Tropical cyclones reaching an intensity of approximately 120 kts have repeatedly formed more than one eyewall as mapped by passive microwave imagery. The eye and eyewall shrink in diameter as the storm approaches 120 kts while readily observed via both visible and infrared (IR) imagery. However, passive microwave data has now shown how a secondary eyewall forms with a larger radius than the inner eyewall. This secondary eyewall completely encircles the inner eye and cuts off the inflow to the inner eyewall as the evolution continues. The inner eyewall weakens at the expense of the outer eyewall, which then contracts and eventually replaces the inner eyewall. The secondary eyewall can shrink in size and become similar in size to the original eye.

Passive microwave data from the Special Sensor Microwave/Imager (SSM/I) and more recently the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) clearly map these inner core dynamics. Passive microwave permits the user to view these changes even when cirrus clouds obscure vis/IR imagery, due to its ability to see through most non-raining upper-level clouds. Small cirrus cloud crystals do not impact radiation at 85 GHz, but larger frozen hydrometeors associated with intense convection in the rainbands and eyewall are heavy scatters and thus dramatically lower brightness temperatures. Double eyewalls or concentric eyewalls are thus easy to map with the three SSM/Is and one TMI and can even be seen with Advanced Microwave Sounding Unit (AMSU-B) data. Time series examples for multiple storms will be reviewed and the majority of tropical cyclones in both the Atlantic and Pacific basins reaching 120 kts or higher will be summarized for the last six years.