J8.4 Factors affecting 'Critical Cloud Fraction'?

Tuesday, 8 July 2014: 11:30 AM
Essex North (Westin Copley Place)
Kruthika Eswaran, Indian Institute of Science, Bengaluru, India; and S. K. Satheesh and J. Srinivasan

Due to high spatial and temporal variability of aerosol properties, it is uncertain whether they exert a warming or cooling effect. The different physical and optical properties of various types of aerosols make the aerosol-radiation interaction a complex problem. Recent studies have defined a parameter called the 'critical cloud fraction' as the point where the aerosol top of the atmosphere (TOA) forcing changes sign from negative to positive. However over regions influenced by different aerosol sources, this parameter varies with seasons due to the change in the properties of aerosols present above the region. Bay of Bengal (BOB) was chosen as the region of interest due to its important role on Indian Monsoon. This small oceanic region surrounded by landmass acts as a sink for a variety of aerosols and air mass types. Our results indicate that aerosol absorption can be determined indirectly through measurements of TOA fluxes. Using one year satellite data, the 'optically equivalent' aerosol type over a particular region could be inferred as more absorptive or more scattering based on the slope of the TOA aerosol forcing (from CERES data) versus aerosol optical depth (AOD) (from MODIS) plots, which substantially varies as a function of single scattering albedo (SSA). The data was binned and then plotted to remove the large scatter and get a more clear relationship between TOA forcing and AOD for three different seasons December-January-February (DJF), September-October-November (SON) and March-April-May (MAM). This is carried out over a pristine oceanic region where it is expected that SSA and AOD have a dominant effect on the aerosol forcing compared to aerosol size. However over regions influenced by multiple aerosols having different SSA, the controlling factor in determining the aerosol forcing is not just a single property but a combination of all aerosol properties including asymmetry parameter (g), angstrom exponent (AE) and also the surface reflectance. Determining the individual impacts of each property on the TOA aerosol forcing makes the problem even more challenging and this is further investigated using a radiative transfer model.

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