In this work, we investigate the dynamics of wind turbine tip-vortex breakdown in a conventionally neutral atmospheric boundary layer (ABL). To this end, high-resolution data are collected from large-eddy simulations of a
wind turbine operating within a neutral ABL and studied by means of proper orthogonal decomposition (POD)
and Fourier analysis. The high resolution of the generated data in both space and time allows us to gain insight
into the tip-vortex breakdown mechanisms by (i) capturing the energy modes of the coherent structures, (ii) studying their contribution to the tip-vortex breakdown through their power spectra functions and mean kinetic energy
(MKE) flux, and (iii) analysing the growth rate of each contributing perturbation frequency along tip vortices.
Our analysis shows that under a fully turbulent scenario, the growth rate of perturbations along the tip vortices is
largest for low wave numbers, i.e. long-wave perturbations. Additionally, the MKE flux reaches its highest value
at two diameters downstream of the rotor plane, a behaviour that can be attributed to the coexistence of multiple
interacting POD modes, with the streamwise vortex roller mode being the primary contributor to the total MKE
flux budget, contributing approximately 24%. Finally, comparisons with a laminar, uniform flow scenario subject
to a single-frequency perturbation highlight the differences between the two ambient flow conditions. In the nonturbulent, uniform flow scenario, the growth rate attains its maximum value at a wave number corresponding to
the out-of-phase mutual-inductance mechanism, whereas the MKE flux exhibits local minima and maxima along
the wake and at different downstream locations depending on the perturbation frequency. Our analyses suggest
that the breakdown of the wind turbine tip vortices under a fully turbulent neutral ABL inflow is due to complex
interactions across a range of excitation frequencies, in which the mutual-inductance instability may not be the
dominant one.