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Comparative Analysis of Supercell Environment in Hurricanes Harvey and Irma

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Comparative Analysis of Supercell Environment in Hurricanes Harvey and Irma Comparative Analysis of Supercell Environment in Hurricanes Harvey and Irma IVY MACDANIEL Affilitation/Fall 2020: Austin Peay State University [email protected] Significant Opportunities in Atmospheric Research and Science Science Mentor: Christopher Rozoff, Dereka Carroll, and Jonathan Vigh Writing/Communication Mentor: Chris Davis Coach: Kristen Aponte July 31, 2020 Abstract Although severe weather forecasting has improved over time, small rotating thunderstorms, some- times called miniature supercells, often pose challenges with tracking and issuing tornado warnings. Being smaller than typical supercells, those storms are more difficult to discern on radar, especially at greater distances. Those storms are also found in tornado-producing tropical cyclones, and tornadoes produced by tropical cyclones, while they tend to be weak, can still pose a threat to lives and properties in the Southeast U.S. The most likely tropical cyclone to produce tornadoes is a formerly intense tropical cyclone that weakens at landfall. Tornadoes within Hurricanes Harvey and Irma from the 2017 Atlantic season were examined in this study. In Harvey’s case, the majority of tornadoes formed inland from southeast Texas to Tennessee. Mesoanalysis showed moderately high convective available potential en- ergy (CAPE) and more moderate vertical wind shear than was found in Irma. Irma, however, produced the majority of tornadoes near the shoreline. Environmental analysis showed higher CAPE over water than over the Florida peninsula during Irma’s landfall. Yet, the shear was higher over land. However, supercells met their demise over the land when the environment became unfavorable. While it was still unclear why the supercells died over land, a key observation showed that there was a significant decrease of CAPE over land compared to offshore despite favorable shear. However, supercells in post-tropical Harvey formed and sustained themselves far inland while being driven by buoyancy. Thus, additional investigation in the future would be necessary. This work was performed under the auspices of the Significant Opportunities in Atmospheric Research and Science Program. SOARS is managed by the University Corporation for Atmospheric Research and is funded by the Na- tional Science Foundation, the National Center for Atmospheric Research, the National Oceanic and Atmospheric Administration, and the University of Colorado at Boulder. 1 Introduction Over the decades, the severe weather research has helped save lives thanks to advanced technology and improved warnings. However, the research has been based heavily based on classic Great Plains supercells with a limited scope for miniature supercells regardless whether they were part of tropical cyclones (TCs) or not. Yet, miniature supercells, especially those in TCs, should not be dismissed since they are capable of producing tornadoes that pose threat to lives and properties. The occurrence of tornadoes within tropical cyclones also complicates the warning process because of the presence of multiple hazards at the same time. Early research into TC tornadoes from hurricanes and typhoons (Novlan and Gray, 1974) revealed key fundamentals about the location and general occurrence of TC tornadoes within TCs. The Novlan and Gray study examined US hurricanes from 1948 to 1972 and Japanese typhoons from 1950 to 1971. Tornadoes were found most likely to occur during landfalls amid the increasing vertical wind shear as observed in some hurricanes and typhoons. Here, vertical wind shear refers to the variation of the horizontal wind with height. Overall, TC tornadoes were found to be weaker than their Great Plains counterparts. However, 10% of TC-related deaths from that period were attributed to tornadoes. In observed TCs, tornadoes usually form in the front right sector of TCs (oriented relative to the move- ment of the TC) (Novlan and Gray, 1974). The most prolific tornado-producing TCs are dissipating, but formerly intense TCs. They have on average three times the filling rate, (as measured by a change in the minimum central sea-level pressure of roughly 30 mb/12 hr), compared with non-tornadic TCs that weak- ened at a more moderate (10 mb/12 hr). The shear was also much higher on average for tornado-producing TCs than non-tornadic TCs, with the average vertical shear of tornado-producing environments being greater than 40 kt from surface to 1524 m (5,000 feet). Non-tornadic TCs, however, tend to show minimal to no shear for same layer of atmosphere. It also suggested that dry air intrusion plays a role in TC tornadoes as observed in Hurricanes Jeanne and Ivan (Baker et al., 2008). Ivan experienced a dry air