SO442 – Tropical Meteorology - PowerPoint Presentation

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SO442 – Tropical MeteorologyTropical Cyclone Lifecycle

http://glossary.ametsoc.org/wiki/Tropical_cyclone

http://glossary.ametsoc.org/wiki/Tropical_cyclone

TC Lifecycle Stages & CharacteristicsKey structural features (i

) boundary layer inflow

(ii) eyewall

(iii) cirrus shield

(iv) rainbands

(v) upper tropospheric outflow

(vi) eye (usually > 100 kts)

Classification

IntensityStructural

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 circulation

Hurricane(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

kts Super TyphoonThe most symmetric of TCs; an eye is likely to form near 100 kts; severe TC classification varies by basin.Decay / 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.

http://www.meted.ucar.edu/tropical/textbook_2nd_edition/navmenu.php?tab=9&page=2.1.0

TC Lifecycle – Atlantic example

http://www.britannica.com/EBchecked/topic/606551/tropical-cyclone

N

E

z

D

ynamic requirements

(specifics of a particular disturbance)

Necessary Conditions for TC

Formation (Gray 1968)

Sufficient ocean thermal energy: 26°C to a depth of >60 m

Enhanced mid-level (700-500 hPa) relative humidity

Conditional instability (i.e., some amount of CAPE)

Enhanced low-level relative vorticity

Weak vertical shear of the horizontal wind

Displacement off the equator (~5°)

Thermodynamic requirements

(background state; ability to support DMC)

s

hear vector

Vertical shear is the vector difference between the horizontal winds at two different levels

Strong winds aloft are associated with large shear values

Incipient Disturbance: StructureLocalized, convective cells form frequently in the tropics. As we’ve learned, the 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 intensityThunderstorms 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 > 26oCMoist

and unstable mid-tropospherePositive relative vorticity at low levelsMinimal vertical wind shear (deep shear < 10 m/s

)Non-zero Coriolis forceDO 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##

Necessary Conditions for TC Formation

An enhanced region of low-level cyclonic vorticity is also necessary to lower what is known as the

Rossby

radius of deformation

to a value that will permit the longevity of the incipient disturbance

is the vertical depth of the temperature anomaly associated with the disturbance

is the relative vorticity

is the Coriolis parameter

is the Brunt-

Vaisaila

frequency: a measure of static stability (larger

 more stable)

We’ll take a look at a couple of hypothetical disturbances to see how this works…

300 hPa

5

00 hPa

7

00 hPa

For the first case, we’ll consider how a relatively small (in length) disturbance, like a thunderstorm, perturbs the height field

is the Brunt-

Vaisala

frequency: a measure of static stability (larger

 more stable)

is the vertical depth of the temperature anomaly associated with the disturbance

is the relative vorticity

is the Coriolis parameter

Latent

heating

~10 km

At small spatial scales, the horizontal PGF is easily able to adjust the mass (height) field back toward the pre-disturbed state and the disturbance decays (assuming the initial trigger mechanism ceases)

Coriolis needs time to work (hours+)…the mass field has already been readjusted and the horizontal motions ceased before Coriolis can work

PGF

300 hPa

5

00 hPa

7

00 hPa

For the next case, we’ll consider how a relatively large (in length) disturbance, like a complex of thunderstorms, perturbs the height field

is the Brunt-

Vaisala

frequency: a measure of static stability (larger

 more stable)

is the vertical depth of the temperature anomaly associated with the disturbance

is the relative vorticity

is the Coriolis parameter

Latent

heating

~500 km

At large spatial scales, the horizontal PGF acts over a long distance and mass has to move a long distance (at relatively slow speed) to try to restore the pre-disturbed state

Coriolis has sufficient time to operate on the horizontally moving air and deflects it into cyclonic rotation at low-levels

The deflection of the initially inward moving mass into cyclonic rotation means the mass field does not adjust and the disturbance is able to persist

Effectively, the atmosphere comes into

thermal wind balance

with the perturbation in the height field

PGF

is the Brunt-

Vaisala

frequency: a measure of static stability (larger

 more stable)

is the vertical depth of the temperature anomaly associated with the disturbance

is the relative vorticity

is the Coriolis parameter

Latent

heating

~500 km

The

Rossby

radius of deformation is the “threshold” boundary between disturbances that tend to persist (larger than

) and those that tend to dissipate (smaller than

)

If

is large, the atmosphere’s vertical restoring force is strong and the disturbance is dampened quickly

If

is large, for a disturbance of finite magnitude, its ability to perturb the pressure surfaces is reduced since it’s “spread out” vertically

Both of these conditions imply that disturbances would tend to dissipate since most would be smaller than

is the Brunt-

Vaisala

frequency: a measure of static stability (larger

 more stable)

is the vertical depth of the temperature anomaly associated with the disturbance

is the relative vorticity

is the Coriolis parameter

Latent

heating

~500 km

The

Rossby

radius of deformation is the “threshold” boundary between disturbances that tend to persist (larger than

) and those that tend to dissipate (smaller than

)

If

is large,

is reduced so “smaller” (mesoscale?) disturbances may be able to persist

What happens to

in the tropics?

