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Gary Dantinne July 27, 1999 02:44

pictures of turbulence

I am looking for some pictures of turbulent flows. Does anyone know where I can find any?



Patrick Godon July 27, 1999 11:10

Re: pictures of turbulence
Hi there, hoe gaat het met je?

What kind of turbulent flows are you looking for? Two-dimensional? Three-dimensional? Compressible? Rotating? Convective? etc...

What kind of pitures do you want? On the Web? From papers? Original flow from experiments? or from simulations?

Are you interested in the density of the flow? The vorticity? The velocities? etc...

Are you looking for some specific patterns like the Kelvin-Helmholtz instability? (nice spiral pattern repeating itself)?

You can also take pictures by yourself: vapors coming out of the boiling pot; cigarettes smoke; cloud patterns; wavy patterns in the water behind a boat; water flowing over rocks in a river, etc... Most of the flows are tubulent!

I'll give you some references here:

for nice vortices both in simulations and real flows, see

Bayly and Orszag, 1988, Annual Review of Fluid Mechanics, vol.20, p.359 (section 5. Discusssions).

For wavy flows that you can reproduce at home (water table experiments) see:

Daviaud, Hegseth, Berge, 1992, Physical Review Letters, vol.69, number 17. page 2511;

Hamilton, Abernathy, J. Fluid Mech., 1994, 264, p.185.

You can have some beautifull pictures on the web. NASA Jet Propulsion Laboratory ( ) has nice pictures of turbulence in the atmosphere of Jupiter. Just make a search (for exmaple in yahoo) with (again for example) the word "Kelvin-Helmholtz". Or just go to the library and check the periodicals like Journal of Fluid Mechanics, Computational Physics, etc...

Bonne chance!


Patrick Godon July 27, 1999 15:19

Re: pictures of turbulence: that's a nice one!
Here is a nice picture of turbulence due to the development of a Kelvin-Helmholtz instability (simulations in 2D).

<LI> a picture ;

and more (animations) on the web page of <LI> phil armitage .

Gary Dantinne July 29, 1999 10:47

Re: pictures of turbulence
zeer goed dank u

I 'm looking for pictures from 'natural' turbulence (clouds, wake of a ship, ...). Of course I can take pictures by myself but I am not sure to get good results.

Anyway thank you for your quick answer

A +


Patrick Godon July 29, 1999 11:38

Re: pictures of turbulence: some more..
The reference I gave you from Daviaud, Hegseth and Berge have nice picture of real turbulence similar to the pattern done by the wake of a ship. You can maybe try to get the original from them (they are in France).

There is also a good piture of clouds in the book of Drazin and Reid, hydrodynamics stability (a KH instability indeed), Cambridge University Press, page 21 (1984 edition). You can also try to get some original from Camb. Press.

There online picutre of clouds from space:

Hurricane Luis

and others at .

There a is Von Karman vortex street developing in the atmosphere, seen from the Space Shuttle at the ESA web site: click here .


Tot zien,


John C. Chien July 29, 1999 13:08

Re: pictures of turbulence: some more..
(1). Nice picture about the hurricane. (2). Those are clouds shapes. (3). When an aircraft enters the eye of hurricane, the reported message was there were clouds formation, but it was very quite. (4). Also from the ground observation, when a town is in the middle of the hurricane, it is also extremely quite and there is no wind, for a few hours. There are always dark clouds in the sky even in the middle of the hurricane's eye. (5). So, my question is , it is really hard to tell the turbulence from the cloud formation. I am not sure how one could relate the cloud picture to the air turbulence just by looking at the picture. What do you think? ( unless the clouds are moving quickly and changing shapes rapidly.)

Patrick Godon July 29, 1999 13:32

Re: pictures of turbulence: some more..
I look at the Hurricane as a vortex on the large scale (two-dimensional and rotating) of the atmosphere. Vortices are feature of turbulence. The great Red Spot of Jupiter is also a huge vortex sustain by the merging of smaller vortices. I consider the vortices to be (one of) the display of turbulence in 2D flows. This is on the large scale of thousands of km. The particular Eddy turnover time is long for the large scale turbulence and cannot be felt on the small scale.

