems need to be ~5º off the equator in order
to have a chance for development
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TC Genesis
Favorable Wind Shear Pattern:
• Wind shear is often defined as the vector difference
between winds at two altitudes (850 and 200 mb)
• Low magnitudes of shear (< 20 knots) are desired
Bad – convection
torn apart
High westerly shear
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Good – latent
heat can
concentrate in
one area
Low easterly shear
M. D. Eastin
TC Genesis
Moist Mid-Troposphere:
• Dry air will lead to evaporation and cooling
• Cooling produces a surface high pressure,
low-level divergence, sinking air, and a
suppression of convection
Gray/Blue Areas = Moist
Strong downdrafts = Outflow Boundaries
Red Areas = Dry
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GOES Water Vapor Image
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TC Genesis
Warm Ocean:
• Allows for sensible and latent heat fluxes from
the ocean in order to sustain deep convection
• SSTs > 26.5ºC is the rule
Standard Flux Equations
Deep
Convection
SH   c p CS (TSST  Tair ) v
LH   LvCL (qSST  qair ) v
L
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The inflowing air gains heat and
moisture only if the ocean is
warmer and moister than the air
M. D. Eastin
TC Genesis
Deep Oceanic Mixed Layer:
• Mixed layer: Nearly isothermal ocean layer from
the surface to a depth where temperatures cool
rapidly (the thermocline)
Mixed Layer
• Strong winds churn up cool water from the
thermocline or below
• Deeper mixed layers prevent the cooling of
surface waters
• Cold surface waters limit (or reverse) sensible
and latent heat fluxes, reducing convection
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TC Genesis
Conditionally Unstable Atmosphere:
Sounding on a Skew-T
• Lapse rate between the dry adiabatic
and moist adiabatic lapse rates
• Parcels become unstable only when lifted
to their Level of Free Convection (LFC)
• Further ascent produces latent heat
release and locally warm air
(lowers surface pressure)
• Frictional convergence produces lift
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M. D. Eastin
Easterly Waves
Origin: Develop over sub-Saharan Africa from
instabilities along the African Easterly Jet
Basics:
• Wavelengths of ~3000 km
• Move westward at 6-8 m/s
• 60-80 easterly waves cross the Atlantic
each year between July and October
• 7-9 develop into tropical cyclones
Why do we care about easterly waves?
• Often emerge over warm waters with convection
• Like mid-latitude synoptic waves, have preferred
regions of lift (east of the trough): helps generate
persistent convection in the same location
• Often contain mid-level (but not surface) vortices
• Systems “pre-conditioned” for successful genesis
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Mesoscale Convective Vortices (MCVs)
Origin: Develop within persistent mesoscale
convection from heating aloft (convection)
and cooling below (cold downdrafts)
Basics:
• Confined to mid-levels with little or no
signature at the surface
• Often present in easterly waves
• Dynamically stable (last several days)
• Multiple convective cycles
• Can emerge from the continental U.S.
and developed into tropical cyclones
(e.g. Hurricane Danny 1997)
Typical MCV Cross-Section
Positive
Vorticity
Warm
Negative
Vorticity
Why do we care about MCVs?
• Often emerge over warm waters with convection
• Systems “pre-conditioned” for successful genesis
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Cold
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TC Genesis
One of the greatest enigmas of tropical meteorology:
How do we transform a cold-core synoptic-scale disturbance with a
mid-level vortex to a warm-core system with a surface vortex?
“This question has been asked at every tropical cyclone conference
since the first one in 1960” ---- Dr. William (Bill) Gray, 2014
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M. D. Eastin
Genesis via the CISK Mechanism
Convective Instability of the Second Kind (CISK):
• First proposed by Jule Charney in 1964
• Assumes the atmosphere is conditionally unstable
• Requires the presence of a finite amplitude synoptic
scale disturbance (easterly wave)
• Assumes latent heat release results from synoptic-scale
frictional convergence
Jule Charney
Remaining question: How does the surface vortex form?
