[Eoas-seminar] Meteorology PhD Defense for Kyle Ahern, August 8th, 3:30 PM, Love 353

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Wed Jul 24 13:24:53 EDT 2019

Meteorology Seminar

Kyle Ahern

PhD Meteorology Candidate

Title:  Hurricane Boundary Layer Structure During Intensity Change: An Observational and Numerical Analysis

Major Professor:  Dr. Mark Bourassa and Dr. Robert Hart

Date: August 8th, 2019                           Time: 3:30 PM

Location: Werner A. Baum Seminar Room (353 Love Building)
(Please join us for refreshments served outside room 353 Love @ 3:00 PM)


A combination of observational and numerical analyses is used to investigate hurricane boundary layer (BL) structure in the context of intensity change. These analyses refer to hurricanes in three modes of intensity change: intensifying (IN), steady-state (SS), and weakening (WE). Observations from GPS dropwindsondes launched in Atlantic tropical cyclones between 1998 and 2015 are collected for compositing based on intensity change. After quality control and sorting, 3,091 dropwindsondes were composited. In non-intensifying hurricanes, lower-tropospheric tangential winds were stronger than IN storms outside the radius of maximum winds (RMW), which suggests greater inertial stability [cid:image001.png at 01D54223.2CA2E820]  outside the RMW in non-intensifying hurricanes. The BL radial inflow is of similar thickness across the three composites, and all composite groups have an inflow maximum situated at the RMW. Non-intensifying hurricanes are associated with stronger near-surface inflow outside the eyewall, implying more frictionally forced ascent out of the BL at radii outside the RMW. At greater radii, inflow layer [cid:image002.png at 01D54223.2CA2E820]  is relatively low in the WE composite, suggesting locally enhanced subsidence or downdrafts.
High-resolution numerical case studies of Hurricane Irma in 2017 and Hurricane Earl in 2010 are used to check results found in the composite analysis and highlight BL azimuthal structure. The Weather Research and Forecasting Model for Advanced Research (WRF-ARW) is employed for these full-physics simulations. Irma's strong tangential winds were relatively confined to the RMW, leading to weak [cid:image001.png at 01D54223.2CA2E820]  outside the eyewall. Aside from land interactions, Irma tended to steadily intensify, with an inflow maximum at the RMW and BL ascent isolated inward of the RMW. A brief WE period in Irma was associated with shear- and motion-induced asymmetry, whereby drier air was able to descend into the BL inflow near the RMW.
Hurricane Earl had a broader tangential wind field, with high [cid:image001.png at 01D54223.2CA2E820]  outside the eyewall. Earl's strong BL inflow spread over a large radial band, which was associated with widespread BL convergence and shallow ascent outside the RMW. During a prolonged and progressive decay in Earl's intensity, two regions of BL convergence became apparent: one inward of the RMW, and the other well outside the RMW. Descent of low-enthalpy air into the BL near the RMW occurred during Earl's WE phases. Despite shear and storm motion of comparable magnitude to Irma, asymmetries were more pronounced in Earl's BL. Earl's intensity decline was also associated with strong low-level outflow in the upshear-right quadrant, which may have led to structural evolution that promoted an outer region of BL convergence, as well as an inner-eyewall collapse and coincident secondary eyewall formation.

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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