Using passive remote sensing to retrieve the vertical variation of cloud droplet size in marine stratocumulus: An assessment of information content and the potential for improved retrievals from hyperspectral measurements

We present a theoretical study into the information content of solar reflectance measurements at multiple near-infrared wavelengths in the context of deriving the vertical variation of cloud droplet size from marine stratocumulus. We employ a Bayesian optimal estimation approach to retrieve a profile of droplet effective radius as a function of optical depth in the cloud. This allows an assessment of the information content of the measurement of reflectance at different wavelengths and the potential for a new generation of hyperspectral sensors to improve the retrieval. Our results show that using the three absorbing near-infrared channels of the Moderate Resolution Imaging Spectroradiometer (MODIS) centered at 1.6, 2.1 and 3.7 m a retrieval of droplet sizes lower in the cloud is highly sensitive to small changes in reflectance. Consequently instrument and modeling errors must be reduced to