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Ann. Geophys., 21, 615-626, 2003
www.ann-geophys.net/21/615/2003/
© European Geosciences Union 2003
Photon path length distributions for cloudy skies – oxygen A-Band measurements and model calculations
O. Funk and K. Pfeilsticker
Institut für Umweltphysik, University of Heidelberg, Germany
Abstract. This paper addresses the
statistics underlying cloudy sky radiative transfer (RT) by inspection of the
distribution of the path lengths of solar photons. Recent studies indicate that
this approach is promising, since it might reveal characteristics about the
diffusion process underlying atmospheric radiative transfer (Pfeilsticker,
1999). Moreover, it uses an observable that is directly related to the
atmospheric absorption and, therefore, of climatic relevance. However, these
studies are based largely on the accuracy of the measurement of the photon path
length distribution (PPD). This paper presents a refined analysis method based
on high resolution spectroscopy of the oxygen A-band. The method is validated
by Monte Carlo simulation atmospheric spectra. Additionally, a new method to
measure the effective optical thickness of cloud layers, based on fitting the
measured differential transmissions with a 1-dimensional (discrete ordinate) RT
model, is presented. These methods are applied to measurements conducted during
the cloud radar inter-comparison campaign CLARE’98, which supplied detailed
cloud structure information, required for the further analysis. For some
exemplary cases, measured path length distributions and optical thicknesses are
presented and backed by detailed RT model calculations. For all cases,
reasonable PPDs can be retrieved and the effects of the vertical cloud
structure are found. The inferred cloud optical thicknesses are in agreement
with liquid water path measurements.
Key words. Meteorology and
atmospheric dynamics (radiative processes; instruments and techniques)
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