Spectral Values of Droplet Absorption Coefficients in Marine Stratocumulus Clouds: Measurements Versus Theory

The parameterization of the infrared radiative effects of low level stratocumulus clouds in Global Circulation Models (GCMs) is of interest due to the large areal coverage of the cloud type and the resulting potential influence on estimated atmospheric cooling rates and the surface radiative budget of the planet. The methods used in such parameterizations usually employ a value of the droplet absorption coefficient based on two assumptions: (1) that the absorption approximation is valid (i.e. no scattering in the IR); and (2) that the Mie absorption efficiency parameter is linear in particle radius. Published values for the cloud mass absorption coefficients vary from between 0.08 to 0.13 m² g^-l depending on the spectral interval over which the values are averaged or measured. During the ASTEX Phase II Intensive Field Observation in June, l992, Colorado State University sampled the downwelling infrared radiance field under marine stratocumulus cloud layers using a mid-infrared emission interferometer with a spectral resolution of 1.0 cm^-1. These data and the supporting information from a cloud laser ceilometer and a rawinsonde system will be used to determine the droplet absorption coefficient consistent with the parameterization. The values will be compared to their theoretically determined counterparts in order to further verify the approximation.