On the small scale there is, in the atmosphere, 3D homogeneous turbulence like strong local wind changing from place to place and time to time. The particular Eddy turnover time is very short and can be felt very easily on the 'human' scale.

Of course the clouds pattern are not always associated with the pattern of the flow, but for the hurricane the vortex is apparent.

John C. Chien July 29, 1999 22:03

Re: pictures of turbulence: some more..
(1). Interesting. I have never looked at the hurricane in that way as a large scale vortex. (2). I think if the physical scale is large, then the time scale must also be different. In stead of micro-second time scale, it will be in terms of hours perhaps. (3). At a smaller scale, the time scale of a tornado would be much shorter I think. Because the life of a tornado is rather short. It is a new point of view that the turbulence time scale slows down when the large scale motion grows bigger. So, at the size of a galaxy, the turbulence scale is so large that it is almost stationary in our clock.

Patrick Godon July 30, 1999 10:19

Re: pictures of turbulence: some more..
Yes John,

One way of looking at it is by assuming that the turbulence cannot be supersonic, because otherwise it dissipates quickly by shocks. Therefore you are left with a limit velocity, which is the sound speed. Then the timescale for each length scale is roughly the ratio of the legnth scale, say l, to the velocity, say v, t = l/v. since v is limited to c (the sound speed) the time (Eddy turn over time) can only grow with the lenght scale (scale of the Eddy).

Conserning the eye of the cyclone, the same happens also with the Great Red Spot, where the velocity in the core of the vortex is in fact very small. So here also the eye of the cyclone is rather quite.

Md. Ziaul Islam July 31, 1999 13:13

Re: pictures of turbulence: some more..
I came across an interesting research paper regarding the mathematical modeling of a tornado in fluid dynamics. If anybody is interested then please go through the research paper -

Shtern, V., and Hussain, F., "Hysteresis in a Swirling Jet as a Model Tornado," Physics of Fluids, Vol.5, No. 9, 1993, pp. 2183-2195.

Patrick Godon August 2, 1999 08:30

Re: pictures of turbulence: some more..
Hi There,

Tornado is very different than a hurricane. It is much smaller and it started as a 'rolling' (wind) cylinder on the ground that eventually'stands' up.

Md. Ziaul Islam August 2, 1999 23:14

Re: pictures of turbulence: some more..
Hurricane is a cyclone with very large vortex. On the other hand, tornado is a smaller vortex. Both are created by the natural turbulence phenomena which are not controllable i.e.far away from equilibrium. They eventually decay toward equilibrium in a long period or a short period of time no matter what are the natural controlling devices that created them.

clifford bradford August 6, 1999 01:35

Re: pictures of turbulence
there's a nice book called the "album of fluid motion" it's mostly windtunnel stuff but you might find something you like. also i've looked at some nice fluid picture books by the Jap. Soc. of Mech. Eng. that have natural examples of turbulence (very pretty) these'll will be in the same shelf as the above ref.

Md. Ziaul Islam August 11, 1999 17:19

Re: pictures of turbulence: some more..

Turn! Turn! Turn!

Scientists unravel the

twisted ways of tornadoes.

Tornado Simulations prepared by NCSA's Atmosphere and Oceanic Science group for the IMAX movie Stormchasers.


Tornadoes are unpredictable and deadly--witness the set of

twisters that tore through Bangladesh on May 13, killing

atleast 500 and injuring up to 50,000 people (and look at

recent tornado statistics.

Tornadoes are also irresistible, it seems. Stout-hearted

"storm chasers" race to put themselves right in the path of

danger, both for the sheer thrill of it and to gather

information that will clarify how tornadoes form, gather

strength and dissipate. The obsession seems to be catching:

thrill-seekers are flocking to see real and imagined

tornadoes blast across movie screens throughout the U.S.

