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July 16, 2005

Ask Bryan: What is eyewall replacement?

Could you explain "eyewall replacement"?
jeff | 07.15.05 - 10:23 am
Eyewall replacement is the process through which hurricanes can be Category 5 during one advisory, down to Category 3 at the next, and then back to Category 5 again. This can be a little complicated but I will give it my best shot. For those of you looking for a simpler explanation, you can check out articles from USA Today and NOAA.

To understand what eyewall replace is, you must first understand how an eyewall forms. In any weather system, winds are forced by the pressure gradient from areas of high pressure to low pressure. However, as the wind travels it is deflected by the Coriolis effect to the right in the northern hemisphere and to the left in the southern hemisphere. The Coriolis effect is not an actual force but rather can be best expressed as "a frame of reference problem." If anyone needs a clear explanation of this, simply ask and I will make another post. Otherwise check here for a quick explanation. The easiest way to summarize this is that as the wind blows, the Earth moves out from underneath it.

At upper levels the Coriolis effect perfectly balances the pressure gradient force which causes the air to rotate perfectly around the area of low pressure, always perpendicular to the pressure gradient. The circulation is cyclonic (counterclockwise in the northern hemisphere) for a low and anticyclonic (clockwise in the northern hemisphere) for a high. However, at the surface friction counteracts the Coriolis turning and causes the winds to blow in slightly down the pressure gradient cyclonically towards the low. This flow of air towards the low causes a convergence of the flow at the center of circulation. This convergence forces the air to rise upward creating an area of heavy convection at the center of the low pressure system as the warm and moist air at the surface becomes unstable due to the lifting. This convection will draw more and more air in at the surface to replace the air rising in the center of the low. Eventually the low can strengthen into a hurricane as the updraft gets increasingly strong.

As all of this area rises, it has to go somewhere. This causes an area of diverging air at upper levels. As this air moves out and away from the convection, it is again turned by the Coriolis effect and creates an anticyclonic flow aloft. Yes, a tropical cyclone is rotating in different directions in different levels of the storm. While this outflow process is significant enough to support a tropical depression or storm, the convection in a hurricane can become too great to be supported by this outflow.

In this case, an area of air from the updraft outflow will begin to be forced down in the center of the hurricane. As this air sinks it is warmed by adiabatic compression and dries. This unsaturated, subsiding air kills any convection and causes the clear skies that we refer to as an eye. Since this air in the eye is warmer than the air around it due to the latent heat from condensation added when the air was lifted, we refer to tropical cyclones as "warm core" systems. This warm and dry air is then entrained back into the updraft of eyewall which actually helps to choke the convection by diluting the moist air from the inflow with drier air from the eye. If the convection grows stronger, more air will subside in the eye and be recycled back into the eyewall, weakening the updraft, until an equilibrium is reached.

Ok, so we know how an eye forms, now let's talk about the replacement cycles. As I discussed before, there is a significant low level flow towards the center of a hurricane. This flow causes bands of convection to wrap into the center of the storm. Eventually these bands will form into a ring as they wrap in tighter towards the center. Once a ring is formed, it will actually cut off the low level flow and rob the eyewall of the moist air it needs to survive. Without a flow of moist air at the surface, the eyewall will collapse. This will leave the outer ring of steady convection acting as a new eyewall with the outflow from this ring forming a new eye! This new eye will obviously be wider than the old one. Accordingly, the sustained winds will decrease as the flow will conserve angular momentum. For an example of what the double eyewall phases looks like, check out the very recent Dvorak IR image below from Hurricane Emily. If you look closely, you can clearly see a pair of concentric eyewalls within the central convective core.
The final part of the cycle is that the surface convergence and strengthening ring of convection will cause the new eyewall to shrink in towards the center of circulation. As the diameter of the eyewall shrinks, winds will again increase in order to conserve angular momentum. Often the hurricane will be stronger at this phases than when it had its original eyewall. Now the hurricane will find itself back where it left off at the formation of its original eyewall and the cycle will repeat. Often this will lead to repeated fluctuations in intensity during the lifetime of a hurricane. For example, Hurricane Allen (1980) that was mentioned recently underwent several fluctuations between Categories 3 and 5 during its march west.
Tracking God's Fury:
ATLANTIC
Tropical Storm Maria
Tropical Storm Nate
Hurricane Ophelia
EASTERN PACIFIC
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    Ask Bryan:
    What steers hurricanes?
    What is eyewall replacement?
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