Mission Statement

A focus on polar clouds by GCSS is motivated by:

• poor understanding of the physical processes at work in the polar cloudy boundary layer
• poor simulation of polar cloud, radiation, and boundary layer models by current climate models
• predicted amplification of greenhouse warming in the Arctic

Several features of the polar climate contribute to the difficulties in simulation of the cloud and radiation environment by climate models:

• cold temperatures and low humidities, characterized by complex vertical structure including inversions
• unusual cloud types such as diamond dust, persistent mixed phase clouds, thin multiple cloud layers, and convection from leads in sea ice
• highly reflecting snow/ice surface, which may be highly heterogeneous

The motivation for a focus on polar clouds at this time arises from a wealth of data on arctic clouds and radiation that is recently becoming available.

• Surface Heat Budget of the Arctic Ocean (SHEBA), a field experiment deployed in the Arctic Ocean during the period October 97 through October 98.
• FIRE III Arctic Clouds Experiment, which deployed research aircraft during the period April through July 1998 over the SHEBA surface observations.
• Atmospheric Radiation Measurement (ARM Program), which is deploying instrumentation at Barrow, Alaska for a period of up to 10 years, beginning in March 1998.

These data are providing the basis for the initial case studies that WG5 is considering. Older datasets may also be considered. And of course WG5 will actively pursue additional datasets that may be possible in conjunction with planned and future field programs, particularly in the Antarctic.

The following science issues have been identified related to polar clouds and radiation:

• What is the influence of leads and other open water on cloud properties when large surface-air temperature differences exist?
• How does the extreme static stability and low atmospheric water vapor content of the lower troposphere, especially during winter, affect the flow of energy across the air-sea interface?
• What is the mechanism that leads to the spectacular multiple-layering of summertime cloud systems over the Arctic Ocean?
• How does the transition of low clouds from liquid to crystalline depend on temperature and aerosol characteristics, and how does the springtime transition differ from the autumnal transition?
• What is the spectral distribution of longwave radiation? In particular, what is the role of the 20 mm rotation-band "window" region in regulating the surface and atmospheric temperature in the Arctic?
• What are the effects of springtime "arctic haze" on the absorption of solar radiation in polar clouds?
• How do the transmittance and reflectance of solar radiation by clouds and the surface depend on the low solar zenith angles typical of the polar regions?
• What is the role of diamond dust in determining the radiation fluxes?
• What are the shortwave radiative effects of the horizontally inhomogeneous stratocumulus clouds over the inhomogeneous, highly-reflecting snow/ice surface?
• How do the optical properties of the arctic surface vary in response to changes in snow and ice characteristics (including melt ponds)?

The general strategy for using observations to improve the treatment of arctic clouds and radiation in climate models is illustrated in the figure below. (Click on image to increase size.)

The following parameterizations are targeted by GCSS WG 5 activities:

• microphysics of mixed-phase clouds
• radiative transfer in cloudy atmosphere
• formation and dissipation of boundary layer clouds
• stable boundary layer

The predicted amplification of greenhouse warming in the Arctic are associated with positive radiation feedbacks in the climate models. To insure accurate climate model predictions, the parameterizations of arctic clouds and related processes must provide the correct feedback to the climate in a perturbed simulation. Hence a special effort must be made to compare the model parameterizations against observations in such a way that the physical process (i.e., relationships among variables) that contribute to the feedbacks are examined, rather than by simply examining individual variables.

WG5 projects consist of case studies that can be used to evaluate the following process models:

• Large Eddy Simulations (LES) and Cloud Resolving Models (CRM)
• Radiative transfer models
• Explicit microphysics models

The same cases can be used to evaluate:

• Single Column Models (SCM)
• Numerical Weather Prediction (NWP) models

In addition, some datasets of longer period (e.g. greater than 3 weeks) are assembled specifically to evaluate single-column models.

The WG5 cases are divided into the following two categories:

• WG5 Projects, which represent complete observational case studies for which model intercomparison studies are being undertaken.
• WG5 Case Studies, which may represent preliminary analyses that eventually become Projects, or cases that are less complete or deemed to be less appropriate for formal model intercomparison studies but may be of interest to modelers who want to examine a larger number of cases.