Assessing aerosol indirect effect on ice clouds in CAM5: the role of updraft velocity and ice nuclei spectra

Ice clouds play an important role in regulating the Earth's radiative budget. Aerosols can act as the ice nuclei for ice crystal formation, influencing the microphysical properties of ice clouds. However, the aerosol indirect effect on ice clouds (ice AIE) hasn't been well quantified. In this study, we performed sets of global climate model simulations to estimate the ice AIE and to investigate its sensitivity to sub-grid updraft velocity and ice nuclei (IN) spectra. A new approach is developed to separate the impact of aerosols on warm and cold clouds by using the prescribed-aerosol capability in the Community Atmospheric Model (CAM5). The same aerosol fields are used for the aerosol activation in warm clouds; while aerosols for the ice nucleation in ice clouds are prescribed to either present-day or preindustrial values.. Therefore, the difference in the simulated climatology indicates the effect of anthropogenic aerosols only on ice clouds.

It is found that removing the upper limit of updraft velocity (which is used in the standard CAM5) can lead to more than twofold increase in the estimated long-wave cloud forcing (LWCF) change between PD and PI, although the change in the net cloud forcing is small because of compensation by the change in shortwave cloud forcing (SWCF). Including the effect of pre-existing ice crystals on ice nucleation reduces the change in LWCF significantly. In comparison, the effect of heterogeneous ice nuclei spectra is smaller, although the perturbations in the LWCF and SWCF between PD and PI are still non-negligible.