Process‐Based Simulation of Aerosol‐Cloud Interactions in a One‐Dimensional Cirrus Model

• Published in: Journal of geophysical research. Atmospheres 2020-03, Vol.125 (6), p.n/a
• Main Author: Kärcher, B.
• Format: Article
• Language: English
• Subjects: Aerodynamics; Aerosol-cloud interactions; Aerosols; cirrus; Cirrus clouds; cloud model; Coefficients; Computer simulation; Contrails; Crystal growth; Crystals; Deposition; Eddy diffusion; Fluxes; Freezing; Geophysics; Gravity; Ice; Ice crystal size; Ice crystals; Ice nucleation; Ice particles; Mathematical models; microphysics; Mixing ratio; Nucleation; Nucleation processes; Optical properties; Profiles; Sedimentation; Simulation; Supersaturation; Turbulent diffusion; Uptake; Vertical mixing; Water vapor; Water vapour
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2019JD031847

A new microphysical cirrus model to simulate ice crystal nucleation, depositional growth, and gravitational settling is described. The model tracks individual simulation ice particles in a vertical column of air and allows moisture and heat profiles to be affected by turbulent diffusion. Ice crystal size‐ and supersaturation‐dependent deposition coefficients are employed in a one‐dimensional model framework. This enables the detailed simulation of microphysical feedbacks influencing the outcome of ice nucleation processes in cirrus. The use of spheroidal water vapor fluxes enables the prediction of primary ice crystal shapes once microscopic models describing the vapor uptake on the surfaces of cirrus ice crystals are better constrained. Two applications addressing contrail evolution and cirrus formation demonstrate the potential of the model for advanced studies of aerosol‐cirrus interactions. It is shown that supersaturation in, and microphysical and optical properties of, cirrus are affected by variable deposition coefficients. Vertical variability in ice supersaturation, ice crystal sedimentation, and high turbulent diffusivity all tend to decrease homogeneously nucleated ice number mixing ratios over time, but low ice growth efficiencies counteract this tendency. Vertical mixing induces a tendency to delay the onset of homogeneous freezing. In situations of sustained large‐scale cooling, natural cirrus clouds may often form in air surrounding persistent contrails. Key Points Column model with supersaturation‐dependent deposition coefficients, nonspherical ice crystal shapes, and turbulent diffusivity is presented Feedback between water vapor attachment kinetics on ice crystal surfaces and growth from the vapor influences ice nucleation in cirrus Homogeneous freezing at cloud tops transforms contrails into contrail cirrus in meteorological conditions supporting contrail persistence