A solar reflectance method for retrieving the optical thickness and droplet size of liquid water clouds over snow and ice surfaces

Cloud optical thickness and droplet effective radius retrievals from solar reflectance measurements are traditionally implemented using a combination of spectral channels that are absorbing and nonabsorbing for water particle s. Reflectances in nonabsorbing channels (e.g., 0.67, 0.86, 1.2 mum spectra l window bands) are largely dependent on cloud optical thickness, while lon ger-wavelength absorbing channels (1.6, 2.1, and 3.7 mum window bands) prov ide cloud particle size information. Cloud retrievals over ice and snow sur faces present serious difficulties. At the shorter wavelengths, ice is brig ht and highly variable, both characteristics acting, to significantly incre ase cloud retrieval uncertainty. In contrast, reflectances at the longer wa velengths are relatively small and may be comparable to that of dark open w ater. A modification to the traditional cloud retrieval technique is presen ted. The new algorithm uses only a combination of absorbing spectral channe ls for which the snow/ice albedo is relatively small. Using this approach, retrievals have been made with the MODIS airborne simulator (MAS) imager fl own aboard the high-altitude NASA ER-2 from May to June 1998 during the Arc tic FIRE-ACE field deployment. Data from several coordinated ER-2 and in si tu University of Washington Convair-580 aircraft observations of liquid wat er stratus clouds are examined. MAS retrievals of optical thickness, drople t effective radius, and liquid water path are shown to be in good agreement with in situ measurements. The initial success of the technique has implic ations for future operational satellite cloud retrieval algorithms in polar and wintertime regions.