Colloquium: Dr. Ivy Tan, McGill University

At temperatures between 0°C and ~-38°C in Earth’s atmosphere, ice crystals may form with the aid of aerosols known as ice-nucleating particles (INPs).  Ice nucleation via INPs may occur via a number of different mechanisms, however the immersion freezing mechanism, whereby ice formation is induced by INPs immersed in supercooled liquid droplets, has been shown to be the dominant ice nucleation mechanism in mixed-phase clouds in the actual atmosphere.  The impact of a set of four immersion freezing parameterizations on Arctic amplification is examined in NASA’s GEOS-5 model.  Standalone atmosphere model simulations clearly show that immersion freezing, and in particular, the dependence of INP concentration on temperature plays an important role in the shortwave cloud feedback during the Arctic summer.  Consistent fully-coupled model simulations with preindustrial and quadrupled CO₂ concentrations demonstrate that immersion freezing has a large impact on Arctic amplification, with a range spanning 30% that of those of the current generation of models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6).  The apparent mechanism is triggered by the summertime cloud phase feedback, which strongly regulates the amount of incoming solar radiation and therefore melting of Arctic sea ice.  The extent of Arctic sea ice melt determines the degree of latent and sensible heat flux exchange with the lower troposphere and its thermodynamic stability, which in turn control low-cloud fraction and the lapse rate in autumn and winter.  This study isolates and highlights the importance of ice nucleation for Arctic amplification.    

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