Observations and Characteristics of Small-Scale, Wave-Like Perturbations on the Inner Edge of the Tropical Cyclone Eyewall

One of the most salient problems in the study of the tropical cyclone (TC) boundary layer is the characterization and quantification of coherent eddies believed to transport heat and momentum, endanger aircraft reconnaissance missions, and produce localized swaths of enhanced surface damage. Some observational studies of intense TCs have noted the existence of small-scale asymmetries along the inner edge of the eyewall in Doppler radar reflectivity associated with phenomena known variously as misocyclones, eyewall vorticity maxima, and tornado-scale vortices. These asymmetries appear in the low- to mid-levels as filamentary, cellular, lobed, or scalloped structures with approximately regular azimuthal spacings of only a few km. An examination of ground-based Doppler radar reflectivity in nineteen North Atlantic and Northwest Pacific TCs reveals that these wave-like echoes 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 spacings between echoes is also somewhat correlated with radial distance from the TC center. In addition, radar observations of rapidly intensifying Hurricanes Irma (2017), Michael (2018), and Dorian (2019) suggest a characteristic evolution in which the wave-like echoes emerge toward the end of a period 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. These findings, supported by three-dimensional and azimuthal-mean kinematic fields from high-resolution numerical simulations, indicate that the wave-like echoes are 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.