The basic atmospheric physics that gives rise to tornadoes

is well understood by now. Thunderstorms usually contain

updrafts, large rising swells of warm, moist air. As the

updraft moves, it rotates; if the rotation grows sufficiently

intense, the storm can evolve into a tornado or funnel cloud

(a tornado whose bottom does not touch the ground).

Supercomputer simulations depict this process quite


Most tornadoes form within an especially intense weather

system known as a supercell. Supercell thunderstorms

occur when the warm updraft punches through an overlying,

stable layer and continues upward into a zone of cool, dry

air. The resulting instabilities produce powerful vortex

motions, the lifeblood of tornadoes (a pair of

computer-generated images depicts the difference between

supercell and non-supercell storms). Within the fiercest

tornadoes, wind speeds can approach 300 miles per hour.

Air rushing in to fill the low-pressure void left by the

tornado creates additional fierce, potentially damaging

winds. Staying out of danger is no easy task when a

tornado is anywhere near.

One of the most dangerous aspects of tornadoes is their

capriciousness. Sometimes an updraft gives rise to a

tornado; sometimes it does not. Scientists are still hard

pressed topredict exactly when and where a tornado will

appear; that uncertainty makes it difficult to raise the alarm

in time tosave lives.

In order to nail down some of the tricky details of tornado

formation, Erik Rasmussen of the National Severe Storms

Laboratory in Norman, Okla., banded together with several

of his colleagues to undertake storm studies of

unprecedented accuracy. The researchers called their

project Verification of the Origins of Rotation in Tornadoes

Experiment, usually shortened to its handy acronym of

VORTEX.During tornado season (April 1 to June 15) of

1994 and 1995, the VORTEX scientists intercepted and

monitored 10 severe twisters.

Those studies were greatly aided by a tool known as

Doppler radar, which can measure local wind speeds at

very high resolution. During the Dimmitt, Tex., tornado of

June 2, 1995, the researchers used a single, portable radar

device (the "Doppler on Wheels") that produced sharp

images showing wind features just 200 feet across--good

enough to show clearly the tornado, its evacuated core and

the surrounding cloud of debris. This year Joshua Wurman

of the University of Oklahoma and his collaborators plan to

use a pair of Doppler radars, which will enable them to

assemble more complete, three-dimensional maps of wind


At present, the VORTEX researchers are still digesting their

massive files of data. A preliminary report from the project

participants describes some of the remarkably intricate

observations of the Dimmitt tornado, generally considered

the most thoroughly studied twister in history.

One frustration voiced in that paper is the extreme speed of

these weather outbursts. The time from formation of a

tornado to touch down is no more than a few tens of

minutes, "providing little time for operational observation,

identification and warning," the VORTEX team notes.

The Optical Transient Detector, an experimental NASA

detector recently lofted into Earth orbit, offers hope that

longer advance warnings may be possible. A group led by

Hugh J. Christian of the Marshall Space Flight Center used

the satellite to tally the rate of lightning flashes in large

storms. One provocative finding was that the flash rate often

peaks shortly before tornadoes appear.

The Optical Transient Detector also detected far more

flashes than were observed from the ground--a sign that the

storms that produce tornadoes create mostly

cloud-to-cloud strikes. These intra-cloud bolts seem to

occur primarily while the storm is building in strength, so

space-based observations could help alert people on the

ground to the mounting tornado risk.

Tornadoes are not yet tamed, of course--how could they

be? But clever research and new technologies promise at

least to take a little wind out of their sails.

--Corey S. Powell, Staff Writer


"Tornadoes," by Robert Davies-Jones, from the August

1995 issue of Scientific American. Read about tornado

studies from one of the leading researchers in the field.


Christian Science Monitor

NOAA Severe Weather Spotter's Guide

Severe Storm descriptions from the University of Illinois

Tornado photo gallery from the University of Illinois

Supercell photo gallery from the University of Illinois

Current weather news from the Storm Prediction Center

Tornado resources from USA Today

Md. Ziaul Islam August 22, 1999 21:23

Re: pictures of turbulence: some more..


