Above-cloud absorbing aerosols: effects on MODIS marine boundary layer cloud optical property retrievals and direct aerosol radiative forcing

Clouds, aerosols, and their interactions are widely considered to be key uncertainty components in our current understanding of the Earth's atmosphere and radiation budget. The work presented here is focused on the quasi-permanent marine boundary layer (MBL) clouds off the southern Atlantic coast of Africa and the effects on MODIS cloud optical property retrievals (MOD06) of an overlying absorbing smoke layer. During much of August and September, a persistent smoke layer resides over this region, produced from extensive biomass burning throughout the southern African savanna. The resulting absorption, which increases with decreasing wavelength, potentially introduces biases into the MODIS cloud optical property retrievals of the underlying MBL clouds. This effect is more pronounced in the cloud optical thickness retrievals, which over ocean are derived from the wavelength channel centered near 0.86 μm (effective particle size retrievals are derived from the short and mid-wave IR channels at 1.6, 2.1, and 3.7 μm). Here, the MODIS cloud optical property retrievals are adjusted to account for the above-cloud aerosol attenuation using AOD from the CALIOP 5km aerosol layer product. The above-cloud direct aerosol radiative forcing will then be estimated, and its sensitivity to the bias-adjusted cloud retrievals will be investigated.