Index Geophysics

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

The impact of horizontal heterogeneities, liquid water path (LWP from AMSR-E), and cloud fraction (CF) on MODIS cloud effective radius (<i>r</i>_e), retrieved from the 2.1 μm (<i>r</i>_e2.1) and 3.8 μm (<i>r</i>_e3.8) channels, is investigated for warm clouds over the southeast Pacific. Values of <i>r</i>_e retrieved using the CERES algorithms are averaged at the CERES footprint resolution (&sim;20 km), while heterogeneities (<i>H</i>_&sigma;) are calculated as the ratio between the standard deviation and mean 0.64 μm reflectance. The value of <i>r</i>_e2.1 strongly depends on CF, with magnitudes up to 5 μm larger than those for overcast scenes, whereas <i>r</i>_e3.8 remains insensitive to CF. For cloudy scenes, both <i>r</i>_e2.1 and <i>r</i>_e3.8 increase with <i>H</i>_&sigma; for any given AMSR-E LWP, but <i>r</i>_e2.1 changes more than for <i>r</i>_e3.8. Additionally, <i>r</i>_e3.8–<i>r</i>_e2.1 differences are positive (<1 μm) for homogeneous scenes (<i>H</i>_&sigma; < 0.2) and LWP > 45 gm⁻², and negative (up to −4 μm) for larger <i>H</i>_&sigma;. While <i>r</i>_e3.8–<i>r</i>_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.

Painemal, D.

Minnis, P.

Sun-Mack, S.

2018-01-15

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10.5194/acp-13-9997-2013

https://acp.copernicus.org/articles/13/9997/2013/

eISSN: 1680-7324

eng