Heterogeneous ice nucleation in the WRF-Chem 3.9.1.1 model and its influence on cloud response to volcanic aerosols
Résumé
Heterogeneous ice formation on aerosols is the main primary cloud ice formation process above temperatures of -38°C, and as
a consequence it plays a major role in the formation of mixed-phase and ice clouds. Improving our understanding of ice
processes could help better constrain the radiative forcing of cloud aerosol interactions, which remains a major source of
uncertainty in climate projections. Despite their importance, most atmospheric models do not represent aerosol-cloud ice
processes explicitly.
We extend in the WRF-Chem 3.9.1 model a recent parameterization of deposition-mode ice nucleation to also include
immersion-mode nucleation, based on the classical nucleation theory (CNT) description. We also couple this parameterization
with the aerosol-liquid cloud parameterization of Abdul Razzak and Ghan already included in WRF-Chem 3.9.1. This allows us to
model the effect of aerosols on mixed-phase and ice clouds. We use volcanic eruptions as case studies, especially focusing on
the 2014/2015 Holuhraun/Bárðarbunga eruption in Iceland. Specifically, we investigate how volcanic aerosols influence
modeled cloud microphysical properties with and without the explicit ice nucleation parameterization, comparing the model
against MODIS satellite observations. We also investigate the effect of these processes on the cloud response in terms of
optical properties, radiative fluxes, and precipitation during the eruptions