Passive microwave satellite imagery has revealed a wealth of tropical cyclones exhibiting more than one eyewall, in direct contrast to that observed via traditional visible and Infrared (IR) imagery. Upper-level cloud obscuration often shields vital storm structure as the central dense overcast blocks eye, eyewall and rainband details. Passive microwave (PMW) imagery permits users to “see through” non-raining clouds and map rainbands and eyewall evolution not feasible with vis/IR data sets. Early PMW data indicated multiple or concentric eyewalls form when storms reach ~ 120 kts. Thus, a ten (10) year data set (1993-2003) of Special Sensor Microwave/Imager (SSM/I) digital data has been created to systematically study intense tropical cyclones that may undergo the formation of multiple eyewalls.

The ten-year data set includes all storms with intensities from the best track indicating they reached or surpassed a maximum sustained wind speed of 120 kts. Storms that approached near 120 kts, say 110 kt was also included. The digital data was then accessed and processed using the resampling method of Poe (1990). The special processing with “storm centered” imagery permitted more details within the storm center to be extracted on a more reliable basis.

The data set clearly shows that more than 50% of all storms that reach 120 kts have multiple eyewalls present during their lifespan. The percent of storms with concentric eyewalls varies by basin, with the western Pacific the highest and the southern Hemisphere the lowest. These numbers make generic sense when one considers the frequency of long lasting, intense storms within the western Pacific (WPAC) basin. WPAC storms can often reach super typhoon status (> 130kt) and maintain intense state for days.

The passive microwave data set suffers the obvious limitations of temporal coverage from a polar orbiter sensor. Access to multiple SSM/Is partially mitigates this issue, but the % frequency derived here is likely a minimum number, which may go higher with higher sampling information. PMW data from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and the Advanced Microwave Sounding Unit (AMSU-B) were also studied from the NRL TC web page when available to help bridge the temporal SSM/I gaps.

Data from all three sensors (SSM/I, TMI, AMSU-B) now illustrate the sequence of multiple eyewall formation and the many different configurations and time spans associated with these unique features. Information on eyewall cycles as storms evolve from one to two and back to one eyewall are now being qualitatively used by TC warning centers around the globe. Examples from select storms in several basins will illustrate these remarkable natural phenomena.