2010-05-29: The impact of broken and inhomogeneous clouds on the retrieval of effective radius and cloud phase

In the retrieval of cloud properties from space, a generally applied assumption is that each pixel labeled as cloud-filled contains overcast clouds that are horizontally homogeneous. However, in nature a considerable amount of the clouds are broken and inhomogeneous. Wolters et al. (2010) investigated the effect of broken clouds and inhomogeneous overcast clouds on the cloud particle effective radius (re) and cloud-phase retrieval.
Retrievals performed with the Cloud Physical Properties (CPP) algorithm at high (1 × 1 km2) and low (3 × 3 km2) resolution MODIS-observed 0.6-µm and 1.6-µm reflectances indicate that low-resolution re is overestimated by up to 5 µm and up to 2 µm relative to high-resolution re for broken clouds and inhomogeneous overcast clouds, respectively. For cloud phase, the fraction of water clouds is underestimated by about 2%. The re and cloud-phase retrievals are more affected over ocean than over land surfaces, because of the larger contrast between clouds and the underlying surface and hence a larger contribution from cloud-free parts to the observed reflectance at sub-pixel level. In the future, SEVIRI cloud property retrievals might benefit from a correction for broken cloudiness using the High Resolution Visible (HRV) channel.
Mean retrieved re at high (black line) and low resolution (dashed grey line) for water clouds as retrieved from MODIS reflectances as function of cloud fraction over (top) Europe and (bottom) the Atlantic Ocean. The vertical bars denote the standard deviations within the respective cloud fraction bins. For clarity of presentation, the vertical bars for low resolution are slightly shifted to the right.