Aerosol effects on cirrus through ice nucleation in the Community Atmosphere Model CAM5 with a statistical cirrus scheme



A statistical cirrus scheme that tracks ice saturation ratio in the clear-sky and cloudy portion of a grid box separately has been implemented into the Community Atmosphere Model CAM5 to provide a consistent treatment of ice nucleation and cloud formation. Simulated ice supersaturation and ice crystal number concentrations strongly depend on the number concentrations of heterogeneous ice nuclei (IN), subgrid temperature formulas, and the number concentration of sulfate particles participating in homogeneous freezing, while simulated ice water content is insensitive to these perturbations. Allowing 1–10% of dust particles to serve as heterogeneous IN is found to produce ice supersaturation in better agreement with observations. Introducing a subgrid temperature perturbation based on long-term aircraft observations produces a better hemispheric contrast in ice supersaturation compared to observations. Heterogeneous IN from dust particles alter the net radiative fluxes at the top of atmosphere (TOA) (−0.24 to −1.59 W m−2) with a significant clear-sky longwave component (0.01 to −0.55 W m−2). Different cirrus treatments significantly perturb the net TOA anthropogenic aerosol forcing from −1.21 W m−2 to −1.54 W m−2, with a standard deviation of 0.10 W m−2. Aerosol effects on cirrus exert an even larger impact on the atmospheric component of the radiative fluxes (2 or 3 times the changes in the TOA radiative fluxes) and therefore through the fast atmosphere response on the hydrological cycle. This points to the urgent need to quantify aerosol effects on cirrus through ice nucleation and how these further affect the hydrological cycle.