Mitchell, DL; d'Entremont, RP (2012). Satellite retrieval of the liquid water fraction in tropical clouds between -20 and -38 degrees C. ATMOSPHERIC MEASUREMENT TECHNIQUES, 5(7), 1683-1698.
This study describes a satellite remote sensing method for directly retrieving the liquid water fraction in mixed phase clouds, and appears unique in this respect. The method uses MODIS split-window channels for retrieving the liquid fraction from cold clouds where the liquid water fraction is less than 50% of the total condensate. This makes use of the observation that clouds only containing ice exhibit effective 12-to-11 mu m absorption optical thickness ratios (beta(eff)) that are quasi-constant with retrieved cloud temperature T. This observation was made possible by using two CO2 channels to retrieve T and then using the 12 and 11 mu m channels to retrieve emissivities and beta(eff). Thus for T < -40 A degrees C, beta(eff) is constant, but for T > -40 A degrees C, beta(eff) slowly increases due to the presence of liquid water, revealing mean liquid fractions of 10% around -22 A degrees C from tropical clouds identified as cirrus by the cloud mask. However, the uncertainties for these retrievals are large, and extensive in situ measurements are needed to refine and validate these retrievals. Such liquid levels are shown to reduce the cloud effective diameter D-e such that cloud optical thickness will increase by more than 50% for a given water path, relative to D-e corresponding to pure ice clouds. Such retrieval information is needed for validation of the cloud microphysics in climate models. Since low levels of liquid water can dominate cloud optical properties, tropical clouds between -25 and -20 A degrees C may be susceptible to the first aerosol indirect effect.