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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 calm eye of the tropical cyclone shares many qualitative characteristics with other vortical systems such as tornadoes, waterspouts, dust devils and whirlpools and such a feature appears to be fundamental to many rotating fluid flows.

The formation of the eyewall is related to the convergence of air in a shallow layer some 500 m to 1 km deep adjacent to the sea surface. This layer is referred to as the boundary layer, or friction layer. Above this layer, the swirling winds are approximately in gradient wind balance (Willoughby 1990b, 1991), that is, the inward-directed pressure gradient force is approximately balanced by the sum of the outward-directed centrifugal and Coriolis forces. Frictional stresses in the boundary layer reduce the tangential wind speed and thereby the centrifugal and Coriolis forces, while the pressure gradient force remains largely unchanged. As a result there is a net inward force in the layer which drives the convergence in this layer. Calculations (e.g. Smith 1968, 2003) show that the inflow in the boundary layer turns upwards before it reaches the hurricane center with the maximum upflow near the radius of maximum tangential wind speed just above the boundary layer. The inflowing air is very moist and as it rises out of the boundary layer the vater vapor condenses to form the eyewall clouds. The air flows outwards above the boundary layer (there is essentially nowhere else for it to go, although there may be a little mixing across the inner edge of the eye) and the eyewall clouds tilt outwards with height. As the rising air leaves the friction layer it approximately conserves its absolute angular momentum and as it moves outwards, it spins more slowly and the maximum tangential wind speed occurs at ever increasing radii. Calculations by Emanuel (1997) show that the eyewall has some likeness to an atmospheric front and suggest that the eye is a passive response to processes in the eyewall and beyond.

Outside the eyewall, convection in tropical cyclones is organized into long, narrow rainbands which seem to spiral into the center of the cyclone: these 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).

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 November XX, 2004