TC Lifecycle and Intensity Changes Part I: Genesis

Tropical M. D. Eastin

TC Lifecycle and Intensity ChangesPart 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


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


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)

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

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

TC Genesis

High westerly shear Low easterly shear

Bad – convectiontorn apart

Good – latentheat can concentrate inone area


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

TC Genesis

Red Areas = Dry

Gray/Blue Areas = Moist

GOES Water Vapor Image

Strong downdrafts = Outflow Boundaries


Warm Ocean:

• Allows for sensible and latent heat fluxes from the ocean in order to sustain deep convection

• SSTs > 26.5ºC is the rule

TC Genesis

Deep Convection

L

vTTCcSH airSSTSp )(

vqqCLLH airSSTLv )(

Standard Flux Equations

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


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

TC Genesis

Mixed Layer


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

TC Genesis

Sounding on a Skew-T


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


Mesoscale Convective Vortices (MCVs)

Origin: Develop within persistent mesoscaleconvection 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 Cold

Typical MCV Cross-Section

Warm

Positive Vorticity Negative

Vorticity


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)


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?

Genesis via the CISK Mechanism

Jule Charney


Genesis via the CISK Mechanism

1 Friction with surface causes inflow intothe 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 thuslatent 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)

2

LatentHeatRelease

L

Charney and Eliassen (1964) showed that CISK developed a TC with a diameter of 100 km in 2.5 days (similar to observations)


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)


Genesis via the WISHE Mechanism

Kerry Emanuel


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


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


Genesis via the VHT Mechanism

Mike Montgomery



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



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

Shear Vector

Low-level vortices



z = 0.67 km

• 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.


TC Lifecycle and Intensity ChangesPart 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)


ReferencesCharney, J. G., and A. Eliassen, 1964: On the growth of the hurricane depression. J. Atmos. Sci., 21,

68-75.

Emanuel, K. A., 1986: An air-sea interaction theory for tropical cyclones. Part I: Steady-statemaintenance., 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 information 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 routeto tropical cyclogenesis. J. Atmos. Sci., 63, 355-386.

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TC Lifecycle and Intensity Changes Part I: Genesis