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

Abstract

The impact of horizontal heterogeneities, liquid water path (LWP from AMSR-E), and cloud fraction (CF) on MODIS cloud effective radius ( r e), retrieved from the 2.1 μm ( r e2.1) and 3.8 μm ( r e3.8) channels, is investigated for warm clouds over the southeast Pacific. Values of r e retrieved using the CERES algorithms are averaged at the CERES footprint resolution (∼20 km), while heterogeneities ( H σ) are calculated as the ratio between the standard deviation and mean 0.64 μm reflectance. The value of r e2.1 strongly depends on CF, with magnitudes up to 5 μm 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 σ 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 μm) for homogeneous scenes ( H σ < 0.2) and LWP > 45 gm-2, and negative (up to -4 μm) for larger H σ. 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. © Author(s) 2013.