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Dominant ice nucleation modes in synoptic and anvil cirrus clouds and their effect on ice morphology and fall speeds

 

Authors

David L. Mitchell — Desert Research Institute
Subhashree Mishra — DOE - SunShot Initiative, AAAS S&T Policy Fellow
Paul Lawson — SPEC, Inc.
Xiaohong Liu — Texas A&M University

Category

Cloud Properties

Description

Temperature dependence of mass-normalized (N(D)/total IWC) synoptic cirrus PSD during SPARTICUS. PSD were averaged over 5-degree C intervals, and PSD measured at temperatures below -40 degrees C are in blue. The higher concentrations of small ice crystals in the blue PSD are consistent with homogeneous nucleation that occurs at T < -38 degrees C. The bimodality of warmer (red) PSD may be due to aggregation and lower ice nucleation rates associated with heterogeneous nucleation processes.

Observational studies on the relative roles of heterogeneous and homogeneous ice nucleation have yielded conflicting results regarding which process dominates at temperatures T < -40°C. This may at least partially be due to limitations of the probes used to measure the ice particle size distribution (PSD), where ice particles shatter upon entering the probe inlet tube. The resulting swarm of small ice crystal artifacts will obscure the natural concentration of small ice crystals, veiling useful information relating to ice nucleation. The TC4 and SPARTICUS field campaigns measured PSD using the 2D-S probe that was designed to minimize ice particle shattering, greatly reducing the veil of ice artifacts. Comparisons against traditional probe measurements show small ice crystal concentrations from the 2D-S are generally lower by a factor of 10 to 100 or more. Cirrus clouds during SPARTICUS were classified as either synoptic or anvil cirrus clouds while all TC4 cirrus were anvil cirrus.

Synoptic cirrus clouds were discriminated from anvil cirrus via flight notes and satellite imagery of the sampled cirrus. The 174 PSD from 13 cirrus clouds consisted of 1–2 minute samples of microphysically stable regions of cloud that contained no liquid water. Going from warmer to colder T, the PSD analysis revealed the following abrupt changes near the onset T of homogeneous freezing nucleation (-38°C): (1) a change in PSD shape from bimodal to unimodal, (2) an increase in mass-normalized number concentration, (3) a decrease in mean and effective size, (4) a change in ice particle shape, and (5) a decrease in ice fall speed. All changes are consistent with homogeneous freezing nucleation dominating ice production for T < -40°C. In addition, these results agree with the T dependence of effective size and fall speed derived recently from satellite measurements of cirrus clouds. Results for anvil cirrus during TC4 and SPARTICUS were similar to the synoptic results, although the noted changes near -40°C were less abrupt with the exception of PSD shape.

The results imply that cirrus cloud lifetime and coverage (through the ice fall speed) and optical properties (through crystal size and shape changes) are sensitive to the nucleation mode. These observations provide clear constraints for modeling ice nucleation processes and morphology, which in turn affect the effective size and fall speed, in GCMs and RCMs. Work is underway in applying these results to CAM5.