Modeling the Hydrological Cycle in the Atmosphere of Mars: Influence of a Bimodal Size Distribution of Aerosol Nucleation Particles

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 and tested assuming monomodal and bimodal lognormal 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 on board Mars orbiters. The accounting for bimodality of aerosol particle distribution improves the simulations of the annual hydrological cycle, including predicted ice clouds mass, opacity, number density, and particle radii. The increased number density and lower nucleation rates bring 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. Plain Language Summary Water is a minor but very important component of the Martian atmosphere. It affects the Martian climate, mostly through radiative effects of water clouds and scavenging dust from the atmosphere. It is a sensitive marker of transport processes. Water is the main source of hydrogen in the atmosphere of Mars, and its quantification is necessary for understanding the outgassing. Accurate reproduction of the planet's hydrological cycle is important for predicting areas that may or might sustain life and for predicting consequences of events like collisions with asteroids. We present a new implementation of the hydrological cycle scheme into a Max Planck Institute Martian general circulation model (also known as Martian Atmosphere Observation and Modeling). The key advantage of the new hydrological scheme is in the accurate parameterization of microphysical processes. In particular, it takes into account the fine fraction of airborne dust that serves as nuclei for forming water ice clouds. We found that employing the so-called bimodal dust size distribution significantly improves simulations of the water cycle bringing the results closer to available observations.