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

Abstract

Abstract. 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 particles. Reflectances in nonabsorbing channels (e.g., 0.67, 0.86, 1.2 •m spectral window bands) are largely dependent on cloud optical thickness, while longer-wavelength absorbing channels (1.6, 2.1, and 3.7 •m window bands) provide cloud particle size information. Cloud retrievals over ice and snow surfaces present serious difficulties. At the shorter wavelengths, ice is bright and highly variable, both characteristics acting to significantly increase cloud retrieval uncertainty. In contrast, reflectances at the longer wavelengths are relatively small and may be comparable to that of dark open water. A modification to the traditional cloud retrieval technique is presented. The new algorithm uses only a combination of absorbing spectral channels for which the snow/ice albedo is relatively small. Using this approach, retrievals have been made with the MODIS airborne simulator (MAS) imager flown aboard the high-altitude NASA ER-2 from May to June 1998 during the Arctic FIRE-ACE field deployment. Data from several coordinated ER-2 and in situ University of Washington Convair-580 aircraft observations of liquid water stratus clouds are examined. MAS retrievals of optical thickness, droplet effective radius, and liquid water path are shown to be in good agreement with in situ measurements. The initial success of the technique has implications for future operational satellite cloud retrieval algorithms in polar and wintertime regions. Previous algorithms for retrieving cloud optical thickness and effective particle radius from reflectance measurements have used spectral bands that are both absorbing and nonabsorbing for water particles. In particular, these are generally bidirectional reflectance measurements, i.e., a function of both solar and viewing angles (as opposed to flux reflectance or albedo measurements). Reflectances in nonabsorbing channels (e.g., 0.67, 0.86, 1.2 •m spectral window bands) are largely dependent on cloud optical thickness. Only a relatively small decrease in reflectance occurs with increasing particle size due to a corresponding slight increase in the particle forward scattering (characterized by the scattering asymmetry parameter). However, reflectances at longer wavelengths (1.6, 2.1, and 3.7 •m window bands, collectively referred to as near-infrared in this paper) are extremely sensitive to cloud particle absorption. The single-scattering albedo (difference between unity and fractional particle absorption) decreases in the 2.1 •m band from about 0.99 to 0.96 as effective radius increases from 5 to 20 •m. Since particle absorption is proportional to effective radius, this sensitivity to single-scattering albedo provides information on cloud particle size. Such a combination of absorbing and nonabsorbing reflectance measurements is advan-15,185

3-D polarised simulations of space-borne passive mm/sub-mm midlatitude cirrus observations: a case study

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