  We present a new implementation of the hydrological cycle scheme into a
general circulation model of the Martian atmosphere. The model includes a
semi-Lagrangian transport scheme for water vapor and ice, and accounts for
microphysics of phase transitions between them. The hydrological scheme
includes processes of saturation, nucleation, particle growth, sublimation and
sedimentation under the assumption of a variable size distribution. The scheme
has been implemented into the Max Planck Institute Martian general circulation
model (MPI--MGCM) and tested assuming mono- and bimodal log-normal
distributions of ice condensation nuclei. We present a comparison of the
simulated annual variations, horizontal and vertical distributions of water
vapor and ice clouds with the available observations from instruments onboard
Mars orbiters. The accounting for bi-modality of aerosol particle distribution
improves the simulations of the annual hydrological cycle, including predicted
ice clouds mass, opacity, number density, particle radii. The increased number
density and lower nucleation rates brings the simulated cloud opacities closer
to observations. Simulations show a weak effect of the excess of small aerosol
particles on the simulated water vapor distributions.
