  Local-box hydrodynamical model atmospheres provide statistical information
about the spatial dependence, as well as temporal evolution, of a star's
emergent radiation field. Here, we consider late-type stellar atmospheres for
which temporal changes of the radiative output are primarily related to
convective (granular) surface flows. We derived relations for evaluating the
granulation-induced, disk-integrated thus observable fluctuations of the
stellar brightness and location of the photocenter from radiation intensities
available from a local model. Apart from their application in the context of
hydrodynamical stellar atmospheres, these formulae provide some broader insight
into the nature of the fluctuations under consideration. Brightness
fluctuations scale inversely proportional to the square root of the number of
convective cells (the statistically independently radiating surface elements)
present on the stellar surface and increase with more pronounced
limb-darkening. Fluctuations of the stellar photocentric position do not depend
on the number of cells and are largely insensitive to the degree of
limb-darkening. They amount to a small fraction of the typical cell size, and
can become a limiting factor for high-precision astrometry in the case of
extreme giants. The temporal brightness and positional fluctuations are
statistically uncorrelated but closely related in magnitude.
