  Planetary-scale waves at the Venusian cloud-top cause periodic variations in
both winds and ultraviolet (UV) brightness. While the wave candidates are the
4-day Kelvin wave and 5-day Rossby wave with zonal wavenumber 1, their temporal
evolutions are poorly understood. Here we conducted a time series analysis of
the 365-nm brightness and cloud-tracking wind variations, obtained by the UV
Imager onboard the Japanese Venus Climate Orbiter Akatsuki from June to October
2017, revealing a dramatic evolution of planetary-scale waves and corresponding
changes in planetary-scale UV features. We identified a prominent 5-day
periodicity in both the winds and brightness variations, whose phase velocities
were slower than the dayside mean zonal winds (or the super-rotation) by >35 m
s$^{-1}$. The reconstructed planetary-scale vortices were nearly equatorially
symmetric and centered at ~35{\deg} latitude in both hemispheres, which
indicated that they were part of a Rossby wave. The amplitude of winds
variation associated with the observed Rossby wave packet were amplified
gradually over ~20 days and attenuated over ~50 days. Following the formation
of the Rossby wave vortices, brightness variation emerges to form rippling
white cloud belts in the 45{\deg}-60{\deg} latitudes of both hemispheres.
~3.8-day periodic signals were observed in the zonal wind and brightness
variations in the equatorial region before the Rossby wave amplification.
Although the amplitude and significance of the 3.8-day mode were relatively low
in the observation season, this feature is consistent with a Kelvin wave, which
may be the cause of the dark clusters in the equatorial region.
