Estimation of the Impacts from Cloud Vertical Heterogeneity on Passive Ice Cloud Retrievals

Spaceborne passive instruments are widely used to infer long term ice cloud properties due to their large temporal and spatial coverage. Although ice clouds observed from active instruments (e.g., CALIOP and CloudSat) demonstrate the vertically inhomogeneous nature of such clouds, a prevailing assumption in passive cloud retrieval algorithms is that the observed scene consists of a single-layered plane-parallel cloud. Many theoretical studies showed that, on the pixel level, cloud vertical heterogeneity can have an important impact on cloud optical and microphysical property retrievals, e.g., cloud optical thickness (COT) and cloud effective radius (CER). However, it is still unclear how and to what degree cloud heterogeneity biases ice cloud macrophysical and radiative passive data records on a global scale. In this study, based on cloud particle size profiles derived from one-year of the CloudSat 2C-Ice product, we estimate potential biases of ice water path (IWP), reflective shortwave (SW) flux, and outgoing longwave radiation (OLR) from the MODIS Level 3 (MOD08) monthly mean products. Results show that ice cloud vertical heterogeneity has strong impact on the magnitude of global IWP and relatively small impact on cloudy sky radiation. In deep convection regions, IWP from MODIS retrievals have negative biases up to 50%.