Cirrus clouds have a net warming effect on the atmosphere and cover about 30% of the Earth’s area. Aerosol particles initiate ice formation in the upper troposphere through modes of action that include homogeneous freezing of solution droplets, heterogeneous nucleation on solid particles immersed in a solution, and deposition nucleation of vapor onto solid particles. However, the efficacy with which particles act to form cirrus particles in a model depends on the representation of updrafts. Here, for the first time we extend the NCAR CAM5 climate model using a representation of updrafts based on observations of gravity waves, and follow ice formation/evaporation during both updrafts and downdrafts. Various versions of the model are compared with observations. However, there is no single representation that fits all observations. We examine the possible change in ice number concentration from secondary organic aerosols (SOA) whose concentrations are expected to increase in the future as a result of climate change. We contrast the radiative effects associated with SOA with that of aerosols originating from surface sources of fossil fuel and biomass burning and from aircraft particles that have previously formed ice in contrails. Results show that if 30% of SOA acts as a heterogeneous ice nuclei, it exerts a large positive radiative effect (0.6 Wm-2) while that fossil fuel and biomass burning soot aerosols exert a negative radiative forcing of about − 0.15±0.02 Wm-2 and aircraft aerosols exert a forcing of -0.20±0.06 Wm−2. These estimates of forcing depend on the fraction of dust particles that act as heterogeneous ice nuclei as well as the treatment of updraft velocities.

Climate impact of secondary organic aerosols on cirrus clouds

255th National Meeting (2018)

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