Photon Path Lengths Distributions for Cloudy Skies : Their first and second-order moments inferred from high resolution oxygen A-Band spectroscopy
Modelling radiative transfer in cloudy skies is one of the most challenging tasks in climate modelling. The goal of this thesis is to achieve an improved understanding of photon transport inclouds. For this purpose, photon path length distributions are retrieved from spectroscopic oxygen A-band measurements (760-780 nm). An existing measurement method is improved and optimized. In comparison to former studies, it is now possible to measure single skylight spectra within a few seconds of one another using modern CCD detectors and optimized instruments. This enables a reduction of the spatial resolution to scales smaller than the radiative smoothing scale. Two different studies are performed for the validation of the method. It is shown, that a distribution function must be assumed for the correct description of the measured spectra. Furthermore, an absorption spectrum is calculated using a Monte Carlo radiative transport model. This spectrum is used to retrieve a path length distribution, which is subsequently compared to the model result. The spectroscopic measurements for the photon path length distribution retrieval were performed simultaneously with other in-situ measurements during two dedicated European cloud research campaigns (BBC1 and BBC2) held at the KNMI experimental site in Cabauw/Netherlands in September 2001 and May 2003. The measurement of other cloud parameters like optical thickness, liquid water content and geometrical cloud extension makes it possible to experimentally prove a new theory of photon diffusion of cloudy sky for the first time. The presented observations provide (further) evidence for the anomalous diffusion transport of solar photons being transmitted by complicated cloud covers. It is also shown, that Levy walks are well suited for the description of photon transport in clouds.
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