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Tropical Cyclone Lifecycle Summary

http://glossary.ametsoc.org/wiki/. Tropical_cyclone. . http://glossary.ametsoc.org/wiki/. Tropical_cyclone. . Necessary, but not sufficient, conditions for tropical cyclogenesis. (Gray, 1968). Although the initial mechanisms by which tropical cyclones might form can vary from ocean basin to ocean basin, the process by which TCs develop should be similar globally.

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Tropical Cyclone Lifecycle Summary

Presentation on theme: "Tropical Cyclone Lifecycle Summary"— Presentation transcript:


Tropical Cyclone Lifecycle SummarySlide2







Necessary, but not sufficient, conditions for tropical cyclogenesis

(Gray, 1968)

Although the initial mechanisms by which tropical cyclones might form can vary from ocean basin to ocean basin, the process by which TCs develop should be similar globally

Gray (1968) proposed several necessary, but not sufficient, conditions necessary for tropical cyclogenesis:

Strong moisture convergence into the vortex caused by frictionally-forced low-level convergence (e.g., Ekman turning)

Accompanying upper-tropospheric divergence that leads to deep cumulus convectionSlightly more net divergence than convergence in the vortex columnHorizontal wind shear present in the lower troposphere, but minimal vertical wind shear Sea surface and deeper ocean temperature at or exceeding 26.5°C

Position poleward of at least 5° to invoke Coriolis turningA pre-existing low-level vorticity disturbanceSlide5

TC Lifecycle Stages & Characteristics





) boundary layer inflow (ii) eyewall (iii) cirrus shield (

iv) rainbands(v) upper tropospheric outflow

(vi) eye (usually > 100 kts)


Structural CharacteristicsTropical Depression(“incipient disturbance” in the text)

Up to 17 m s-1 (<

34 kts)Disorganized convection; Individual thunderstorms; a closed circulation is unlikely

Tropical Storm18-32 m s-1 (34-63

kts)Increased convection and organization; typically a closed circulationHurricane

(also Typhoon or Cyclone)> 33 m s-1 (>

64 kts)

Increased symmetry; a “mature” system; includes key structural features, and well-defined primary and secondary circulations.Severe Tropical Cyclone(Major

Hurricane or Super Typhoon)>100 kts Major Hurricane>135


Super Typhoon

The most symmetric of TCs; a

n eye is likely to form near 100


; severe TC classification varies by basin.


/ Extratropical TransitionVariousIf a TC encounters land while in the tropics, the storm will decay.If a TC exits the tropics (becoming extratropical), it may decay or re-intensify as an extratropical system.




TC Lifecycle – Atlantic example


Slide7 Incipient Disturbance

1. Definition, Structure, and Characteristics

2. Necessary (but not sufficient) conditions for TC Formation

3. Upscale Development

4. Formation Mechanisms by RegionSlide8

Incipient Disturbance: Structure

Localized, convective cells form frequently in the tropics. As we’ve



conditionally unstable

environment is conducive to the formation of thunderstorms. Tropical Cyclones are not instantaneous.“Intermediate, weak disturbances” must form first“Intermediate” refers to scale and duration, while “weak” refers to intensity

Thunderstorms satisfy this requirementOrganize into tropical depressions/storms/hurricanesThe initial disturbance can be very asymmetric prior to TC formation.Must have necessary but not sufficient conditions for TCs to develop

SST > 26oC

Moist mid-tropospherePositive relative vorticity at low levels

Minimal vertical wind shear (deep shear < 10 m/s)DO THIS: View the movie clip beneath Fig. 8.28 in your text. The clip shows the evolution of temperature and precipitation through the lifecycle of a convective cell.

Source: https://www.meted.ucar.edu/sign_in.php?go_back_to=http%253A%252F%252Fwww.meted.ucar.edu%252Ftropical%252Ftextbook_2nd_edition%252Fprint_8.htm## Slide9

Necessary (but not sufficient


) conditions for Tropical Cyclone formation


> 26

oCThe ocean provides heat (sensible heat flux) and moisture (latent heat flux) to the atmosphere immediately above the surface of the ocean.This warm air right above the surface is then less dense than the surrounding air, and therefore rises – releasing the latent heat as the parcel cools and the moisture condenses into clouds

Moist mid-troposphereTall towers of deep convective clouds will only develop if moisture extends vertically into the atmosphere. If rising warm, moist air from just above the sea surface runs into a layer of dry air (such as the Saharan Air Layer), the lapse rate will change (from moist to dry) and the parcel will be much more apt to become neutral (i.e. and stop rising) or stable (and sink), and the convection will cease. However, if the column is moist, parcels will still continue to rise developing deep towers of spinning the atmosphere.

