Abstract

This study investigates effects of drizzle and cloud horizontal inhomogeneity on cloud effective radius ( r e ) retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS). In order to identify the relative importance of various factors, we developed a MODIS cloud property retrieval simulator based on the combination of large‐eddy simulations (LES) and radiative transfer computations. The case studies based on synthetic LES cloud fields indicate that at high spatial resolution (∼100 m) 3‐D radiative transfer effects, such as illumination and shadowing, can induce significant differences between retrievals of r e based on reflectance at 2.1 μ m ( r e,2.1 ) and 3.7 μ m ( r e,3.7 ). It is also found that 3‐D effects tend to have stronger impact on r e,2.1 than r e,3.7 , leading to positive difference between the two (Δ r e,3.7−2.1 ) from illumination and negative Δ r e,3.7−2.1 from shadowing. The cancellation of opposing 3‐D effects leads to overall reasonable agreement between r e,2.1 and r e,3.7 at high spatial resolution as far as domain averages are concerned. At resolutions similar to MODIS, however, r e,2.1 is systematically larger than r e,3.7 when averaged over the LES domain, with the difference exhibiting a threshold‐like dependence on both r e,2.1 and an index of the sub‐pixel variability in reflectance ( H σ ), consistent with MODIS observations. In the LES cases studied, drizzle does not strongly impact r e retrievals at either wavelength. It is also found that opposing 3‐D radiative transfer effects partly cancel each other when cloud reflectance is aggregated from high spatial resolution to MODIS resolution, resulting in a weaker net impact of 3‐D radiative effects on r e retrievals. The large difference at MODIS resolution between r e,3.7 and r e,2.1 for highly inhomogeneous pixels with H σ > 0.4 can be largely attributed to what we refer to as the “plane‐parallel r e bias,” which is attributable to the impact of sub‐pixel level horizontal variability of cloud optical thickness on r e retrievals and is greater for r e,2.1 than r e,3.7 . These results suggest that there are substantial uncertainties attributable to 3‐D radiative effects and plane‐parallel r e bias in the MODIS r e,2.1 retrievals for pixels with strong sub‐pixel scale variability, and the H σ index can be used to identify these uncertainties.