intrusion and produced several tornadoes. Jeanne, however, did not experience a dry air intrusion and did not produce any tornadoes. Research has shown that there are distinctive differences between TC and classical Great Plains en- vironments (Novlan and Gray, 1974). Though most of the Great Plains and TC tornadoes are produced by supercell storms, long lived thunderstorms with rotating updrafts, the environments of tornadic storms within TCs are different from the tornadic Great Plains storms. Usually, the Great Plains environment fea- tures high convective available potential energy (CAPE) and moderate vertical shear over a deep layer. TC environments may have lower overall CAPE but strong vertical wind shear at low altitudes. The majority of TC tornadoes were spawned by supercells where low level shear was usually generated in the rainbands during the landfall as a result of surface friction (Green et al., 2011). Surface friction dramatically slows the winds near the ground, and also changes the flow direction inward toward low pressure, whereas winds 500-1000 m above the ground are less affected. Therefore, vertical wind shear in the very lowest levels of the atmosphere increases significantly upon landfall. Approximately 79% of all tornadoes produced by TCs were produced by right moving supercells (Ed- wards et al., 2012), meaning that they moved to the right of the vector wind shear averaged over a deep layer. Miniature supercells, characteristic of most TC tornado-producing storms, are smaller than classi- cal supercells over the Great Plains. Average cloud top heights are 24,000 to 32,000 feet for miniature storms (NWS Louisville, n.d.). Classical supercells are known to produce intense severe weather usually have 40-50,000 feet cloud tops (up to 16 km). While the tornadoes produced in tropical cyclones are weak, roughly 7% are rated at EF-2 or greater, where EF refers to the enhanced Fujita scale and ranges from 0 (minimal) to 5 (catastrophic) damage (Edwards et al., 2012). Tools for non-TC supercells can be used for their TC counterparts (Baker et al., 2008), and these include parameters derived from atmospheric soundings and surface meso-analyses that depict fine-scale structures in the surface winds. Common tool for severe weather forecasting is sounding which can be used in TCs. A couple exampes of those forecasting tools 1 are sounding and mesoanalysis. According to reports by from the National Hurricane Center, Category 4 Hurricane Harvey produced 52 tornadoes from Texas to Tennessee (Blake and Zelinsky, 2018). Irma, on the other hand, produced 25 tornadoes along the Florida and South Carolina coasts (Cangialosi et al., 2018). As a Cape Verde hurricane, Irma was a Category 5 storm that tracked through Caribbean islands and along the Florida coast. Both Harvey and Irma were selected from a pool of candidate TCs. The whole scope of the project this summer and onward was to study miniature supercells and their environments within tropical cyclones and outside tropical cyclones. Some key objectives were examining different tropical cyclones to examine environment the supercells were in, taking detailed observations of supercells, and examining nontropical environments. It could be expanded to study how miniature supercells behave in tropical cyclones and in various non-tropical scenarios. Two possible expansions were miniature supercells with history of significant severe (EF-2+ tornadoes, 75+ mph/121 kph, or 2+”/5 cm hailstones) and features of supercells in various sectors of tropical cyclones at different stages. Therefore, the findings described in the paper thus serve as a preliminary study that can be expanded on through future studies. 2 Methods After investigating different potential TC cases, Harvey and Irma were determined to be best cases because they were prolific tornado producers and also strong TCs. The pool of candidate TCs for this study was major Atlantic hurricanes from 2017 to 2019. Those that did not make landfall in U.S. or Puerto Rico were eliminated. Originally, the cases were Harvey, Irma, Maria, Michael, and Dorian, but they were reduced to Harvey and Irma due to time constraints of the project. Harvey and Irma were prolific tornado producers, but they spawned tornadoes in different regions, primarily inland for Harvey and near the immediate coast for Irma. This difference warranted investigation primarily based on observations. There is no single comprehensive source of observations for tropical cyclone tornadoes. Therefore, I had to piece together information from various sources. The Storm Prediction Center (SPC) Severe Weather Event Archive and Preliminary Reports contained the information needed to identify pinpoint the time and location of each tornado. This was useful for identifying which upper air soundings could be considered as representative of the environment of the
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