Robert F Cahalan
NASA/Goddard Space Flight Center
Code 613.2
Greenbelt, MD 20771

E-mail: robert.f.cahalan@nasa.gov
Phone: 301-614-5390
Fax: 301-614-5493

We propose to use MODIS cloud and aerosol products to study correlations of aerosol with cloud properties while properly accounting for three-dimensional (3D) radiative effects. With this we will address the important Earth science question of cloud-aerosol interactions, and provide improved assessment of indirect forcing by aerosols. This work will reduce uncertainty in indirect aerosol radiative forcing and improve understanding of cloud and aerosol climatology. Clouds are the most strikingly complex 3D objects in the Earth's atmosphere. One consequence is the radiative properties of each cloudy pixel are effected by its neighbors. Current operational retrievals of cloud and aerosol properties treat each pixel independently of its neighbors, assuming no radiative interactions between them. We propose to quantify the influence of 3D radiative effects on MODIS-derived cloud optical thickness, particle size, and aerosol optical thickness. We organize our efforts in 3 tasks:

1. For aerosol retrievals, we will provide a new product: cloud-related enhancement of clear-sky reflected radiance. The enhancement will then be converted into a correction to operational retrievals of aerosol optical thickness.
2. For cloud retrievals, we will provide 3D-related uncertainties in cloud optical thickness and droplet effective radius. In combination with operationally supplied 1D uncertainties, new 3D uncertainties will help reject unreliable retrievals and keep good retrievals.
3. Using these improved products, we will study correlations of cloud and aerosol properties as a function of cloud fraction, and cloud and aerosol types, and determine how observed correlations are consistent with aerosol indirect effects. Applying 3D radiative transfer calculations, we will combine multi-spectral MODIS information on spatial variability with MISR multi-angle information and ASTER sub-pixel information. This proposal builds on our solid foundation of cloud modeling and 3D cloud radiative transfer expertise developed during the I3RC initiative and subsequently applied to issues of MODIS cloud product validation.