Abstract
The inference of ice cloud properties from remote sensing data depends on the assumed forward ice particle model, as they are used in the radiative transfer simulations that are part of the retrieval process. The Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6 (MC6) ice cloud property retrievals are produced in conjunction with a single-habit ice particle model with a fixed degree of ice particle surface roughness (the MC6 model). In this study, we examine the MC6 model and five other ice models with either smoother or rougher surface textures to determine an optimal model to reproduce the angular variation of the radiation field sampled by the Multi-angle Imaging Spectroradiometer (MISR) as a function of latitude. The spherical albedo difference (SAD) method is used to infer an optimal ice particle model. The method is applied to collocated MISR and MODIS data over ocean for clouds with temperatures ≤233 K during December solstice from 2012–2015. The range of solar zenith angles covered by the MISR cameras is broader at the solstices than at other times of the year, with fewer scattering angles associated with sun glint during the December solstice than the June solstice. The results suggest a latitudinal dependence in an optimal ice particle model, and an additional dependence on the solar zenith angle (SZA) at the time of the observations. The MC6 model is one of the most optimal models on the global scale. In further analysis, the results are filtered by a cloud heterogeneity index to investigate cloudy scenarios that are less susceptible to potential 3D effects. Compared to results for global data, the consistency between measurements and a given model can be distinguished in both the tropics and extra-tropics. The SAD analysis suggests that the optimal model for thick homogeneous clouds corresponds to more roughened ice particles in the tropics than in the extra-tropics. While the MC6 model is one of the models most consistent with the global data, it may not be the most optimal model for the tropics.
Highlights
The inference of ice cloud optical thickness τ and effective particle size reff from passive spaceborne radiometric measurements requires an assumed ice particle model that provides the bulk scattering and absorption properties
A Voronoi particle model was suggested for use with geostationary satellite data [3] and the inhomogeneous hexagon model (IHM) was defined for the Polarization and Directionality of the Earth’s Reflectances (POLDER) on the Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) satellite ice cloud property retrievals
The overarching goal of this study is to identify an appropriate degree of surface roughness adopted for ice particle models through the use of collocated Multi-angle Imaging Spectro Radiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) data
Summary
The inference of ice cloud optical thickness τ and effective particle size reff from passive spaceborne radiometric measurements requires an assumed ice particle model that provides the bulk scattering and absorption properties. Because the phase function is fundamental to the remote sensing of global cloud properties, a better understanding of the appropriate degree of ice particle surface roughness for a given ice particle habit is important for improving the consistency of the retrievals based on observations by different sensors. The spherical albedo difference (SAD) method [6] was developed to quantify the comparison between spherical albedo values computed with an assumed ice particle model and their counterparts derived from multi-angle satellite measurements. This algorithm was used to validate cloud particle models and their single-scattering properties [11,12,13].
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