Eyewall Replacement Cycles

Mature tropical cyclones will sometimes undergo a process where their original eyewall collapses inward and a new eyewall develops from convection surrounding the old eyewall. This process is known as an Eyewall Replacement Cycle, or ERC. During ERCs, tropical cyclones typically weaken slightly as the old eyewall collapses into the eye and disrupts the low pressure center. The size of the wind field also usually increases during an ERC as a new, larger eyewall develops farther away from the eye. Once an ERC is complete and the new eyewall has fully developed, the tropical cyclone may begin strengthening again as the eye of the hurricane clears. The satellite imagery above shows a tropical cyclone losing its original eyewall as it collapses into the eye and developing a new eyewall with a clearing eye, which will allow it to reintensify.

The reason for the occurrence of ERCs is not fully understood. Some research suggests that the inner eyewall begins to collapse when strong convection develops beyond the eyewall and a second outer eyewall begins to develop. The secondary eyewall may develop as the WISHE process results in a strong heat flux away from the inner eyewall. However, some research has shown that WISHE is not necessary to generate a secondary eyewall.

Once a tropical cyclone has finished an ERC, it usually develops a large, symmetric eyewall with a well-defined eye. This often results in the development of an annular hurricane, which has a large symmetric eye and does not have strong outer rainbands.