Haze Particle Nucleation Simulations in Cirrus Clouds, and Applications for Numerical and Lidar Studies

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  • 1 Department of Meteorology, University of Utah, Salt Lake City, Utah
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Abstract

A one-dimensional cloud microphysical model is applied to exploring the basic conditions under which ice crystal nucleation, from the homogeneous freezing of ammonium sulfate haze particles, can occur in cirrus clouds at temperatures ≲ −35°C. Cirrus generating regions maintained by uniform updrafts of 0.1–0.25 m s−1, and an idealized ice crystal precipitation mechanism dependent on vertical wind shear are treated in the model. The findings indicate that ice crystals are generated in a pulse-like fashion as a result of water vapor competition effects from ice crystals nucleated within an updraft, followed by precipitation. Water saturation is not required for ice crystal nucleation at ≲ −35°C, and the relative humidities required at decreasing temperatures gradually decrease. The temperature dependency of the relative humidities associated with ice production does not depend significantly on model inputs, suggesting that cirrus cloud processes follow an adjusted pseudoadiabat, which produces ice mass contents that become increasingly smaller than those possible from a pseudoadiabatic process involving nucleation at water saturation. Finally, to determine whether polarization lidar observations can identify haze particles in cirrus generating regions, as has been suggested by recent studies, Mie scattering simulations were performed for the properties of the model-generated haze particles.

Abstract

A one-dimensional cloud microphysical model is applied to exploring the basic conditions under which ice crystal nucleation, from the homogeneous freezing of ammonium sulfate haze particles, can occur in cirrus clouds at temperatures ≲ −35°C. Cirrus generating regions maintained by uniform updrafts of 0.1–0.25 m s−1, and an idealized ice crystal precipitation mechanism dependent on vertical wind shear are treated in the model. The findings indicate that ice crystals are generated in a pulse-like fashion as a result of water vapor competition effects from ice crystals nucleated within an updraft, followed by precipitation. Water saturation is not required for ice crystal nucleation at ≲ −35°C, and the relative humidities required at decreasing temperatures gradually decrease. The temperature dependency of the relative humidities associated with ice production does not depend significantly on model inputs, suggesting that cirrus cloud processes follow an adjusted pseudoadiabat, which produces ice mass contents that become increasingly smaller than those possible from a pseudoadiabatic process involving nucleation at water saturation. Finally, to determine whether polarization lidar observations can identify haze particles in cirrus generating regions, as has been suggested by recent studies, Mie scattering simulations were performed for the properties of the model-generated haze particles.

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