title: View angle dependence of MODIS liquid water path retrievals in warm oceanic clouds.
abstract: 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 ∩ shape to stronger ∪ shape as heterogeneity, sun angle, or latitude increased. The 2.2 µm effective radius variations always had a ∪ 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 ∪-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.
  
  
  CONCLUSIONS
  Microwave LWP shows small overall and cross-swath variationsMODIS in-cloud LWP also shows good view angle consistency in most casesMODIS retrievals show strong overall increase with heterogeneity and sun angle.
authors.affiliation: Leibniz Institute for Tropospheric Research Leipzig, Germany.
authors.forename: Ákos
authors.initials: Á
authors.lastname: Horváth
authors.affiliation: Scripps Institution of Oceanography La Jolla, California, USA.
authors.forename: Chellappan
authors.initials: C
authors.lastname: Seethala
authors.affiliation: Leibniz Institute for Tropospheric Research Leipzig, Germany.
authors.forename: Hartwig
authors.initials: H
authors.lastname: Deneke
keywords: cloud remote sensing; liquid water path; plane-parallel bias; three-dimensional effects; view angle dependence
references.citation: J Geophys Res Atmos. 2014 Jul 16;119(13):8304-8328
references.pmid: 25821665