Characterizing the Co-Existence of Water and Ice in Arctic Clouds

McFarquhar, G., University of Oklahoma

Cloud Distributions/Characterizations

Cloud Properties

Greg M. McFarquhar and Stewart G. Cober. 2004: Single-Scattering Properties of Mixed-Phase Arctic Clouds at Solar Wavelengths: Impacts on Radiative Transfer. Journal of Climate: Vol. 17, No. 19, pp. 3799-3813.

Analyses of in-cloud observations obtained during FIRE-ACE showed that over one-third (33%) of the clouds were mixed-phase, about half of the clouds were ice phase, and just 16% were liquid phase.

Analyses of in-cloud observations obtained during FIRE-ACE showed that over one-third (33%) of the clouds were mixed-phase, about half of the clouds were ice phase, and just 16% were liquid phase.

Radiative feedbacks in the sensitive Arctic climate are primarily the result of complex interactions involving sea ice, snow cover, and clouds. Understanding the properties of mixed-phase clouds (clouds composed of both ice and water) is critical for simulating the Arctic environment because the radiative properties of these clouds are thought to differ substantially from clouds composed exclusively of liquid or ice. Mixed-phase clouds may contain regions of pure water and pure ice interspersed with regions in which both phases coexist. The detail of the distribution of these phases within the cloud and the total amount of the cloud that is characterized by each phase affects the radiative impact of the cloud on the surface and the atmosphere. As reported in the Journal of Climate (October 2004), researchers sponsored by the DOE Atmospheric Radiation Measurement (ARM) Program used in situ measurements obtained during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE) Arctic Cloud Experiment (ACE) and the Surface Heat Budget of the Arctic (SHEBA) project to examine the nature of mixed-phase clouds in an effort to determine the relative importance of each phase upon the radiative properties of an Arctic cloud field. Results of their analyses corroborated a previous study showing that a large percentage of the clouds in typical Arctic cloud field consisted exclusively of ice crystals or supercooled water droplets, even though they occurred in the temperature range (between 0º and -30ºC) where mixed-phase (both phases coexisting) clouds are prescribed by some theories. This finding is inconsistent with the implementation of many parameterizations currently used in large-scale models.

The radiative properties of the particles within a cloud that affect cloud-radiation interactions at the solar wavelengths are (1) the single scattering albedo, (2) asymmetry parameter, and (3) extinction cross section. These properties must be diagnosed in large-scale models to enable the radiative transfer characteristics to be computed. However, existing large-scale model schemes that diagnose or predict liquid and ice properties and their radiative effects in mixed-phase cloud fields have not been well evaluated against observations. Using observational data collected from 18 flights during FIRE-ACE, the researchers analyzed measurements of drop and ice crystal particle size distribution, water content, and icing rate. They also used the observations from FIRE-ACE to determine how the properties of the mixed-phase clouds differed from the clouds consisting just liquid or ice, and compared those findings against a parameterization scheme that predicts the radiative characteristics of mixed-phase cloud fields. The scheme predicted a substantially larger contribution by ice to the overall radiative impacts of the cloud than was found in the observations.

The study also demonstrated the organized nature of Arctic mixed-phase clouds, a structural characteristic that is likely to be important in determining the evolution of the cloud field and its radiative properties. The scale of the observed organization was smaller than the spatial scales representative of grid boxes (~100km) used in global climate models and is, therefore, not represented in current parameterization schemes. The FIRE-ACE data showed that 90% of the observed mixed-phase clouds appeared to have sub-grid scale organization that could impact the evolution of the cloud field and its radiative impacts.

The researchers' findings support the observational evidence to date about the importance of water in determining the radiative properties of mixed-phase clouds. For temperature ranges where water and ice may coexist, an accurate representation of not only the average liquid water content of the cloud field, but also of the details of the distribution of liquid water and the nature of its organization is needed for accurate simulation of Arctic mixed-phase clouds in large-scale models. To further investigate these issues, researchers are now analyzing data collected during the Mixed-Phase Arctic Cloud Experiment conducted in October 2004, which resulted in the largest sample of Arctic mixed-phase cloud observations ever collected.