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TC Lifecycle and Intensity Changes Part I: Genesis. Hurricane Katrina (2005) August 24-29. Outline. Tropical Cyclone Genesis Large-Scale Factors Easterly Waves and MCVs CISK Mechanism WISHE Mechanism VHT Mechanism MP Mechanism. TC Genesis.

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TC Lifecycle and Intensity Changes

Part I: Genesis

Hurricane Katrina (2005)

August 24-29

M. D. Eastin


Outline

  • Tropical Cyclone Genesis

    • 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

M. D. Eastin


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)

M. D. Eastin


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

M. D. Eastin


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

Good – latent

heat can

concentrate in

one area

Bad – convection

torn apart

High westerly shear 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

GOES Water Vapor Image

M. D. Eastin


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

The inflowing air gains heat and moisture only if the ocean is warmer and moister than the air

L

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)

  • 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

Mixed Layer

M. D. Eastin


TC Genesis

  • Conditionally Unstable Atmosphere:

  • 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

Sounding on a Skew-T

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

M. D. Eastin


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)

  • Why do we care about MCVs?

  • Often emerge over warm waters with convection

  • Systems “pre-conditioned” for successful genesis

Typical MCV Cross-Section

Positive

Vorticity

Negative

Vorticity

Warm

Cold

M. D. Eastin


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 dawn of time.” (Dr. Bill Gray, 2003)

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

  • Remaining question:How does the surface vortex form?

Jule Charney

M. D. Eastin


Genesis via the CISK Mechanism

1

2

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”

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

L

Charney and Eliassen (1964) showed that CISK developed a TC with a

diameter of 100 km in 2.5 days (similar to observations)

M. D. Eastin


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)

  • Remaining question:How does the surface vortex form?

Kerry Emanuel

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

M. D. Eastin


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

  • 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?

Mike Montgomery

M. D. Eastin


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

M. D. Eastin


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

  • Episodic convective bursts (hot towers)

  • developed multiple low-level vortices that

  • rotated around to the northeast

Low-level vortices

M. D. Eastin


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

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

  • Assumes the atmosphere is conditionally unstable

  • Requires the presence of a movingandmature finite

  • amplitude disturbance (an easterly wave) with a closed

  • central circulation in the wave-relative framework

  • (also called the “marsupial pouch”)

  • 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?

Tim Dunkerton

Zhou Wang

Mike Montgomery

Captain Kangaroo

M. D. Eastin


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

  • 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/storms2014.html

Streamlines in the wave-relative

reference frame

Wave Axis

M. D. Eastin


Genesis via the MP Mechanism

Tropical Storm Fabio (2000)

Precipitation Rate

Thin black contours: Wave-relative streamlines at 600-mb

Thin red contours: Pouch boundaries at 600-mb

Thick black line: Trough (wave) axis

Shading:Precipitation Rate (mm/day)

Large Black Dot: Genesis time and location

M. D. Eastin


Genesis via the MP Mechanism

Tropical Storm Fabio (2000)

Vertical Vorticity 850-mb

Thin black contours: Wave-relative streamlines at 850-mb

Thin red contours: Pouch boundaries at 850-mb

Thick black line: Trough (wave) axis

Shading:Vertical vorticity (10-5 s-1) at 850-mb

Large Black Dot: Genesis time and location

M. D. Eastin


Genesis via the MP Mechanism

Tropical Storm Fabio (2000)

Relative Humidity 850-mb

Thin black contours: Wave-relative streamlines at 850-mb

Thin red contours: Pouch boundaries at 850-mb

Thick black line: Trough (wave) axis

Shading:Relative Humidity (%) at 850-mb

Large Black Dot: Genesis time and location

M. D. Eastin


Genesis via the MP Mechanism

Tropical Storm Fabio (2000)

200-850-mb Vertical Shear

Thin black contours: Wave-relative streamlines at 850-mb

Thin red contours: Pouch boundaries at 850-mb

Thick black line: Trough (wave) axis

Shading:Vertical Shear (m s-1)

Large Black Dot: 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)

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.

M. D. Eastin


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