An assessment of cloud liquid water path and droplet number concentration retrievals derived from MODIS-like observations: Comparison with large-eddy simulation and implications for aerosol indirect effect study

The cloud liquid water path (LWP) and cloud droplet number concentration (CDNC) are two key parameters in the study of aerosol-cloud interaction. The objective of this research is to identify and understand the uncertainty sources in MODIS-like LWP and CDNC retrievals. First, we use radiative transfer models (both 1D and 3D) to simulate the MODIS observation from large-eddy simulations (LES). Then, LWP and CDNC are retrieved from the simulated observation. Finally, we compare the retrieved and LES cloud fields to identify sources of uncertainties. Three major sources of uncertainty are identified: 1) cloud vertical structure, 2) cloud horizontal inhomogeneity and 3) 3-D radiative transfer effects. An intriguing finding is that in many cases these uncertainties act to cancel one another leading to reduced impacts on the retrieval. The implications for aerosol-cloud interaction study, in particular Twomey effect, will be discussed.