Sensitivity of Satellite-Retrieved Cloud Properties to the Effective Variance of Cloud Droplet Size Distribution

Cloud reflectance models currently used in cloud property retrievals from satellites have been developed using size distributions defined by a set of fixed effective radii with a fixed effective variance. The satellite retrievals used for the Atmospheric Radiation Measurement (ARM) program assume droplet size distributions with an effective variance value of 0.10 (Minnis et al. 1998); the International Satellite Cloud Climatology Project uses 0.15 (Rossow and Schiffer 1999); and the Moderate Resolution Imaging Spectroradiometer (MODIS) team uses 0.13 (Nakajima and King 1990). These distributions are not necessarily representative of the actual sizes present in the clouds being observed. Because the assumed distributions can affect the reflectance patterns and near-infrared absorption, even for the same droplet effective radius reff, it is desirable to use the optimal size distributions in satellite retrievals of cloud properties. Collocated observations of the same clouds from different geostationary satellites, at different viewing angles, indicate that the current models may not be optimal (Ayers et al. 2005). Similarly, hour-to-hour variations in effective radius and optical depth reveal an unexplained dependence on scattering angle. To explore this issue, this paper examines the sensitivity of the cloud reflectance at 0.65 and 3.90-{micro}m to changes in the effective variance, or the spectral dispersion, of the modeled size distributions. The effects on the scattering phase functions and on the cloud reflectances are presented, as well as some resultant effects on the retrieved cloud properties