Solar Radiative Transfer in Three-Dimensional Clouds: Exact Computations and Development of Parameterizations

Kurzfassung

The 4DCLOUDS project was established to gain a more accurate understanding of the role clouds play in radiative exchange processes between the Earth's surface and the atmosphere. To carry out this challenging task it is neccessary to tackle the issue from both sides, experimentally and theoretically. On the one hand two measurement campaigns, the Baltex Bridge Campaigns BBC1 and BBC2, have been carried out. On the other hand cloud fields have been generated statistically and physically, and multi-dimensional high-spectral-resolved radiative transfer (RT) simulations have been performed. Additionally, one-dimensional (1D) RT calculations by means of the perturbation theory (PT) have been carried out in order to derive an improved parameterization of the radiative properties of the atmosphere. Calculations of the upward, downward irradiances and absorbed fluxes at arbitrary atmospheric levels have been computed for cloud fields retrieved by combining radar, lidar, microwave and wind velocity measurements. The results of these computations have been validated with in-situ measurements of spectral flux densities performed during the campaigns mentioned above. Moreover, simulations using generated cloud fields have been performed and the results have been stored as a reference database for PT realizations. Concerning the perturbation theory, starting from 1D RT for one (or more) base cases, perturbations with respect to absorption-, scattering- and expansion-coefficients of the phase function have been dealt with so far. Results for azimuthally independent effects namely upwelling, downwelling, net and actinic flux densities have been derived. The introduction of a second base case and successive application of the so-called Hermite interpolation yields results with an error of just a few percent with respect to exact values computed by DISORT.