Impact of broken and inhomogeneous clouds in satellite cloud-phase retrieval

The influence of broken and inhomogeneous clouds on satellite cloud-phase retrievals is investigated. The research comprises two parts: First, an assessment of the effect of broken and inhomogeneous water clouds on the cloud-phase retrieval is performed through an evaluation of the errors in retrieved optical thickness (τ) and effective radius (reff) using synthetic datasets. Second, a comparison of MODIS low-resolution (3x3 km) cloud properties against corresponding high-resolution (1x1 km) retrievals for low-resolution cases of broken and inhomogeneous clouds was carried out. The evaluation of the synthetic datasets reveal that, for water clouds with high values of τ and reff, a shift in cloud phase from ‘water’ to ‘ice’ can be expected at cloud fractions between 0.4 and 0.8. For overcast fields of inhomogeneous water clouds, reff is overestimated by ~3-5 μm at low-resolution as compared to high-resolution. Depending on the amount of cloud variability and the underlying surface this overestimation may lead to a shift in cloud phase from ‘water’ to ‘ice’. Although the MODIS observations do not reveal a sharp increase in reff with decreasing cloud fraction, the expected increase in reff for inhomogeneous overcast water clouds is clearly manifested in the observations.