On the Structure and Frequency of Secondary Eyewall Formation in HWRF Simulations of Tropical Cyclone Harvey (2017)

• Other Authors: Di Catarina, Federico (author)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Atmospheric sciences; Meteorology
• Online Access: http://purl.flvc.org/fsu/fd/2018_Fall_DiCatarina_fsu_0071N_14783

Hurricane Harvey (2017) spawned from a westward propagating tropical wave in the Atlantic and then tracked across the southern Caribbean Sea, the Yucatán Peninsula, and lastly over the Gulf of Mexico, where it quickly intensified into a category 4 (on the Saffir-Simpson Scale) tropical cyclone. As a mature hurricane, Harvey underwent an eyewall replacement cycle which led to structural and intensity changes hours before making landfall over the Texas central coast. This study investigates the structure and frequency of secondary eyewalls in 20 forecast simulations of Tropical Cyclone Harvey (2017) as produced by the 2017 operational Hurricane Weather Research and Forecast (HWRF) System. To understand the predictability of secondary eyewalls, the secondary eyewall-producing simulations must be distinguished from the non-secondary eyewall-producing simulations. Thus, a diagnostic method of subjectively detecting secondary eyewalls in forecast data is developed. The diagnostic method identifies specific secondary eyewall traits that have been studied and documented in literature. The results show that most of the simulations (~80%) produce a secondary eyewall. While the all secondary eyewall-producing simulations are initialized over the ocean, the unsuccessful simulations, on the other hand, are initialized over or just west of the Yucatán Peninsula. To study the relationship between land-storm interaction and secondary eyewall simulation, a comparison is made between the successful simulations initialized over the Caribbean Sea (which tracked over the Yucatán Peninsula) and the unsuccessful runs. For both sets of simulations, the effect of land-storm interaction led to temporary storm weakening while over the Yucatán Peninsula. However, this interaction has respectively a greater negative effect on vortex spin-up and organization on those simulations initialized over land. A comparison between the over land evolution of a non-SE producing and aSE-producing simulation is made. The results show that both storms maintain a similar dynamic structure as they move west over the Yucatán Peninsula. However, the SE-producing simulation is in a more favorable thermodynamic environment with higher RH values above the storms and more convective activity near its center when compared to the non-SE producing simulation. Based on these results, it is speculated that deep moist convective feedback processes enhanced by a thermodynamically favorable conditions within and near the Caribbean Sea initialized storms act as an additional intensification mechanism which lacks in the over land initialized storms. The relatively drier air mass and less convective activity associated with the land simulations produces a less favorable environment and limits the intensification rate of these storms over once over water. It is speculated that slower intensification rates inhibit these storms from reaching an adequate TC intensity and structure conducive for SEF before making landfall over Texas/Mexico and weakening. === A Thesis submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science. === Summer Semester 2018. === July 5, 2018. === 2017, Harvey, Hurricane, HWRF, Secondary, Structure === Includes bibliographical references. === Jeffrey Chagnon, Professor Directing Thesis; Robert E. Hart, Committee Member; Philip Sura, Committee Member.