Ice Cloud Optical Thickness, Effective Radius, And Ice Water Path Inferred From Fused MISR and MODIS Measurements Based on a Pixel-Level Optimal Ice Particle Roughness Model

The Multi-angle Imaging SpectroRadiometer (MISR) provides measurements over a wider scattering-angle range for a given location than a cross-track scanning sensor such as the MODerate resolution Imaging Spectroradiometer (MODIS). Based on a full year (2013) of fused MISR-MODIS datasets, we develop a variable surface roughness model for ice particles with the goal of identifying the optimal degree of roughness in the ice model for a given pixel containing single-layer ice clouds. For the MISR-MODIS observations over oceans, severe roughness values are often selected for a pixel when optical thickness (τ) and particle effective radius (R_eff) are large in conjunction with larger cloud heterogeneity index (H_σ) or a warmer cloud top temperature. Furthermore, τ, R_eff, and ice water path are retrieved with the optimal model and compared to operational MODIS Collection 6 (MC6) products that assume a constant roughness. In general, the retrievals based on the present optimal model lead to greater consistency with MISR measurements, and result in larger median τ by 10.1% and smaller median R_eff by 6.5% but almost identical ice water path in comparison with the MC6 counterparts. The higher average τ value is caused by a slightly larger number of large τ cases, but the smaller average R_eff value is due to the shifting of the retrieved R_eff value toward smaller values by approximately 2–4 μm in comparison to the MC6 distribution over all seasons. Both τ retrievals have similar regional and monthly variations, but a larger annual cycle of R_eff is associated with the optimal model.