Detection of Non-spherical Particles - Cirrus and Dust - With MISR

The Multi-angle Imaging SpectroRadiometer (MISR) onboard the EOS Terra platform has been providing information about aerosols globally over both land and ocean since March 2000. Aerosol attributes inferred from MISR data include optical depth, Angstrom exponent, relative amounts of small, medium and large particles, relative amounts of spherical and non-spherical particles, and single scattering albedo. From this list only optical depth has been fully validated; the remaining products products are in the process of systematic verification through independent studies. The overall distribution, characteristics and behavior of the aerosol products are best seen in MISR Level 3 maps (global, lat-long at 0.5 deg resolution, monthly and seasonal) which show spatial patterns and time trends which are difficult to discern when viewing the Level 2 data only. In particular, we noticed in these Level 3 maps that the relative amount of non-spherical particles displayed globally was highly dependent on the type and accuracy of the particular modeled non-spherical (dust) aerosol components used in the aerosol retrieval process. In early versions of the retrieval algorithm, the dust models used were selected infrequently, mainly for optically thick dust clouds over the ocean originating in the Sahara. The dust models were subsequently improved, which resulted in better dust retrieval coverage, both over land and ocean. But an additional consequence of the dust model improvement was that significant amounts of non-spherical particles appeared around the globe in locations where they had previously not been detected. The spatial distribution of the newly identified particles can be described primarily as zonal bands, one in each hemisphere.The southern band appears dominant, and shifts in latitude with season. These bands are generally not in the same location as known dust events so we suspect that these features are actually cirrus, being misinterpreted as dust in the retrieval process, due to a lack of other non-spherical particle models in the current retrieval data base that better match the cirrus phase functions. We will show some MISR Level 3 maps, illustrating retrieved aerosol attributes, with emphasis on the non-spherical nature of the aerosols. Also, we will present results of studies of selected areas, both within and adjacent to these non-spherical particle bands, using a variety of non-spherical cirrus models and spherical aerosol models, to explore whether these bands do indeed have a significant cirrus component.