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Modeling Cirrus Clouds. Part I: Treatment of Bimodal Size Spectra and Case Study Analysis

David L. MitchellDesert Research Institute, Reno, Nevada

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Steven K. ChaiDesert Research Institute, Reno, Nevada

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Yangang LiuDesert Research Institute, Reno, Nevada

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Andrew J. HeymsfieldNational Center for Atmospheric Research, Boulder, Colorado

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Yayi DongDivision of Environmental Quality, Boise, Idaho

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Abstract

A model has been developed that predicts the evolution of bimodal size spectra in cirrus clouds. This was done by predicting two size distributions: one for ice particles less than about 150 µm and another for larger particles. The sum of these two distributions yielded the composite, bimodal size distribution, which was predicted from the growth processes of vapor deposition and aggregation. Predicted size spectra were directly compared with size spectra measured during a cirrus cloud case study sampled during a Lagrangian spiral descent. Favorable agreement was obtained between predicted and measured size distributions, especially at ice particle sizes < 150 µm.

The aircraft sampling technique and conditions characterizing the case study were well suited for evaluating the effect of aggregation on ice particle growth. Model calculations indicated that aggregation growth increased mean ice particle sizes by up to 50 µm and reduced concentrations by up to 60% during the case study. The cloud-averaged aggregation efficiency was estimated to be 0.5, which is comparable with values estimated for frontal clouds. It was suggested that ice crystals referred to as planar polycrystals, which exhibit complex three-dimensional shapes, may be precursors for aggregation in cirrus clouds.

The physical mechanisms governing the evolution of the small particle size distribution were investigated using ice particle replicator data from two case studies. Combining the replicator data with theoretical work on the evolution of size distributions, evidence indicates that the mean size in the small ice particle size distribution is relatively small and constant when vapor deposition growth dominates, implying pristine hexagonal crystal types. When aggregation growth involving planar polycrystals occurs, this mean size tends to become larger and can vary widely. Aggregation growth in the small particle distribution may act to diminish bimodal behavior.

Estimated contributions of unmeasured small ice particles (D < 50 µm) to cloud optical depth, τ, were shown to vary widely with conditions and were at least about a factor of 5 less than estimates from an earlier study. The overall impact of the aggregation process on τ, relative to τ for diffusion growth only, was estimated. For the case study considered, aggregation attenuated τ by 10% to 20%.

Finally, the microphysical properties were predicted from analytical expressions. This may make it possible to incorporate this model into large-scale models without excessive increases in computation time.

Abstract

A model has been developed that predicts the evolution of bimodal size spectra in cirrus clouds. This was done by predicting two size distributions: one for ice particles less than about 150 µm and another for larger particles. The sum of these two distributions yielded the composite, bimodal size distribution, which was predicted from the growth processes of vapor deposition and aggregation. Predicted size spectra were directly compared with size spectra measured during a cirrus cloud case study sampled during a Lagrangian spiral descent. Favorable agreement was obtained between predicted and measured size distributions, especially at ice particle sizes < 150 µm.

The aircraft sampling technique and conditions characterizing the case study were well suited for evaluating the effect of aggregation on ice particle growth. Model calculations indicated that aggregation growth increased mean ice particle sizes by up to 50 µm and reduced concentrations by up to 60% during the case study. The cloud-averaged aggregation efficiency was estimated to be 0.5, which is comparable with values estimated for frontal clouds. It was suggested that ice crystals referred to as planar polycrystals, which exhibit complex three-dimensional shapes, may be precursors for aggregation in cirrus clouds.

The physical mechanisms governing the evolution of the small particle size distribution were investigated using ice particle replicator data from two case studies. Combining the replicator data with theoretical work on the evolution of size distributions, evidence indicates that the mean size in the small ice particle size distribution is relatively small and constant when vapor deposition growth dominates, implying pristine hexagonal crystal types. When aggregation growth involving planar polycrystals occurs, this mean size tends to become larger and can vary widely. Aggregation growth in the small particle distribution may act to diminish bimodal behavior.

Estimated contributions of unmeasured small ice particles (D < 50 µm) to cloud optical depth, τ, were shown to vary widely with conditions and were at least about a factor of 5 less than estimates from an earlier study. The overall impact of the aggregation process on τ, relative to τ for diffusion growth only, was estimated. For the case study considered, aggregation attenuated τ by 10% to 20%.

Finally, the microphysical properties were predicted from analytical expressions. This may make it possible to incorporate this model into large-scale models without excessive increases in computation time.

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