Satellite retrieval of the liquid water fraction in tropical clouds between −20 and −38 °C

[摘要] This study describes a satellite remote sensing methodfor directly retrieving the liquid water fraction in mixed phase clouds, andappears unique in this respect. The method uses MODIS split-window channelsfor retrieving the liquid fraction from cold clouds where the liquid waterfraction is less than 50% of the total condensate. This makes use of theobservation that clouds only containing ice exhibit effective12-to-11 μm absorption optical thickness ratios (β_eff) that arequasi-constant with retrieved cloud temperature T. This observation was madepossible by using two CO₂ channels to retrieve T and then using the 12and 11 μm channels to retrieve emissivities and β_eff. Thusfor T < −40 °C, β_eff is constant, but for T > −40 °C,β_eff slowly increases due to thepresence 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 insitu measurements are needed to refine and validate these retrievals. Suchliquid levels are shown to reduce the cloud effective diameter D_e suchthat cloud optical thickness will increase by more than 50% for a givenwater path, relative to D_e corresponding to pure ice clouds. Suchretrieval information is needed for validation of the cloud microphysics inclimate models. Since low levels of liquid water can dominate cloud opticalproperties, tropical clouds between −25 and −20 °C may besusceptible to the first aerosol indirect effect.

[学科分类] 几何与拓扑