Aerosol Indirect Effects and Their Effective Representation in Climate Models from the New ASR Shortwave Array Spectroradiometer

This project will address aerosol-cloud indirect effects using new measurements from the Shortwave Array Spectroradiometer (SAS) instruments recently procured under the American Recovery and Reinvestment Act (ARRA). The SAS instruments measure downwelling hemispheric irradiance or zenith radiance with full spectral resolution across the wavelength range 340-1700 nm. These instruments have been deployed primarily at the Southern Great Plains (SGP) site, but are also to be deployed in approved DOE field programs.
This project has three research objectives:

• We will develop a radiative transfer retrieval algorithm for use with the new Shortwave Array Spectroradiometer (SAS) data. This will provide high-time-resolution retrievals of cloud thermodynamic phase and optical depth, and, for single-phase clouds, liquid water droplet effective radius or ice water effective particle size. The SAS instruments operate over a wavelength range that is very sensitive to cloud properties. A key advantage of this SAS-based retrieval is that we can retrieve the specific optical properties at high-time resolution from a single instrument, for either overcast skies or broken clouds. Here, we are building on our previous work with shortwave North Slope of Alaska (NSA) spectroradiometer data from the DOE-supported Indirect and Semi-Direct Aerosol Campaign (ISDAC).
• We will apply the algorithm to year-round SAS data at the SGP site, and other (already approved) deployments of the SAS instruments. We will create a PI Product of thermodynamic phase and cloud optical depth (all clouds) and effective particle size (when a predominant single phase can be identified).
• Using aerosol data at SGP, back trajectory analysis, and other ancillary data, we will identify case studies in which aerosol indirect effects appear to be manifesting. The high-time resolution capability of the SAS instruments allows the possibility of identifying true process studies in aerosol-cloud-climate interaction. We will publish and make these cases available to the climate modeling community.

The scientific motivation for this project involves the recently evolved capability in global climate models to incorporate aerosol chemical and microphysical properties as they influence cloud activation and hence manifestation of indirect effects. These new models need testing, validation, and refinement. Experimental atmospheric science programs must therefore move beyond the task of merely confirming that aerosol indirect effects occur in nature, and must progress toward identifying high-time-resolution process studies specific to various categories of tropospheric aerosol. The new SAS instruments provide an excellent opportunity for this type of progress toward improving climate model performance.

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This work was supported by the U.S. Department of Energy’s Office of Science, through the Biological and Environmental Research program as part of the Atmospheric System Research program.