Mechanisms of three types of roll structures in the typhoon boundary layer

The boundary layer is crucial to understanding the potential damage caused by typhoons, as the organized motions within it significantly affect the transfer of momentum, heat, moisture, and other substances. In the present study, three types of roll structures are identified in the typhoon boundary layer. The Type-A roll structure, caused by shear instability (0 < Ri < 0.25), has two modes. Mode-I is associated with the tangential wind, located slightly inside the radius of maximum wind and oriented vertically upward, with the roll axis nearly parallel to the tangential wind. Mode-II is associated with the radial wind, located at the radius of maximum wind and tilted upward, with the roll axis nearly parallel to the radial wind. Furthermore, at the same radius, Mode-I dominates at a higher height, while Mode-II dominates at a lower height. The dominant mode shifts from Mode-I to Mode-II as typhoon intensity increases, as more intense typhoons tend to exhibit annular characteristics with a more tilted eyewall structure. The Type-B roll structure, caused by inertial instability, is reflected in the flat region of total angular momentum and is located just outside the radius of maximum wind, with the roll axis nearly parallel to the radial wind. The Type-C roll structure, caused by inflection point instability, is located outside the Type-B roll structure, with the roll axis nearly parallel to the tangential wind. An accurate understanding of the mechanisms of these three types of roll structures plays a significant role in research on typhoon structure, numerical prediction models, disaster assessment models, and wave and storm surge dynamics.