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Genesis via the CISK Mechanism
1
Friction with surface causes inflow into
the disturbance to be “deflected” inward
toward the surface center. Mass continuity
dictates upward motion must result. This
process is called “Ekman Pumping”
L
2
Upward motion causes saturation and thus
latent heat release. If conditionally unstable,
upward motion will continue and enhance
secondary circulation. Vortex will stretch,
which will develop and intensify low-level
cyclonic vorticity (through conservation of
angular momentum)
Latent
Heat
Release
Charney and Eliassen (1964) showed that CISK developed a TC with a
diameter of 100 km in 2.5 days (similar to observations)
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Genesis via the WISHE Mechanism
Wind Induced Surface Heat Exchange (WISHE):
• First proposed by Kerry Emanuel in 1986
• Assumes the tropical atmosphere is not conditionally
unstable, but rather near neutral to moist convection
(i.e. the thermodynamic profile is moist adiabatic)
• Assumes the primary instability is the thermodynamic
difference between ocean and the boundary layer air
(i.e. sensible and latent heat fluxes are crucial)
• Genesis requires the presence of a finite amplitude
disturbance (i.e. an easterly wave or MCV)
Kerry Emanuel
Remaining question: How does the surface vortex form?
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M. D. Eastin
Genesis via the WISHE Mechanism
a. Prior convective cycle creates a
MCV. Continued stratiform rain
leads to cooling and a mesoscale
downdraft, which transports the
mid-level vorticity and low-θe air
to the surface
b. New surface cyclone envokes
sensible and latent heat fluxes.
Frictional driven inflow begins to
warm and moisten, and develop
new convection.
c. Downdrafts disappear, convection
regularly occurs in near neutral
air, warm core gradually develops,
further vortex intensification near
the surface
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Genesis via the VHT Mechanism
Vortical Hot Towers (VHT):
• First proposed by Mike Montgomery in 2004
• Assumes the atmosphere is conditionally unstable
• Assumes the preferred route to genesis is from multiple
“merger events” between convective-scale cumulonimbus
towers that possess intense cyclonic vorticity
Mike Montgomery
• Genesis requires the presence of a finite amplitude
disturbance (easterly wave or MCV) for a background
vorticity source
Remaining question: How does the surface vortex form?
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Genesis via the VHT Mechanism
a. Hot towers (buoyant updrafts) develop
and feed off the conditional instability.
Minimal low-level vorticity.
b. Upward acceleration leads to vorticity
stretching and low-level convergence
(via angular momentum conservation)
of background vorticity
Considerable low-level vorticity
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Genesis via the VHT Mechanism
Shear Vector
Observational Evidence:
Tropical Storm Gustav (2002)
Vertically sheared from the northeast
• Exposed low-level circulation
• Convection confined to the southwest
Low-level vortices
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Episodic convective bursts (hot towers)
developed multiple low-level vortices that
rotated around to the northeast
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Genesis via the VHT Mechanism
• Low-level vorticity maxima associated with two
distinct hot towers are present
• Roughly 0.5 hrs later the maxima have merged
into a single stronger low-level vorticity maximum
• The low-level vortex develops through multiple
merger events.
z = 0.67 km
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M. D. Eastin
Genesis via the MP Mechanism
Marsupial Pouch (MP):
• First proposed by Tim Dunkerton, Zhou Wang, and
Mike Montgomery in 2009
• A special case for the VHT Mechanism
• Most applicable in the Atlantic basin
Tim Dunkerton
Zhou Wang
• Assumes the atmosphere is conditionally unstable
• Requires the presence of a moving and mature finite
amplitude disturbance (an easterly wave) with a closed
central circulation in the wave-relative framework
(also called the “marsupial pouch”)
Mike Montgomery
• Assumes the preferred route to genesis is from multiple
“merger events” between both shallow and deep VHTs
contained within the re-circulating marsupial pouch
Remaining question: Why is the marsupial pouch desirable?
