  Growing observational evidence has suggested active meteorology in
atmospheres of brown dwarfs (BDs) and directly imaged extrasolar giant planets
(EGPs). In particular, a number of surveys have shown that near-IR brightness
variability is common among L and T dwarfs. Despite initial understandings of
atmospheric dynamics which is the major cause of the variability by previous
studies, the detailed mechanism of variability remains elusive, and we need to
seek a natural, self-consistent mechanism. Clouds are important in shaping the
thermal structure and spectral properties of these atmospheres via large
opacity, and we expect the same for inducing atmospheric variability. In this
work, using a time-dependent one-dimensional model that incorporates a
self-consistent coupling between the thermal structure, convective mixing,
cloud radiative heating/cooling and condensation/evaporation of clouds, we show
that radiative cloud feedback can drive spontaneous atmospheric variability in
both temperature and cloud structure in conditions appropriate for BDs and
directly imaged EGPs. The typical periods of variability are one to tens of
hours with typical amplitude of the variability up to hundreds of Kelvins in
effective temperature. The existence of variability is robust over a wide range
of parameter space, but the detailed evolution of variability is sensitive to
model parameters. Our novel, self-consistent mechanism has important
implications for the observed flux variability of BDs and directly imaged EGPs,
especially those evolve in short timescales. It is also a promising mechanism
for cloud breaking, which has been proposed to explain the L/T transition of
brown dwarfs.
