On the impacts of cloud horizontal inhomogeneity and warm rain processes on passive remote sensing of cloud droplet size: A integrated study based on large-eddy simulation model and satellite observations

Low-level warm marine boundary layer (MBL) clouds have a significant, and yet not well-understood role, in Earth's radiative energy balance and hydrological cycle. Improving our understanding relies heavily on global continuous observation of MBL cloud properties from satellite sensors. A well-documented passive remote sensing method to simultaneously retrieve cloud droplet size and cloud optical thickness is the so-called bi-spectral solar reflective method [Nakajima and King, 1990]. Several widely used satellite cloud products, including MODIS operational cloud product, is based on this method. Another passive method utilizes the multi-angle polarization measurement of supernumerary-rainbow, from for example POLDER, to retrieve MBL cloud droplet size .

Recently, we have developed a satellite retrieval simulator based on the combination of radiative transfer models (both 1-D and 3-D) and large-eddy simulation (LES) model to simulate MODIS-like and POLDER-like MBL cloud property retrievals. Using this simulator, in combination with satellite observations from A-Train sensors, we investigated how cloud horizontal inhomogeneity and warm rain processes influence the passive remote sensing of MBL cloud droplet size. We found that, the cloud horizontal inhomogeneity can lead to various 3-D radiative transfer effects [Zhang and Platnick, 2011; Zhang et al., 2012] that affect the spectral and polarization methods to different degrees and in different ways. We also found that the warm rain process can induce changes in cloud vertical structure and cloud droplet size distribution, which in turn affects the cloud droplet size retrievals from passive sensors.

Nakajima, Teruyuki, Michael D. King, 1990: Determination of the Optical Thickness and Effective Particle Radius of Clouds from Reflected Solar Radiation Measurements. Part I: Theory. J. Atmos. Sci., 47, 1878–1893

Zhang, Z., A. S. Ackerman, G. Feingold, S. Platnick, R. Pincus, and H. Xue (2012), Effects of cloud horizontal inhomogeneity and drizzle on remote sensing of cloud droplet effective radius: Case studies based on large-eddy simulations, J Geophys Res, 117(D19), D19208–, doi:10.1029/2012JD017655.

Zhang, Z., and S. Platnick (2011), An assessment of differences between cloud effective particle radius retrievals for marine water clouds from three MODIS spectral bands, J Geophys Res, 116(D20), D20215, doi:10.1029/2011JD016216.