Back to Basic Definitions
Page | Back
to Main FAQ Page
Subject: A11) What is the "eye"? How is it formed
and maintained ? What is the "eyewall"? What are "spiral bands"?
(Written with major assistance 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 ring 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 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.
Last updated August 13, 2004
Back to Basic Definitions
Page | Back
to Main FAQ Page
|