Abstract
AbstractA 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.
Highlights
Unlike other clouds, cirrus does not reflect much incident sunlight back to space but traps heat originating from the Earth's surface and atmosphere efficiently
As turbulence levels are typically low in the upper troposphere, we focus in the case of ice crystals on tracking sedimentation and capturing effects of water vapor and heat diffusion on nucleation
The ice crystals grow in size due to the ice supersaturation generated in the updraft
Summary
Cirrus does not reflect much incident sunlight back to space but traps heat originating from the Earth's surface and atmosphere efficiently. The latter is influenced by vertical air motions. We make use of a kinetic (non-equilibrium) treatment of the interaction between water vapor, aerosol particles, and ice crystals and a particle tracking approach for the ice phase The latter offers a number of advantages over Eulerian microphysics schemes in cloud simulations (Grabowski et al, 2019). We opt for a 1-D model framework because capturing vertical variations is more important for short-term process studies of aerosol-cirrus interactions than the simulation of horizontal variability. The study concludes with a summary and outlook (section 4)
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