Activity 2: Ice Cloud Formation and Atmospheric Impacts

Small changes in cloud properties can cause a large change in radiative forcing, comparable in magnitude to the radiative forcing caused by an increase in anthropogenic greenhouse gases. Currently, clouds provide some of the greatest uncertainties in predictions of climate change [Lohmann and Feichter, 2005; IPCC, 2007]. This is in large part because the properties of clouds and their formation processes and responses to small changes are poorly understood, especially ice and mixed-phase clouds understood [Cantrell and Heymsfield, 2005; Hegg and Baker, 2009].  The ability of aerosol to nucleate liquid water, i.e. to act as cloud condensation nuclei (CCN), is much better understood.


Ice particles can form in the atmosphere when ice homogeneously nucleates in aqueous aerosol particles or heterogeneously nucleates on solid particles.  Only a very small fraction of atmospheric particles cause heterogeneous ice nucleation, and this subset of particles are referred to as ice nuclei (IN) [DeMott, 2002]. Although homogeneous nucleation in aqueous particles is now relatively well understood [Hegg and Baker, 2009; Koop et al., 2000], heterogeneous ice nucleation on solid particles remains highly uncertain, which translates to large uncertainties in models used to predict radiative forcing [Lohmann and Feichter, 2005]. To improve climate predictions, concentrations of IN in different environments need to be determined and the types of atmospheric particles that act as good IN need to be identified. This information can then be used to constrain and validate ice nucleation and ice cloud formation parameterizations in atmospheric models.