Abstract
This thesis examines the prevalence and characteristics of small-scale coherent structures along the inner edge of the tropical cyclone (TC) eyewall in the lower to mid-troposphere, which appear as filamentary, cellular, lobed, or scalloped structures in Doppler radar reflectivity with approximately regular azimuthal spacings of only a few kilometers. An examination of ground-based radar reflectivity in seventeen North Atlantic and Northwest Pacific TCs reveals that these wave-like coherent structures appear predominantly to the left of the environmental vertical wind shear vector, in the offshore flow, and/or while the eyewall exhibits low-wavenumber asymmetries. The spacing between coherent structures is also somewhat correlated with radial distance from the TC center. Radar observations of rapidly intensifying Hurricanes Irma (2017), Michael (2018), and Dorian (2019) suggest a characteristic evolution in which the coherent structures emerge a few hours after the onset of barotropic instability and right before peak intensity. Geostationary Lightning Mapper data shows that the emergence of these structures in these three cases was accompanied by outbreaks of eyewall lightning. Mesoscale numerical simulations of an idealized TC with varying horizontal and vertical resolution were analyzed to gain insight into the physical mechanisms responsible for the coherent structures. No discernible differences in short-term evolution were found between storms in which the structures were and were not present. Three-dimensional and azimuthal-mean kinematic fields indicate that the coherent structures are likely generated by dynamical instability in the supergradient “corner-flow” region of the TC boundary layer wherein vertical vorticity is concentrated near the surface, stretched by the frictional updraft, and tilted into the horizontal as the flow readjusts to gradient balance aloft.