The impact of horizontal heterogeneities, cloud fraction, and liquid water path on warm cloud effective radii from CERES-like Aqua MODIS retrievals

The impact of horizontal heterogeneities, liquid water path (LWP from AMSR-E), and cloud fraction (CF) on MODIS cloud effective radius (re), retrieved from the 2.1 mu m (r(e2.1)) and 3.8 mu m (r(e3.8)) channels, is investigated for warm clouds over the southeast Pacific. Values of re retrieved using the CERES algorithms are averaged at the CERES footprint resolution (similar to 20 km), while heterogeneities (H-sigma) are calculated as the ratio between the standard deviation and mean 0.64 mu m reflectance. The value of r(e2.1) strongly depends on CF, with magnitudes up to 5 mu m larger than those for overcast scenes, whereas r(e3.8) remains insensitive to CF. For cloudy scenes, both r(e2.1) and r(e3.8) increase with H-sigma for any given AMSR-E LWP, but r(e2.1) changes more than for r(e3.8). Additionally, r(e3.8)-r(e2.1) differences are positive (<1 mu m) for homogeneous scenes (H-sigma < 0.2) and LWP > 45 gm(-2), and negative (up to -4 mu m) for larger H-sigma. While r(e3.8)-r(e2.1) differences in homogeneous scenes are qualitatively consistent with in situ microphysical observations over the region of study, negative differences - particularly evinced in mean regional maps - are more likely to reflect the dominant bias associated with cloud heterogeneities rather than information about the cloud vertical structure. The consequences for MODIS LWP are also discussed.