page_title: Eyewall replacement cycle
section_title: Death of the inner eyewall
hierarchical_section_title: Eyewall replacement cycle / Death of the inner eyewall
caption_attribution_description: English: 0038z 12 October 2013 F-17 P1H microwave pass of Cyclone Phailin in the North Indian Ocean. At this time, Phailin was a potentially catasrophic system packing winds of 160 mph.
context_page_description: Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 185 km/h, or major hurricanes. When tropical cyclones reach this intensity, and the eyewall contracts or is already sufficiently small, some of the outer rainbands may strengthen and organize into a ring of thunderstorms—an outer eyewall—that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. Since the strongest winds are in a cyclone's eyewall, the tropical cyclone usually weakens during this phase, as the inner wall is "choked" by the outer wall. Eventually the outer eyewall replaces the inner one completely, and the storm may re-intensify.
  The discovery of this process was partially responsible for the end of the U.S. government's hurricane modification experiment Project Stormfury. This project set out to seed clouds outside the eyewall, apparently causing a new eyewall to form and weakening the storm. When it was discovered that this was a natural process due to hurricane dynamics, the project was quickly abandoned.
context_section_description: After the secondary eyewall totally surrounds the inner eyewall, it begins to affect the tropical cyclone dynamics. Hurricanes are fueled by the high ocean temperature. Sea surface temperatures immediately underneath a tropical cyclone can be several degrees cooler than those at the periphery of a storm, and therefore cyclones are dependent upon receiving the energy from the ocean from the inward spiraling winds. When an outer eyewall is formed, the moisture and angular momentum necessary for the maintenance of the inner eyewall is now being used to sustain the outer eyewall, causing the inner eye to weaken and dissipate, leaving the tropical cyclone with one eye that is larger in diameter than the previous eye.
  In the moat region between the inner and outer eyewall, observations by dropsondes have shown high temperatures and dewpoint depressions. The eyewall contracts because of inertial instability. Contraction of the eyewall occurs if the area of convection occurs outside the radius of maximum winds. After the outer eyewall forms, subsidence increases rapidly in the moat region.
  Once the inner eyewall dissipates, the storm weakens; the central pressure increases and the maximum sustained windspeed decreases. Rapid changes in the intensity of tropical cyclones is a typical characteristic of eyewall replacement cycles. Compared to the processes involved with the formation of the secondary eyewall, the death of the inner eyewall is fairly well understood.
  Some tropical cyclones with extremely large outer eyewalls do not experience the contraction of the outer eye and subsequent dissipation of the inner eye. Typhoon Winnie (1997) developed an outer eyewall with a diameter of 200 nautical miles (370 km) that did not dissipate until it reached the shoreline. The time required for the eyewall to collapse is inversely related to the diameter of the eyewall which is mostly because inward directed wind decreases asymptotically to zero with distance from the radius of maximum winds, but also due to the distance required to collapse the eyewall.
  Throughout the entire vertical layer of the moat, there is dry descending air. The dynamics of the moat region are similar to the eye, while the outer eyewall takes on the dynamics of the primary eyewall. The vertical structure of the eye has two layers. The largest layer is that from the top of the tropopause to a capping layer around 700 hPa which is described by descending warm air. Below the capping layer, the air is moist and has convection with the presence of stratocumulus clouds. The moat gradually takes on the characteristics of the eye, upon which the inner eyewall can only dissipate in strength as the majority of the inflow is now being used to maintain the outer eyewall. The inner eye is eventually evaporated as it is warmed by the surrounding dry air in the moat and eye. Models and observations show that once the outer eyewall completely surrounds the inner eye, it takes less than 12 hours for the complete dissipation of the inner eyewall. The inner eyewall feeds mostly upon the moist air in the lower portion of the eye before evaporating.
caption: A microwave pass of Cyclone Phailin revealing the moat between the inner and outer eyewalls.
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