Part A : Basic Definitions


Christopher W. Landsea


4301 Rickenbacker Causeway

Miami, Florida 33149

Version 2.8

12 August, 1999


A1) What is a hurricane, typhoon, or tropical cyclone? A2) What are "Cape Verde"-type hurricanes? A3) What is a super-typhoon? A4) Where do these easterly waves come from and what causes them? A5) What is a sub-tropical cyclone? A6) How are tropical cyclones different from mid-latitude storms? A7) How are tropical cyclones different from tornadoes? A8) What does the acronym "CDO" in a discussion of tropical cyclones mean? A9) What is a TUTT?

A10) How do tropical cyclones form ?

A11) What is the "eye" ? How is it formed and maintained ?

Subject: A1) What is a hurricane, typhoon, or tropical cyclone?


The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993).

Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt, 39 mph) are called "tropical depressions". (This is not to be confused with the condition mid-latitude people get during a long, cold and grey winter wishing they could be closer to the equator ;-)) Once the tropical cyclone reaches winds of at least 17 m/s they are typically called a "tropical storm" and assigned a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called: a "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E); a "typhoon" (the Northwest Pacific Ocean west of the dateline); a "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E); a "severe cyclonic storm" (the North Indian Ocean); and a "tropical cyclone" (the Southwest Indian Ocean) (Neumann 1993).

Note that just the definition of "maximum sustained surface winds" depends upon who is taking the measurements. The World Meteorology Organization guidelines suggest utilizing a 10 min average to get a sustained measurement. Most countries utilize this as the standard. However the National Hurricane Center (NHC) and the Joint Typhoon Warning Center (JTWC) of the USA use a 1 min averaging period to get sustained winds. This difference may provide complications in comparing the statistics from one basin to another as using a smaller averaging period may slightly raise the number of occurrences (Neumann 1993).

Subject: A2) What are "Cape Verde"-type hurricanes?

Cape Verde-type hurricanes are those Atlantic basin tropical cyclones that develop into tropical storms fairly close (<1000 km [600 mi] or so) of the Cape Verde Islands and then become hurricanes before reaching the Caribbean. (That would be my definition, there may be others.) Typically, this may occur in August and September, but in rare years (like 1995) there may be some in late July and/or early October. The numbers range from none up to around five per year - with an average of around 2.

Subject: A3) What is a super-typhoon?

A "super-typhoon" is a term utilized by the U.S. Joint Typhoon Warning Center in Guam for typhoons that reach maximum sustained 1-minute surface winds of at least 65 m/s (130 kt, 150 mph). This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin or a category 5 severe tropical cyclone in the Australian basin.

Subject: A4) Where do these easterly waves come from and what causes them?

It has been recognized since at least the 1930s (Dunn 1940) that lower tropospheric (from the ocean surface to about 5 km [3 mi] with a maximum at 3 km [2 mi]) westward traveling disturbances often serve as the "seedling" circulations for a large proportion of tropical cyclones over the North Atlantic Ocean. Riehl (1945) helped to substantiate that these disturbances, now known as African easterly waves, had their origins over North Africa. While a variety of mechanisms for the origins of these waves were proposed in the next few decades, it was Burpee (1972) who documented that the waves were being generated by an instability of the African easterly jet. (This instability - known as baroclinic-barotropic instability - is where the value of the potential vorticity begins to decrease toward the north.) The jet arises as a result of the reversed lower-tropospheric temperature gradient over western and central North Africa due to extremely warm temperatures over the Saharan Desert in contrast with substantially cooler temperatures along the Gulf of Guinea coast.

The waves move generally toward the west in the lower tropospheric tradewind flow across the Atlantic Ocean. They are first seen usually in April or May and continue until October or November. The waves have a period of about 3 or 4 days and a wavelength of 2000 to 2500 km [1200 to 1500 mi], typically (Burpee 1974). One should keep in mind that the "waves" can be more correctly thought of as the convectively active troughs along an extended wave train. On average, about 60 waves are generated over North Africa each year, but it appears that the number that is formed has no relationship to how much tropical cyclone activity there is over the Atlantic each year.

