  Water clouds are expected to form on Y dwarfs and giant planets with
equilibrium temperatures near or below that of Earth, drastically altering
their atmospheric compositions and their albedos and thermal emission spectra.
Here we use the 1D Community Aerosol and Radiation Model for Atmospheres
(CARMA) to investigate the microphysics of water clouds on cool substellar
worlds to constrain their typical particle sizes and vertical extent, taking
into consideration nucleation and condensation, which have not been considered
in detail for water clouds in H/He atmospheres. We compute a small grid of Y
dwarf and temperate giant exoplanet atmosphere models with water clouds forming
through homogeneous nucleation and heterogeneous nucleation on cloud
condensation nuclei composed of meteoritic dust, organic photochemical hazes,
and upwelled potassium chloride cloud particles. We present comparisons with
the Ackerman & Marley parameterization of cloud physics to extract the optimal
sedimentation efficiency parameter (f$_{sed}$) using Virga. We find that no
Virga model replicates the CARMA water clouds exactly and that a transition in
f$_{sed}$ occurs from the base of the cloud to the cloud top. Furthermore, we
generate simulated thermal emission and geometric albedo spectra and find
large, wavelength-dependent differences between the CARMA and Virga models,
with different gas absorption bands reacting differently to the different cloud
distributions and particularly large differences in the M band. Therefore,
constraining the vertically-dependent properties of water clouds will be
essential to estimating the gas abundances in these atmospheres.
