Measurement of Photon Path Length Distributions For Validation of Radiative Transfer Models

High resolution DOAS spectroscopy of the Oxygen A-band (760-780 nm) in zenith- scattered light is a powerful tool to infer path length distributions (PDF) of solar pho- tons transmitted to the ground. Solar photon PDF's thus provide information on mul- tiple scattering statistics of cloudy sky radiative transport (RT). When combining so- lar photon PDF's measurements with simultaneous RADAR measurements of the 3D cloud structure and radiometric measurements of the Liquid Water Path (LWP) effects of the 3-D cloud structure on the RT can be studied thoroughly. One strong indicator for 3-D effects on the cloudy sky RT is the investigation of the mean total photon path length ( <L>) as a function of observed parameters such as the cloud vertical exten- sion (H), and cloud optical thickness () or total LWP, where the asymmetry parame- ter (g) and the effective cloud droplet radius being cloud type dependent parameters. While classical 1-D RT model of homogeneous clouds predict < L >= (1-g)· ·H, real cloud covers often show < L >= [(1 - g) · ] · H with being the Levy- -1