  We investigate the interplay between large-scale patterns, so-called
superstructures, in the fluctuation fields of temperature $\theta$ and vertical
velocity $w$ in turbulent Rayleigh-B\'{e}nard convection at large aspect
ratios. Earlier studies suggested that velocity superstructures were smaller
than their thermal counterparts in the center of the domain. However, a
scale-by-scale analysis of the correlation between the two fields employing the
linear coherence spectrum reveals that superstructures of the same size exist
in both fields, which are almost perfectly correlated. The issue is further
clarified by the observation that in contrast to the temperature, and unlike
assumed previously, superstructures in the vertical velocity field do not
result in a peak in the power spectrum of $w$. The origin of this difference is
traced back to the production terms of the $\theta$- and $w$-variance. These
results are confirmed for a range of Rayleigh numbers $Ra = 10^5$--$10^9$, the
superstructure size is seen to increase monotonically with $Ra$. Furthermore,
the scale distribution of particularly the temperature fluctuations is
pronouncedly bimodal. In addition to the large-scale peak caused by the
superstructures, there exists a strong small-scale peak. This `inner peak' is
most intense at a distance of $\delta_\theta$ from the wall and associated with
structures of size $\approx 10 \delta_\theta$, where $\delta_\theta$ is the
thermal boundary layer thickness. Finally, based on the vertical coherence
relative to a reference height of $\delta_\theta$, a self-similar structure is
identified in the velocity field (vertical and horizontal components) but not
in the temperature.
