Saharan air layer interaction with Hurricane Claudette (2003)
Rothman, Gabriel Samuel
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It is well understood that the Saharan air layer (SAL) influences the variability and the intensity of tropical cyclones and African easterly waves in the Atlantic Ocean Basin. The radiative and conductive properties of the mineral dust are thought to contribute to warming within the SAL, creating an anomalous baroclinic zone in the Tropical North Atlantic. Environmental baroclinic instability is a mechanism by which available potential energy is converted to eddy kinetic energy, leading to wave growth. However, this same baroclinic mechanism, along with the dry properties of the SAL could also lead to asymmetries in tropical cyclones, limiting tropical cyclone intensity. Claudette and its formative wave which occurred during the 2003 Atlantic Basin hurricane season propagated along the southern boundary of a broad but moderate SAL outbreak throughout its evolution. First, it was found that the presence of Saharan dust was well correlated to heating in the environment. Thus, the wave growth potential was diagnosed using the Chamey-Stem condition for combined barotropic-baroclinic instability. It was found that the Chamey-Stem condition was satisfied for the formative tropical wave throughout much of its evolution, corresponding to the mostly progressive wave growth which occurred over time. The lifting potential due to the SAL boundaries and the mid-level easterly jet were then investigated, and the current research suggests that there is a dual-celled set of circulations, forced by the SAL boundaries but modified by the mid-level easterly jet. The SAL continued to remain intact in the storm environment almost throughout Claudette's entire evolution, after tropical cyclogenesis and through its entire track into the Western Gulf of Mexico. Rapid intensification in the last 15 hours prior to landfall was concurrent with the breakdown of Saharan air intrusion that had been persistent in the storm environment. Although the presence of dry air may have limited the intensity of Claudette before the rapid intensification period, external factors from the SAL, such as oceanic hurricane heat potential, may have also played a role in Claudette's intensity fluctuations.