Articles | Volume 17, issue 5
Research article
14 Mar 2017
Research article |  | 14 Mar 2017

Impact of vertical wind shear on roll structure in idealized hurricane boundary layers

Shouping Wang and Qingfang Jiang

Abstract. Quasi-two-dimensional roll vortices are frequently observed in hurricane boundary layers. It is believed that this highly coherent structure, likely caused by the inflection-point instability, plays an important role in organizing turbulent transport. Large-eddy simulations are conducted to investigate the impact of wind shear characteristics, such as the shear strength and inflection-point level, on the roll structure in terms of its spectral characteristics and turbulence organization. A mean wind nudging approach is used in the simulations to maintain the specified mean wind shear without directly affecting turbulent motions. Enhancing the radial wind shear expands the roll horizontal scale and strengthens the roll's kinetic energy. Increasing the inflection-point level tends to produce a narrow and sharp peak in the power spectrum at the wavelength consistent with the roll spacing indicated by the instantaneous turbulent fields. The spectral tangential momentum flux, in particular, reaches a strong peak value at the roll wavelength. In contrast, the spectral radial momentum flux obtains its maximum at the wavelength that is usually shorter than the roll's, suggesting that the roll radial momentum transport is less efficient than the tangential because of the quasi-two-dimensionality of the roll structure. The most robust rolls are produced in a simulation with the highest inflection-point level and relatively strong radial wind shear. Based on the spectral analysis, the roll-scale contribution to the turbulent momentum flux can reach 40 % in the middle of the boundary layer.

Short summary
Tropical cyclones are significantly influenced by the lowest part of atmosphere from sea surface to 1 km because strong winds interacting with ocean surface may provide significant energy to the storms. This work aims to understand how different wind intensity and shape may change the energy transfer. A numerical modeling technique is used to simulate the wind and energy transfer. It is found that the change in wind speed with height is particularly important in generating energy transport.
Final-revised paper