Positive relative vorticity at low levelsThis circulation (CCW in the NH) is indicative of an area of low pressure and is conducive to formation for two reasons:1. There will be convergence into the low that results in upward vertical motion and convection (as long as moisture remains in the column).

2. The vorticity allows the individual convective towers to merge as they spin inward (slightly) toward the center, forming a ring of convection around the low pressure center. Minimal vertical wind shear (deep shear < 10 m/s)

Strong TCs are symmetric horizontally and they are stacked verticallyHorizontal: Think of the “fat tire” or “donut” appearance of a strong hurricane or typhoon from a satellite perspective. The eyewall is evident as a thick ring of bright white (i.e. very tall) clouds, indicating very deep convection,. Winds here are also nearly symmetric about the storm center.

Vertical: The storm stands upright, and the strongest winds exist at the surface. . In regions of vertical wind shear, upper-level and lower-level winds are not identical. They can be different speeds, different directions, or both. Frequently, the upper-level winds are faster than the lower-level winds and the storm tilts over. The vertical integrity of the vertical columns is then disrupted and the columns dry out, reducing nearby convection.

* All conditions must be present simultaneously before tropical cyclogenesis can occur; HOWEVER, even if they are all present simultaneously, tropical

cyclogenesis may not occur…

(from Gray 1969)Slide10

TC Development: An “Upscale” Process

Deep thunderstorms are often


hot towers” to denote the latent heat released, which warms the column –or–

“vortical hot towers” (VHTs) to denote the cyclonic rotation present within the tower.These “incipient disturbances” must grow in size (spatial scale) and duration (temporal scale) in order to become a tropical storm.

The VHTs are convective scale systems, and to grow, they will often merge with larger mesoscale

systems MCV: mesoscale convective vortexMCS: mesoscale convective systemThis upscale growth continues through a process called “

axisymmetrization”denotes convection becoming symmetric about the center of a low-pressure system, which eventually becomes the vertical axis of the TCConvection that occurs on larger spatial scales will have longer temporal scales.

Thunderstorms can last several hoursHurricanes can last several days

(from Houze 2010)Slide11

Formation Mechanisms by Region


African Easterly Waves:

local convection and mesoscale systems that initiate in western Africa, subtropical cyclones.

Convergence upstream (east) of the trough axis results in convection.

Eastern PacificInstabilities in the ITCZ with moist easterly and equatorial waves originating from the Atlantic.West Pacific & Indian Ocean Monsoon trough, equatorial Rossby and mixed Rossby gravity waves, and merger of a number of small mesoscale systems.




Atypical formation locations:

Central Pacific

TUTT-induced formation

Tropical Upper-Tropospheric Troughs (TUTTs) are upper-level lows that, under the right conditions, can extend circulation to the surface and result in TC formation




South Atlantic

Note that the S. Atlantic has SSTs




CWhy do no TCs form there?High vertical wind shearFewer incipient disturbances that occur in regions of positive relative vorticity (at low levels)Slide13 Tropical Storm




Satellite characteristics (organization)Slide14


Becomes a tropical storm once it is self-sustaining

No longer requires external forcing

Can withstand the loss of one or more of the necessary but not sufficient conditions for TC formation

Intensity is based on peak surface wind speed.Warm waters increase energy

Evaporation + heat transfer moistens the tropical storm boundary layerSlide15

Fig 8.8


September 20, 2005 0000 UTC

Tropical Storm Rita

Central pressure= 993


Peak of winds= 30 m/sSlide16 Hurricane

(or Typhoon or Cyclone)


Dynamic: Primary and Secondary Circulations

Thermodynamic: Clouds and Precipitation

Flow BalanceInertial StabilityMotionSlide17

Primary and secondary circulationsSlide18

Thermodynamic Structure

Figure at rightSlide19 Inertial Stability

Inertial stability (


) is a measure of the resistance of a symmetric vortex to “

forcings” acting to change its structure

Note: we said that a TS is “self-sustaining”, meaning that it was able to continue to develop even if one or more of the necessary (but not sufficient) conditions was lacking.The question is then: How can we quantify this?The formula for inertial stability is shown at right.Note the following relationships:

Inertial stability will increase when: relative vorticity,

Coriolis parameter, and

wind speed increaser decreases

(smaller radii, i.e. closer to the TC center).For a given radius and latitude (constant r and f):As wind speed increases, the inertial stability at that location increases.We expect inertial stability to vary with location in the TC.


is inertial stability

is the relative vorticity

is the local Coriolis parameter

The subscript denotes that the value is a constant and is derived from the TC center location

is the relative angular velocity.