While only about 60% of the Atlantic tropical storms and minor hurricanes ( Saffir-Simpson Scale categories 1 and 2) originate from easterly waves, nearly 85% of the intense (or major) hurricanes have their origins as easterly waves (Landsea 1993). It is suggested, though, that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa (Avila and Pasch 1995).

It is currently completely unknown how easterly waves change from year to year in both intensity and location and how these might relate to the activity in the Atlantic (and East Pacific).

Subject: A5) What is a sub-tropical cyclone?

A sub-tropical cyclone is a low-pressure system existing in the tropical or subtropical latitudes (anywhere from the equator to about 50N) that has characteristics of both tropical cyclones and mid-latitude (or extratropical) cyclones. Therefore, many of these cyclones exist in a weak to moderate horizontal temperature gradient region (like mid-latitude cyclones), but also receive much of their energy from convective clouds (like tropical cyclones). Often, these storms have a radius of maximum winds which is farther out (on the order of 100-200 km [60-125 miles] from the center) than what is observed for purely "tropical" systems. Additionally, the maximum sustained winds for sub-tropical cyclones have not been observed to be stronger than about 33 m/s (64 kts, 74 mph)).

Many times these subtropical storms transform into true tropical cyclones. A recent example is the Atlantic basin's Hurricane Florence in November 1994 which began as a subtropical cyclone before becoming fully tropical. Note there has been at least one occurrence of tropical cyclones transforming into a subtropical storm (e.g. Atlantic basin storm 8 in 1973).

Subtropical cyclones in the Atlantic basin are classified by the maximum sustained surface winds:

less than 18 m/s (34 kts, 39 mph) - "subtropical depression",

greater than or equal to 18 m/s (34 kts, 39 mph) - "subtropical storm"

Note that while these are not given names, they are warned on and forecasted for by the National Hurricane Center similar to the treatment received by tropical cyclones in the region.

Subject: A6) How are tropical cyclones different from mid-latitude storms?

The tropical cyclone is a low-pressure system which derives its energy primarily from evaporation from the sea in the presence of high winds and lowered surface pressure and the associated condensation in convective clouds concentrated near its center (Holland 1993). Mid-latitude storms (low pressure systems with associated cold fronts, warm fronts, and occluded fronts) primarily get their energy from the horizontal temperature gradients that exist in the atmosphere.

Structurally, tropical cyclones have their strongest winds near the earth's surface (a consequence of being "warm-core" in the troposphere), while mid-latitude storms have their strongest winds near the tropopause (a consequence of being "warm-core" in the stratosphere and "cold-core" in the troposphere). "Warm-core" refers to being relatively warmer than the environment at the same pressure surface ("pressure surfaces" are simply another way to measure height or altitude).

Subject: A7) How are tropical cyclones different from tornadoes?

While both tropical cyclones and tornadoes are atmospheric vortices, they have little in common. Tornadoes have diameters on the scale of 100s of meters and are produced from a single convective storm (i.e. a thunderstorm or cumulonimbus). A tropical cyclone, however, has a diameter on the scale of 100s of *kilometers* and is comprised of several to dozens of convective storms. Additionally, while tornadoes require substantial vertical shear of the horizontal winds (i.e. change of wind speed and/or direction with height) to provide ideal conditions for tornado genesis, tropical cyclones require very low values (less than 10 m/s [20 kt, 23 mph]) of tropospheric vertical shear in order to form and grow. These vertical shear values are indicative of the horizontal temperature fields for each phenomenon: tornadoes are produced in regions of large temperature gradient, while tropical cyclones are generated in regions of near zero horizontal temperature gradient. Tornadoes are primarily an over-land phenomena as solar heating of the land surface usually contributes toward the development of the thunderstorm that spawns the vortex (though over-water tornadoes have occurred). In contrast, tropical cyclones are purely an oceanic phenomena - they die out over-land due to a loss of a moisture source. Lastly, tropical cyclones have a lifetime that is measured in days, while tornadoes typically last on the scale of minutes.

