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.