Abstract
Mixed-phase clouds (MPCs), which consist of both supercooled cloud
droplets and ice crystals, play an important role in the Earth’s
radiative energy budget and hydrological cycle. In particular, the
fraction of ice crystals in MPCs determines their radiative effects,
precipitation formation and lifetime. In order for ice crystals to form
in MPCs, ice-nucleating particles (INPs) are required. However, a
large-scale relationship between INPs and ice initiation in clouds has
yet to be observed. By analyzing satellite observations of the typical
transition temperature (T*) where MPCs become more frequent than liquid
clouds, we constrain the importance of INPs in MPC formation. We find
that over the Arctic and Southern Ocean, snow and sea ice cover
significantly reduces T*. This indicates that the availability of INPs
is essential in controlling cloud phase evolution and that local sources
of INPs in the high-latitudes play a key role in the formation of MPCs.