An interesting side note is that tropical cyclones at landfall often provide the conditions necessary for tornado formation. As the tropical cyclone makes landfall and begins decaying, the winds at the surface die off quicker than the winds at, say, 850 mb. This sets up a fairly strong vertical wind shear that allows for the development of tornadoes, especially on the tropical cyclone's right side (with respect to the forward motion of the tropical cyclone). For the southern hemisphere, this would be a concern on the tropical cyclone's left side - due to the reverse spin of southern hemisphere storms. (Novlan and Gray 1974)

Subject: A8) What does the acronym "CDO" in a discussion of tropical cyclones mean?

NRL - Monterey

"CDO" is an acronym that stands for "central dense overcast". This is the cirrus cloud shield that results from the thunderstorms in the eyewall of a tropical cyclone and its rainbands. Before the tropical cyclone reaches hurricane strength (33 m/s, 64 kts, 74mph), typically the CDO is uniformly showing the cold cloud tops of the cirrus with no eye apparent. Once the storm reaches the hurricane strength threshold, usually an eye can be seen in either the infrared or visible channels of the satellites. Tropical cyclones that have nearly circular CDO's are indicative of favorable, low vertical shear environments.

Subject: A9) What is a "TUTT"?

Fitzpatrick et al. 1995

A "TUTT" is a Tropical Upper Tropospheric Trough. A TUTT low is a TUTT that has completely cut-off. TUTT lows are more commonly known in the Western Hemisphere as an "upper cold low". TUTTs are different than mid-latitude troughs in that they are maintained by subsidence warming near the tropopause which balances radiational cooling. TUTTs are important for tropical cyclone forecasting as they can force large amounts of harmful vertical wind shear over tropical disturbances and tropical cyclones. There are also suggestions that TUTTs can assist tropical cyclone genesis and intensification by providing additional forced ascent near the storm center and/or by allowing for an efficient outflow channel in the upper troposphere. For a more detailed discussion on TUTTs see the article by Fitzpatrick et al. (1995).

Subject: A10) How do tropical cyclones form ?

To undergo tropical cyclogenesis, there are several favorable pre cursor environmental conditions that must be in place (Gray 1968,1979) :

1.Warm ocean waters (of at least 26.5C [80F]) throughout a sufficient depth (unknown how deep, but at least on the order

of 50 m [150 ft]). Warm waters are necessary to fuel the heat engine of the tropical cyclone.

2.An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection. It is the

thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development.

3.Relatively moist layers near the mid-troposphere (5 km [3 mi]). Dry mid levels are not conducive for allowing the continuing

development of widespread thunderstorm activity.

4.A minimum distance of at least 500 km [300 mi] from the equator. For tropical cyclogenesis to occur, there is a requirement

for non-negligible amounts of the Coriolis force to provide for near gradient wind balance to occur. Without the Coriolis

force, the low pressure of the disturbance cannot be maintained.

5.A pre-existing near-surface disturbance with sufficient vorticity and convergence. Tropical cyclones cannot be generated

spontaneously. To develop, they require a weakly organized system with sizable spin and low level inflow.

6.Low values (less than about 10 m/s [20 kts 23 mph]) of vertical wind shear between the surface and the upper troposphere.

Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt the incipient

tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken or

destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone center.

Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favorable conditions do not develop. Recent work (Velasco and Fritsch 1987, Chen and Frank 1993, Emanuel 1993) has identified that large thunderstorm systems (called mesoscale convective complexes [MCC]) often produce an inertially stable, warm core vortex in the trailing altostratus decks of the MCC. These mesovortices have a horizontal scale of approximately 100 to 200 km [75 to 150 mi], are strongest in the mid-troposphere (5 km [3 mi]) and have no appreciable signature at the surface. Zehr (1992) hypothesizes that genesis of the tropical cyclones occurs in two stages:

stage 1 occurs when the MCC produces a mesoscale vortex.

stage 2 occurs when a second blow up of convection at the mesoscale vortex initiates the intensification process of

lowering central pressure and increasing swirling winds.