Introduction to TC Motion





Typhoon Wipha

We have seen that TCs will initially propagate westward in the NATL. This westward motion is typical in both the N & S hemispheres.

TC motion is highly impacted by the environment around the storm.


NATL: “Bermuda High”; WPAC: “Subtropical Ridge”

Continued movement westward, or a change of direction toward the poles is generally a function of the


of the high (or ridge), as well as the

location of the TC

relative to the high (or ridge).

If a storm tracks northward around the high/ridge, it may “


”, eventually leaving the tropics (a nearly barotropic environment) and entering the midlatitudes (a baroclinic environment).Slide21

Introduction to TC Motion

TC motion is highly impacted by the environment around the storm



The polar front jet (PFJ) located near 300 mb (~30,000’) outlines the “

midlatitude pattern” (a series of ridges and troughs). These alternating high and low pressure features act as a wave guide north of the TC

 impacts the TC’s winds.PGF between the TC, the midlatitude pattern, and the subtropical high will drive winds.

Phasing of the midlatitude pattern is importantThe PFJ is an upper-level feature. As a result, there is substantial vertical wind shear in the jet region  impacts the

TC’s tilt.





Typhoon WiphaSlide22

Winds are driven by the PGF between the systems (the TC,


pattern & STR).

Remember, the tighter the gradient, the faster the winds.

While it is convenient to look at pressure systems on the surface chart like the one above, TCs are steered by winds on more than just one level. In fact, the dominant steering level is 500 mb (~18,000’). Challenging aspect: At every level except the surface, forecasters look at constant-pressure charts (i.e. 300, 500, and 850-mb charts) to determine winds (and other weather features).

Since the entire chart is one pressure (i.e. 500 mb), contours on the chart mark the height of that surface. Higher heights indicate that the pressure surface (500 mb) is located higher in the atmosphere.RIDGE 

think of a mountain on a topography mapLower heights indicate that pressure surface (500 mb) is located lower in the atmosphere.TROUGH  think of a valley on a topography map





Typhoon Wipha

Introduction to TC MotionSlide23

The End of the Tropical Cyclone Lifecycle

In the tropics

Outside the tropicsSlide24

Poleward Motion

Notice that the TC tracks extend much farther


in the northern hemisphere than in the southern hemisphere.

In the southern hemisphere

, the polar jet is much more zonal (E/W). This strong westerly wind constrains the poleward extent of the TC track. (The jet is an upper-level feature and will shear the TC apart).In the northern hemisphere, the increased land mass creates greater thermal discontinuities, resulting in greater variability in the path of the jet. There is greater

meridional (N/S) flow which often facilitates increased poleward motion by TC exiting the tropics.




The End of the Tropical Cyclone Lifecycle


TC Remains in the Tropics

Encounters a “hostile environment” and decays

strong vertical winds

cool ocean tempsdry air intrusionlandfall

Landfall Changes in storm environmentLoss of ocean energy sourceWith less moisture, there is reduced convection; With reduced convection, there is less subsidence in the eyeThis weakens the warm core and raises the central pressure

This leads to a reduced pressure gradient and reduced surface windsIncreased frictionEffects on storm structureWeakening of surface winds

Redistribution of precipitation Go:

TC Exits the TropicsTC will undergo “Extratropical Transition”: Interaction with the Polar Front Jet will impact the symmetry of the TC.Vertical Wind Shear will cause upper levels to tilt eastward

Often this will destroy the coherence of the TC, and it will decay.Occasionally, however, the wave train will be positioned in such a way that the TC is absorbed and actually intensifies as an extratropical system.Decay: can result from the same “hostile” environmental factors noted for systems that remain in the tropics.

Intensification: If TCs evolve into extratropical systems, typically: symmetry decreasesDry air is wrapped into the southwestern sidePrecipitation increases on the northerneaster

sideCold and warm fronts develop