[Eoas-seminar] Meteorology PhD defense for Levi Cowan, November 1, 2:00 PM, Love 353

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Fri Oct 18 15:37:43 EDT 2019

Meteorology Seminar

Levi Cowan

PhD Meteorology Candidate

Major Professor:  Dr. Robert Hart

Date: November 1, 2019                        Time: 2:00 PM

Location: Werner A. Baum Seminar Room (353 Love Building)
(Please join us for refreshments served outside room 353 Love @ 1:30 PM)


Interactions of Atlantic tropical cyclones (TCs) with upper tropospheric flow are identified in 37 years of ERA-Interim reanalysis data and analyzed from multiple perspectives. Upper tropospheric troughs are identified in a more comprehensive way than past methodologies, targeting features on the dynamic tropopause to reduce exclusivity of feature selection and sensitivity to the background environment. As a unique way of characterizing and subsetting environmental flow, upper tropospheric jets are identified in 200-hPa wind fields within 3000 km of TCs using a robust, objective algorithm. The climatology of the resulting dataset of jet axes is explored through various means, including an objective clustering technique, which yielded seven statistically distinct groups of jets associated with recognizable flow patterns near TCs. The mean impact of TC outflow on adjacent jets is also quantified, with along-jet acceleration downstream of the TC found to be nearly ubiquitous across the Atlantic basin, though modulated strongly by the geographically varying background state.

The influence of nearby upper tropospheric troughs and jets on TC intensity is also assessed through a variety of approaches. In order to minimize systematic sampling biases when quantifying this impact, a spatially varying climatology of TC intensification rate is developed using a second-order, generalized least squares regression model, allowing TC intensity responses to external forcing to be evaluated as departures from their expected value. Both troughs and jets are found to be net negative influences on TC intensity, on average, primarily due to increasing vertical shear with proximity to the vortex. Differences between rapidly intensifying (RI) and rapidly weakening (RW) cases during TC-trough-jet interactions depend not only on shear, but on dynamic forcing imposed by baroclinic processes and eddy momentum fluxes that can counter the influence of shear. Intensifying cases are primarily associated with jets that approach the poleward side of the TC and possess entrance regions that amplify over time, increasing dynamic forcing for ascent near the TC core while maintaining enough distance to prevent shear from overwhelming those effects.

This study expands the set of tools for analyzing TC interactions with upper tropospheric flow by improving trough identification and introducing a new perspective through the use of jets. Jets afford greater specificity in describing environmental flow, and allow unique methods of quantifying its impact on TCs. A close relationship is found between jet proximity and vertical shear, as well as jet acceleration and dynamically-forced ascent. Some measures of jet entrance region orientation correlate with the relative magnitude of these influences. Prior research has tended to evaluate such influences individually or relied on case studies to elucidate their collective impact on a single storm. This body of work seeks to illuminate relationships between TCs and upper tropospheric flow that are robust across large samples of TCs and storm environments, utilizing novel approaches such as the jet perspective to extract previously unquantified information.

Shel McGuire
Florida State University
Academic Program Specialist
Department of Earth, Ocean, & Atmospheric Science
1017 Academic Way, 410 Love Building (Meteorology)
Tallahassee, FL 32306

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