Based on direct numerical simulations and symmetry arguments, we show that
the large-scale Fourier modes are useful tools to describe the flow structures
and dynamics of flow reversals in Rayleigh-B\'enard convection (RBC). We
observe that during the reversals, the amplitude of one of the large-scale
modes vanishes, while another mode rises sharply, very similar to the
"cessation-led" reversals observed earlier in experiments and numerical
simulations. We find anomalous fluctuations in the Nusselt number during the
reversals. Using the structures of the RBC equations in the Fourier space, we
deduce two symmetry transformations that leave the equations invariant. These
symmetry transformations help us in identifying the reversing and non-reversing
Fourier modes.Comment: 4 pages, 3 figure
