Tropical cyclone multiple eyewall configurations

Passive microwave satellite imagery enables tropical cyclone (TC) satellite analysts to view storm structure infrequently detected in coincident visible and/or infrared imagery. Upper-level clouds often shield inner-storm structure in geostationary vis/IR imagery that provides the backbone to global TC monitoring. A twelve (12) year Special Sensor Microwave/Imager (SSM/I) digital data set (1993-2005) has been created to systematically study intense tropical cyclones that may undergo the formation of multiple eyewalls. In addition, the Naval Research Laboratory's (NRL) TC web page adds a wealth of other microwave sensors covering the time period 1997-Present.

The twelve-year data set includes all storms with intensities greater than or equal to 120 kts as denoted in the best track files created by the National Hurricane Center (Atlantic and East Pacific) and the Joint Typhoon Warning Center (JTWC) for the rest of the TC basins. Storms that approached 120 kts, ~110 kt were also included. The resampling method of Poe (1990) was used for processing storm-centered SSM/I 85 GHz that enabled enhanced eyewall structure analysis.

The data set clearly shows that a large percentage of all storms reaching 120 kts have multiple eyewalls sometime during their lifespan. The percent of storms with concentric eyewalls varies by basin, with the western Pacific by far the highest due to many long lasting intense systems and favorable environment factors. Although the Atlantic basin typically trails the WPAC in Category 5 systems, the 2005 season included three (Emily, Katrina, and Rita), with two exhibiting distinct multiple eyewall configurations.

The passive microwave data set suffers from obvious limitations of temporal coverage from a polar orbiter sensor. The NRL TC web page has recently benefited from the inclusion of new sensors that include the CORIOLIS WindSat polarimetric radiometer and the Special Sensor Microwave Imager Sounder (SSMIS). Both sensors contribute to more robust TC sampling due in part to their orbital paths and in particular to the SSMIS's 1700-km swath, 300-km wider than the heritage SSMI.

Data from five passive microwave imagers and a sounder (SSM/I, TMI, AMSR-E, WindSat, SSMIS and AMSU-B) were incorporated in this study to monitor multiple eyewall formation and eyewall replacement cycles. Specific eyewall cycle scenarios will be documented and characterized regarding the specific sequence of events leading up to and through eyewall structural changes.