It is

small

which increases

Relatively large

therefore becomes an important determinant in a disturbances ability to persist in the tropics

This is why a source of low-level cyclonic vorticity is one of the necessary conditions for tropical cyclogenesis!

Moreover, for the horizontal scale of most tropical disturbances, there is a threshold value for

that allows the disturbance to exceed

…it is the value associated with a wind speed near ~34

kts

STAGE 2: Tropical Storm (34-63 kts)

Strictly speaking, a disturbance becomes a tropical storm when it no longer requires external forcing to survive (i.e., when it becomes “self-sustaining”)

If the other required conditions (e.g., favorable thermodynamics, low wind shear, etc.) are

maintained

to sufficient degree, the tropical storm can intensify

The storm’s strong winds promote high evaporation rates

from the sea surface consistent with a relationship we looked at previously:

wind velocity

…friction acting on the cyclonic winds in the boundary layer causes convergence toward the axis of rotation

This, together with sensible heat flux from the warm ocean, increases the near surface air’s theta-e and, hence, its buoyancy

…converging air is forced to rise, by continuity, where it cools adiabatically until becoming supersaturated at which point net condensation occurs and latent heat is released

…the heating in mid-troposphere lowers heights below which causes the cyclonic winds to increase which increases evaporation and the cycle continues…

TC Development: An “Upscale” ProcessDeep thunderstorms are often called“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

need to merge with larger mesoscale systems (to increase their Rossby radius

)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 TC

Convection that occurs on larger spatial scales will have longer temporal scales (because it is larger than the Rossby radius)

Thunderstorms can last several hoursHurricanes can last

several days

(from Houze 2010)

STAGE 2: Tropical Storm (34-63 kts)

Again, if the conditions perpetuating the cycle just described persist, then the cyclone can intensify

Tropical Storm Rita at an intensity of ~60

kts

(993

mb

)

Continued maintenance of favorable conditions leads to the development of a symmetric convection pattern and development of a clear subsidence eye in the center…this occurs in the vicinity of ~65

kts

or so…

STAGE 3: Severe Tropical Cyclone (64+ kts)

(a.k.a. hurricane)

In general, parameters must remain favorable to permit further intensification

Hurricane Rita at an intensity of ~90

kts

(967

mb

)

Strong cyclones may be able to tolerate (and even intensify under) a modest amount of vertical wind shear…weak storms cannot survive under shear which is why it is considered a negative for tropical cyclogenesis

Fairly symmetric convective pattern developing along with a clear eye in microwave imagery

STAGE 4: Super Typhoon/Major Hurricane

Peak winds > 100

kts

+

Relatively few TCs reach this status

Generally requires the storm to remain over open ocean…more difficult for storms that are near land

Bay of Bengal, Gulf of Mexico, Western Caribbean are exceptions (high ocean heat content)

Hurricane Rita at an intensity of ~150

kts

(898

mb

)

S

ymmetric inner-core convective pattern…symmetric clear eye

STAGE 5: Decay or Extratropical Transition

Begins to occur when one or more of the necessary conditions becomes hostile to the storm

Sufficient ocean thermal energy: 26°C to a depth of >60 m

Enhanced mid-level (700-500 hPa) relative humidity

Conditional instability (i.e., some amount of CAPE)

Enhanced low-level relative vorticity

Weak vertical shear of the horizontal wind

Displacement off the equator (~5°)

For example…

Moving over cooler water or making landfall

Entrainment of dry air

Movement into an area of unfavorable thermodynamics

Development of strong (i.e., 20+

kts

) deep-layer vertical wind shear over the storm

Interestingly, the development of modest shear over a strong TC can offset worsening thermodynamics (i.e., lower CAPE) somewhat by improving ventilation over the storm and providing a forcing for air to rise independent of buoyancy

STAGE 5: Decay or Extratropical Transition

Remnants of Hurricane Rita after landfall (over Arkansas)

Formation mechanisms by

region

Atlantic

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.

http://maloney.atmos.colostate.edu/galaka/images/Easterly_Waves_fig02.jpg

Atypical formation locations:Central PacificTUTT-induced formationTropical Upper-Tropospheric Troughs (TUTTs) are upper-level lows that, under the right conditions, can extend circulation to the surface and result in TC formation

Rare

http://www.aoml.noaa.gov/hrd/tcfaq/A10.html

South Atlantic

Note that the S. Atlantic has SSTs

>

26

o

C

Why do no TCs form there?

High vertical wind shear

Fewer incipient disturbances that occur in regions of positive relative vorticity (at low levels)

The end of the Tropical Cyclone lifecycle

Stay:

TC Remains in the Tropics

Encounters a “hostile environment” and decays

strong vertical

winds, cool

ocean temps, and/or dry air intrusionLandfall

Changes in storm environmentLoss of ocean energy sourceWith less moisture, there is reduced convection

With reduced convection, there is less latent heatThis weakens the

warm core and raises the central pressureThis leads to a reduced pressure gradient and reduced surface windsIncreased friction

Go: TC Exits the Tropics

TC 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 eastwardOften 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 decreases

Dry air is wrapped into the southwestern sidePrecipitation increases on the northerneaster side

Cold and warm fronts develop