Abstract
Abstract. 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 μm (re2.1) and 3.8 μm (re3.8) 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 (∼20 km), while heterogeneities (Hσ) are calculated as the ratio between the standard deviation and mean 0.64 μm reflectance. The value of re2.1 strongly depends on CF, with magnitudes up to 5 μm larger than those for overcast scenes, whereas re3.8 remains insensitive to CF. For cloudy scenes, both re2.1 and re3.8 increase with Hσ for any given AMSR-E LWP, but re2.1 changes more than for re3.8. Additionally, re3.8–re2.1 differences are positive (<1 μm) for homogeneous scenes (Hσ < 0.2) and LWP > 45 gm−2, and negative (up to −4 μm) for larger Hσ. While re3.8–re2.1 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.
Highlights
Cloud optical thickness (τ ) and effective radius derived from visible and near-infrared satellite instruments have become the standard observational data set in cloud–atmospheric research
In principle re retrieved at 3.8 μm is less sensitive to planeparallel biases and 3-D radiative effects, determining the effects of cloud heterogeneities with the use of multispectral re retrievals is difficult as the different photon penetration of the three MODerate resolution Imaging Spectroradiometer (MODIS) channels in theory should capture physical information of the cloud vertical structure (Platnick, 2000)
We explore the ways that cloud properties retrieved from Aqua MODIS radiances vary for different cloud dynamical configurations and spatial heterogeneities at spatial resolutions typical of synoptic/climate studies
Summary
Cloud optical thickness (τ ) and effective radius (re) derived from visible and near-infrared satellite instruments have become the standard observational data set in cloud–. In principle re retrieved at 3.8 μm is less sensitive to planeparallel biases and 3-D radiative effects, determining the effects of cloud heterogeneities with the use of multispectral re retrievals is difficult as the different photon penetration of the three MODIS channels in theory should capture physical information of the cloud vertical structure (Platnick, 2000). This implies that the 3.8 μm-based re is more influenced by properties closer to the cloud top than the 2.1 and 1.6 μm counterparts. The goal here is to determine the bias magnitude in re due to heterogeneities as well as understanding the physical information that can be obtained from re differences calculated at two wavelengths (3.8 and 2.1 μm)
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