Seminar

Abstract

Atmospheric ice formation impacts climate and the hydrological cycle. Our predictive understanding of primary ice formation from ice-nucleating particles (INPs) is insufficient. Cirrus clouds, consisting solely of ice crystals, form typically at altitudes greater than about 6 km. They can form via different pathways including homogeneous and heterogeneous ice nucleation. At these high altitudes and low temperatures cirrus form at subsaturated conditions with respect to liquid water. In this presentation the case is made that homogeneous and heterogeneous freezing processes can be holistically described in a framework based on condensed-phase water activity, thereby simplifying representation in cloud-resolved modeling. Ice nucleation is often described by a singular, or deterministic (non-time dependent), approach, contrary to classical nucleation theory (CNT) that accounts for the stochastic nature of nucleation. These different viewpoints are discussed, and the application of a water-activity based CNT freezing model to yield a time-dependent and stochastic formulation of ice nucleation relevant for cirrus formation is presented. The choice of freezing parameterization will be discussed in the example of a 1D LES informed mixed-phase cloud modeling study. This model uniquely allows the prognostic treatment of INPs considering both types of ice nucleation descriptions. It turns out that the choice of ice nucleation formalism defines the INP reservoir from which a cloud can draw from to sustain ice crystal formation. These observations also hold for the case of cirrus cloud formation.

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Daniel Knopf is interested in the cloud formation potential and multiphase chemical kinetics of aerosol particles. Daniel pursues this research by means of laboratory experiments, field measurements, and modeling studies. He has advanced the application of nanoscale single-particle analytical techniques to study the phase transition of aerosol particles. Daniel Knopf is a full professor at the School of Marine and Atmospheric Sciences with an affiliated appointment at the Department of Chemistry at Stony Brook University, New York.