View angle dependence of MODIS liquid water path retrievals in warm oceanic clouds

TitleView angle dependence of MODIS liquid water path retrievals in warm oceanic clouds
Publication TypeJournal Article
Year of Publication2014
AuthorsHorvath A., Seethala C., Deneke H.
JournalJournal of Geophysical Research-Atmospheres
Date Published2014/07
Type of ArticleArticle
ISBN Number2169-897X
Accession NumberWOS:000340408000032
Keywords1d; algorithm; droplet effective radius; impact; optical-thickness; passive microwave; reflectance; satellite measurements; solar zenith angle; theory; vertical variation

We investigated the view angle dependence of domain mean Moderate Resolution Imaging Spectroradiometer (MODIS) liquid water path (LWP) and that of corresponding cloud optical thickness, effective radius, and liquid cloud fraction as proxy for plane-parallel retrieval biases. Independent Advanced Microwave Scanning Radiometer-EOS LWP was used to corroborate that the observed variations with sun-view geometry were not severely affected by seasonal/latitudinal changes in cloud properties. Microwave retrievals showed generally small (<10%) cross-swath variations. The view angle (cross-swath) dependence of MODIS optical thickness was weaker in backscatter than forward scatter directions and transitioned from mild boolean AND shape to stronger boolean OR shape as heterogeneity, sun angle, or latitude increased. The 2.2 mu m effective radius variations always had a boolean OR shape, which became pronounced and asymmetric toward forward scatter in the most heterogeneous clouds and/or at the lowest sun. Cloud fraction had the strongest and always boolean OR-shaped view angle dependence. As a result, in-cloud MODIS cloud liquid water path (CLWP) showed surprisingly good view angle (cross-swath) consistency, usually comparable to that of microwave retrievals, due to cancelation between optical thickness and effective radius biases. Larger (20-40%) nadir-relative increases were observed in the most extreme heterogeneity and sun angle bins, that is, typically in the polar regions, which, however, constituted only 3-8% of retrievals. The good consistency of MODIS in-cloud CLWP was lost for gridbox mean LWP, which was dominated by the strong cloud fraction increase with view angle. More worryingly, MODIS LWP exhibited significant and systematic absolute increases with heterogeneity and sun angle that is not present in microwave LWP.

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