Characteristics of Hurricane Lili's Intensity Changes

Babin, A.

LSU Master's Thesis.

Rapid intensity changes of Hurricane Lili in the Gulf of Mexico (GOM) were studied in three distinct ways: climatology, satellite remote sensing, and surface meteorological and oceanographic measurements. Each research methodology provided insight about Hurricane Lili's intensity behavior.

A climatology of rapid and explosive intensifications of hurricanes was developed using minimum central pressure observations for the Atlantic tropical cyclone record. Results showed these events were frequent, especially in the GOM. The majority of intensification events occurred <= 24 h before landfall, with a third to one-half <= 12 h. Lili emerged anomalous as the only hurricane to weaken at a greater rate (+17 hPa over 6 h or +2.83 hPa h-1) than its rapid intensification event rate (-13 hPa over 6 h or -2.16 hPa h-1).

GOES-8 satellite water vapor brightness temperature data were investigated using a -24°C vapor front to delineate a dry air mass west of Lili. Drier air was shown to affect Lili during a rapid weakening phase after the two features were less than 250 km mean or 215 km minimum distance apart. These critical distances are offered as a criterion for a relationship between tropical cyclone weakening and dry air advection. During the time periods where Lili was intensifying or maintaining intensity, this vapor front exhibited more complex signatures of definitive breaks, shape changes, and protrusions. During the rapid weakening phase and when the two features were the greatest distance apart, the vapor front resembled a smooth, strong boundary line. The dry air mass was shown to have the greatest effect upon Lili after the rapid intensification phases.

SST and heat flux calculations illustrated that oceanic contributions to hurricane intensity were more significant during rapid intensification periods. Ocean heat content directly under the center was more vital as seen in Lili's second rapid intensification phase occurring directly over the GOM Loop Current. Although Tropical Storm Isidore cooled surface water temperatures by around 1°C, pre-Isidore water temperature levels were shown to increase latent heat flux values by > 100 W m-2 (+ 40%) over observed values during Lili's weakening phase.

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