Very-large-scale motions in typhoons and one possible original mechanism

Abstract

Intensity typhoons are extreme wind environments encountered by coast and offshore structures, and vortex of different scales have different action mechanisms on the structure. A mature typhoon can be divided into three main parts, and from inner to outer, they are the eye, the eyewall, and the outer region. The spectrum characteristics may be different for the three parts of a typhoon. While nowadays the wind velocity spectra of typhoons in engineering applications are the same as those for the monsoon. In the current study, we conducted field measurements to analyze the characteristics of the wind velocity spectrum of the eye, eyewall, and outer region of a typhoon. For the outer and eye regions, the characteristic of the wind velocity spectrum is the same as the monsoon. For the eyewall region, a −1 subrange (k−1, k is wavenumber) occurs at the wind velocity spectrum, and the vortex structure size even reaches 32.1δ, which illustrates that very-large-scale motions (VLSMs) exist in this region. This is the first study to investigate VLSMs in a typhoon. Furthermore, we found that the −1 subrange of the wind velocity spectrum has a positive correlation with Reynolds shear stress. The phenomena established in the present study were validated by an established theory that states that flow should satisfy two conditions simultaneously for a −1 subrange: strong resonance between average and turbulent motions and a large wind velocity gradient. It may be that this theory is a possible original mechanism for VLSMs. Finally, we confirmed this mechanism with a flat plate shear flow field-measured test.