Subject: A11) What is the "eye"? How is it formed and maintained? (Written with major assitance from Sim Aberson)

NOAA The "eye" is a roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone. Although the winds are calm at the axis of rotation, strong winds may extend well into the eye. There is little or no precipitation and sometimes blue sky or stars can be seen. The eye is the region of lowest surface pressure and warmest temperatures aloft - the eye temperature may be 10 C [18 F] warmer or more at an altitude of 12 km [8 mi] than the surrounding environment, but only 0-2 C [0-3 F] warmer at the surface (Hawkins and Rubsam 1968) in the tropical cyclone. Eyes range in size from 8 km [5 mi] to over 200 km [120 mi] across, but most are approximately 30-60 km [20-40 mi] in diameter (Weatherford and Gray 1988). The eye is surrounded by the eyewall, the roughly circular area of deep convection which is the area of highest surface winds in the tropical cyclone. The eye is composed of air that is slowly sinking and the eyewall has a net upward flow as a result of many moderate - occasionally strong - updrafts and downdrafts. The eye's warm temperatures are due to compressional warming of of the subsiding air. Most soundings taken within the eye show a low-level layer which is relatively moist, with an inversion above - suggesting that the sinking in the eye typically does not reach the ocean surface, but instead only gets to around 1-3 km [ 1-2 mi] of the surface.

The general mechanisms by which the eye and eyewall are formed are not fully understood, although observations have shed some light on the problem. The calm eye of the tropical cyclone shares many qualitative characteristics with other vortical systems such as tornadoes, waterspouts, dust devils and whirlpools. Given that many of these lack a change of phase of water (i.e. no clouds and diabatic heating involved), it may be that the eye feature is a fundamental component to all rotating fluids. It has been hypothesized (e.g. Gray and Shea 1973, Gray 1991) that supergradient wind flow (i.e. swirling winds that are stronger than what the local pressure gradient can typically support) present near the radius of maximum winds (RMW) causes air to be centrifuged out of the eye into the eyewall, thus accounting for the subsidence in the eye. However, Willoughby (1990b, 1991) found that the swirling winds within several tropical storms and hurricanes were within 1-4% of gradient balance. It may be though that the amount of supergradient flow needed to cause such centrifuging of air is only on the order of a couple percent and thus difficult to measure.

Another feature of tropical cyclones that probably plays a role in forming and maintaining the eye is the eyewall convection. Convection in tropical cyclones is organized into long, narrow rainbands which are oriented in the same direction as the horizontal wind. Because these bands seem to spiral into the center of a tropical cyclone, they are sometimes called spiral bands. Along these bands, low-level convergence is a maximum, and therefore, upper-level divergence is most pronounced above. A direct circulation develops in which warm, moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands. Subsidence is distributed over a wide area on the outside of the rainband but is concentrated in the small inside area. As the air subsides, adiabatic warming takes place, and the air dries. Because subsidence is concentrated on the inside of the band, the adiabatic warming is stronger inward from the band causing a sharp contrast in pressure falls across the band since warm air is lighter than cold air. Because of the pressure falls on the inside, the tangential winds around the tropical cyclone increase due to increased pressure gradient. Eventually, the band moves toward the center and encircles it and the eye and eyewall form (Willoughby 1979, 1990a, 1995).

Thus the cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the eyewall and to a forced descent caused by the moist convection of the eyewall. This topic is certainly one that can use more research to ascertain which mechanism is primary.

Some of the most intense tropical cyclones exhibit concentric eyewalls, two or more eyewall structures centered at the circulation center of the storm ( Willoughby et al. 1982,Willoughby 1990a ). Just as the inner eyewall forms, convection surrounding the eyewall can become organized into distinct rings. Eventually, the inner eye begins to feel the effects of the subsidence resulting from the outer eyewall, and the inner eyewall weakens, to be replaced by the outer eyewall. The pressure rises due to the destruction of the inner eyewall are usually more rapid than the pressure falls due to the intensification of the outer eyewall, and the cyclone itself weakens for a short period of time.

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