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Captain Kangaroo
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Genesis via the MP Mechanism
Marsupial Pouch (MP):
• The pouch serves as a “protective barrier”
between the re-circulating inner region with
large vertical vorticity and the bypassing
outer environment with smaller vorticity,
drier air, and stronger vertical shear
Streamlines in the wave-relative
reference frame
Wave Axis
• The pouch prevents intrusions of negative
factors that might prohibit genesis
• Increases the likelihood of genesis
• Stronger easterly waves with pouches tend
to undergo genesis compared to weaker
waves with small pouches
Real-time pouch tracking: http://www.met.nps.edu/~mtmontgo/storms2015-atlantic.html
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M. D. Eastin
Genesis via the MP Mechanism
Tropical Storm Fabio (2000)
Precipitation Rate
Thin black contours:
Thin red contours:
Thick black line:
Shading:
Large Black Dot:
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Wave-relative streamlines at 600-mb
Pouch boundaries at 600-mb
Trough (wave) axis
Precipitation Rate (mm/day)
Genesis time and location
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Genesis via the MP Mechanism
Tropical Storm Fabio (2000)
Vertical Vorticity 850-mb
Thin black contours:
Thin red contours:
Thick black line:
Shading:
Large Black Dot:
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Wave-relative streamlines at 850-mb
Pouch boundaries at 850-mb
Trough (wave) axis
Vertical vorticity (10-5 s-1) at 850-mb
Genesis time and location
M. D. Eastin
Genesis via the MP Mechanism
Tropical Storm Fabio (2000)
Relative Humidity 850-mb
Thin black contours:
Thin red contours:
Thick black line:
Shading:
Large Black Dot:
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Wave-relative streamlines at 850-mb
Pouch boundaries at 850-mb
Trough (wave) axis
Relative Humidity (%) at 850-mb
Genesis time and location
M. D. Eastin
Genesis via the MP Mechanism
Tropical Storm Fabio (2000)
200-850-mb Vertical Shear
Thin black contours:
Thin red contours:
Thick black line:
Shading:
Large Black Dot:
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Wave-relative streamlines at 850-mb
Pouch boundaries at 850-mb
Trough (wave) axis
Vertical Shear (m s-1)
Genesis time and location
M. D. Eastin
TC Lifecycle and Intensity Changes
Part I: Genesis
Summary
• Necessary Large-Scale Conditions
• Pre-existing convection
• Significant planetary vorticity
• Favorable wind shear pattern
• Moist mid-troposphere
• Warm ocean with deep mixed layer
• Conditionally unstable atmosphere
• Easterly Waves (origin, structure, importance)
• Mesoscale Convective Vortices (origin, structure, importance)
• Genesis Mechanisms
• CISK (assumptions, physical processes)
• WISHE (assumptions, physical processes)
• VHTs (assumptions, physical processes)
• MP (assumptions, physical processes)
Tropical
M. D. Eastin
References
Charney, J. G., and A. Eliassen, 1964: On the growth of the hurricane depression. J. Atmos. Sci., 21,
68-75.
Dunkerton, T. J., M. T. Montgomery, and Z. Wang, 2009: Tropical cyclogenesis in a tropical wave
critical layer – easterly waves. J. Atmos. Chem. Phys., 9, 5587-5646.
Emanuel, K. A., 1986: An air-sea interaction theory for tropical cyclones. Part I: Steady-state
maintenance., J. Atmos. Sci., 43, 585-604.
Gray, W. M., 1968: Global view of the origin of tropical disturbances and storms. Mon. Wea. Rev., 96,
669-770.
Hendricks, E. A., M. T, Montgomery, and C. A. Davis, 2004: On the role of “vortical” hot towers in
formation of tropical cyclone Diana (1984), J. Atmos. Sci., 61, 1209-1231.
Montgomery, M. T., M. E. Nicholls, T. A. Cram, and A. B. Saunders, 2006: A vortical hot tower route
to tropical cyclogenesis. J. Atmos. Sci., 63, 355-386.
Tropical
M. D. Eastin