摘要：Polarization lidar observations from the interior of Alaska have revealed
unusual supercooled altocumulus cloud conditions in the presence of boreal
forest fire smoke from local and regional fires. At temperatures of about
−15 °C, the lidar data show ice nucleation prior to liquid cloud formation (i.e. below
water saturation), as well as the occasional glaciation of the liquid layer. Thus the
smoke aerosol appears to act as ice nuclei that become activated in updrafts
before the liquid cloud forms, as the concentrated aqueous organic solutions are
diluted sufficiently to allow them to freeze heterogeneously. This haze particle
freezing process is similar to the production of cirrus ice crystals homogeneously at
much colder temperatures. To test this hypothesis, cloud microphysical model
simulations constrained by the measurements were performed. They indicate that this
heterogeneous ice nucleation scenario can be supported by the cloud model. Although ice
formation in this manner may generally act in the atmosphere, the boreal smoke
particles produce an unusually dramatic effect in the lidar data. We conclude
that smoke-induced ice nucleation occurs at moderate supercooled temperatures
either through the effects of raised soil/dust particles embedded in the smoke
droplets, coated soot aerosol or through the nucleation via certain organic solutions.
