Abstract
Abstract. 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 μm absorption optical thickness ratios (βeff) 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 μm channels to retrieve emissivities and βeff. Thus for T < −40 °C, βeff is constant, but for T > −40 °C, βeff slowly increases due to the presence of liquid water, revealing mean liquid fractions of ~ 10% around −22 °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 De such that cloud optical thickness will increase by more than 50% for a given water path, relative to De 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 °C may be susceptible to the first aerosol indirect effect.
Highlights
While much attention has been paid to boundary layer clouds and cirrus clouds, relatively little research has been conducted on mid-level non-precipitating clouds, even though altocumulus and altostratus clouds cover 22 % of the Earth’s surface (Warren et al, 1988)
A new satellite remote sensing method has been described for retrieving the cloud temperature and emissivities in two split-window channels (11 and 12 μm)
This differed from other studies in that all three of these properties were retrieved independently without estimating temperature from a nearby optically thick cloud evaluated as a “black body” emitter
Summary
While much attention has been paid to boundary layer clouds and cirrus clouds, relatively little research has been conducted on mid-level non-precipitating clouds, even though altocumulus and altostratus clouds cover 22 % of the Earth’s surface (Warren et al, 1988). Satellite remote sensing studies have detected liquid water between −20 and −36 ◦C at all latitudes (Yoshida et al, 2010; Hu et al, 2010), compiling global statistics on the frequency of occurrence of these mixed phase clouds, as well as the temperature dependence of their liquid water content and path. These studies did not retrieve an estimate of the mean liquid water fraction in a cloud field, which is the goal of this study.
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