The Moderate Resolution Imaging Spectroradiometer (MODIS) cloud product provides three separate retrievals of cloud particle effective radii (re), derived from 1.6, 2.1 and 3.7 μm band observations. In this study, differences among the three re retrievals for maritime water clouds (designated as re,1.6 re,2.1 and re,3.7) were systematically investigated through a series of case studies and global analyses. Substantial differences have been found between the re,3.7 and re,2.1 retrievals (i.e., Δre,3.7-2.1), and the difference showed a strong dependence on cloud regimes. The Δre,3.7-2.1 is small, within ±2μm, over the relatively spatially homogeneous costal stratocumulus cloud regions. However, for trade wind cumulus regimes, the re,3.7 was found to be substantially smaller, sometimes by more than 10μm, than the re,2.1. The correlations of re differences with key cloud parameters, including the cloud optical thickness (τ), re and a cloud horizontal heterogeneity index (Hσ) derived from 250μm resolution MODIS 0.86μm band observations, were investigated based on one month of MODIS Terra data. It was found that differences among the three re retrievals for optically thin clouds (τ<5) are highly variable, ranging from −15μm to 10μm, likely due to the large MODIS retrieval uncertainties when the cloud is thin. The re,3.7-2.1 exhibited a threshold-like dependence on both re,2.1 and Hσ. The re,3.7 is found to agree reasonably well with re,2.1 when re,2.1 is smaller than about 15μm, but becomes increasingly smaller than re,2.1 once re,2.1 exceeds this size. All three re retrievals showed little dependence when Hσ< 0.3. However, for Hσ >0.3, both re,1.6 and re,2.1 were seen to increase quickly with increasing Hσ. On the other hand, the re,3.7 statistics showed little dependence on Hσ and remained relatively stable over the whole range of Hσ values. Potential contributing causes to the substantial re,3.7 and re,2.1 differences are discussed. In particular, based on both 1-D and 3-D radiative transfer simulations, we have elucidated mechanisms by which cloud heterogeneity and 3-D radiative effects can cause large differences between re,3.7 and re,2.1 retrievals for highly inhomogeneous clouds.
© 2011 Optical Society of AmericaPDF Article