Determination of optical and microphysical properties of thin warm clouds using ground based hyper-spectral analysis

摘要

Clouds play a critical role in the Earth's radiative budget as they modulatethe atmosphere by reflecting shortwave solar radiation and absorbing longwave IR radiation emitted by the Earth's surface. Although extensivelystudied for decades, cloud modelling in global circulation models is farfrom adequate, mostly due to insufficient spatial resolution of thecirculation models. In addition, measurements of cloud properties still needimprovement, since the vast majority of remote sensing techniques arefocused in relatively large, thick clouds. In this study, we utilize groundbased hyperspectral measurements and analysis to explore very thin waterclouds. These clouds are characterized by liquid water path (LWP) that spansfrom as high as ~50g m?2 and down to 65 mg m?2 with a minimumof about 0.01 visible optical depth. The retrieval methodology relies onthree elements: a detailed radiative transfer calculations in the longwaveIR regime, signal enhancement by subtraction of a clear sky reference, andspectral matching method which exploits fine spectral differences betweenwater droplets of different radii. A detailed description of the theoreticalbasis for the retrieval technique is provided along with a comprehensivediscussion regarding its limitations. The proposed methodology was validatedin a controlled experiment where artificial clouds were sprayed and theireffective radii were both measured and retrieved simultaneously. Thismethodology can be used in several ways: (1) the frequency and opticalproperties of very thin water clouds can be studied more precisely in orderto evaluate their total radiative forcing on the Earth's radiation budget.(2) The unique optical properties of the inter-region between clouds (clouds'"twilight zone") can be studied in order to more rigorously understanding ofthe governing physical processes which dominate this region. (3) Since theoptical thickness of a developed cloud gradually decreases towards itsedges, the proposed methodology can be used to study the spatialmicrophysical behaviour of these edges. (4) A spatial-temporal analysis canbe used to study mixing processes in clouds' entrainment zone.