Concentric Eyewall Formation in Typhoon Sinlaku (2008) – Part II: Further Examination of Axisymmetric and Asymmetric Processes

In part I a new model was proposed for understanding secondary eyewall formation (SEF). In part II we present diagnostic analyses and interpretations to assess this model. Prior to SEF, eddy activities are identified to emanate from the primary eyewall and propagate to larger radii. Eddy activities of different wavenumbers are further investigated to demonstrate the impact of vortex Rossby waves. Budget calculations of the azimuthally-averaged tangential wind field suggest that the formation of the secondary tangential wind maximum in the boundary layer is achieved mainly via the strong radial influx of mean absolute vorticity that more than offsets the loss of tangential momentum due to surface friction. Although the radial influx of asymmetric absolute vorticity in the form of the banded vorticity structures is found to play a positive role in amplifying the mean tangential wind in the boundary layer of the SEF region, this eddy effect is subdominant to the mean influx terms.

To further assess this new SEF model, a diagnosis of the Sawyer-Eliassen balance equation is carried out to understand the balanced/unbalanced responses of the secondary circulation to heat and momentum forcings from the primary eyewall, spiral rainbands and small-scale convective cells outside the eyewall. Other ensemble members and data-denial experiments are examined to evaluate the robustness and uncertainty of the results.

This sequence of studies aims to provide new physical insights into SEF, as well as to improve the design of additional idealized experiments.