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b'This thesis addresses two major problems in the field of radiative transfer (RT) in the\\nearth\\u2019s atmosphere. The first problem is linked with the need for significant computational\\nresources of RT in a three-dimensional (3D) atmospheric model. Although only highly\\nefficient one-dimensional (1D) RT models are employed for each pixel of the model domain\\nseparately and independently, it is still not possible to utilize these models on a frequent\\nbasis, compared to the rate at which meteorological variables are computed. That means\\nthat the calculated radiative properties (RP) are held constant for a longer period of time,\\nwhile the prognostic meteorological variables are updated at a rapid rate. Even though\\nthere is no detailed study about the consequences of this disproportion, an attempt was\\nmade to develop an RT model which permits the fast computation of basic radiative transfer\\nproperties which could be used in the future to update this information more frequently.\\nThe developed model is based on the application of the radiative transfer perturbation\\ntheory to realistic cloud fields column by column. It turned out that the application,\\nintended to replace the Independent Pixel Approximation (IPA), see below, is possible\\nand promising within the assumptions and constraints of the utilized methods. It could\\nbe demonstrated that, depending on the actual case, errors in the pixel transmission and\\nreflection stay bounded to values of up to 10%\\u221215%. In one case the achieved acceleration\\ncould be investigated. It was about a factor of four compared to the direct application of\\nthe usual forward variant of the model, although no numerical optimization was carried\\nout.\\nThe second problem concerns the realistic treatment of the 3D interactions of clouds and\\nsolar radiation. As implied in the above paragraph, 1D RT models are usually employed\\ncolumn by column which suppresses the exchange of radiation between those columns.\\nThus, fundamental 3D effects are neglected by this so-called Independent Pixel Approximation\\n(IPA). These comprise not only small scale contributions due to diffuse radiative\\ntransport, but also large scale patterns like geometric effects of the inclined solar illumination.\\nExamples are blurred radiative structures due to radiative smoothing and the\\nshifted location of shadows and bright areas. To parameterize those effects strong efforts\\nhave been undertaken during the last couple of years. However, no method has proven to\\nbe completely satisfactory and ready for implementation. To carry this research one step\\nfurther two approaches have been adopted and extended. The first is the concept of the\\nTilted Independent Pixel Approximation (TIPA). In contrast to the IPA, which ignores the\\nsolar geometry, this method correctly accounts for the slant illumination due to the correct\\ntracking of the direct beam. As a result, the optical parameters in the slant columns are\\narranged in a more realistic order and the attenuation and the positions of the RP are less\\nerroneous. To further improve this method a transformation has been developed which\\nyields 3D resolution of the RP in the original grid. Since the TIPA still does not include\\nany diffuse radiative exchange as another approach the Nonlocal Independent Pixel Approximation\\n(NIPA) has been explored. This technique uses 1D results and carries out\\na convolution product to distribute RP across column boundaries. In order to arrive at\\na fully independent treatment of this method a simplified derivation of the convolution\\nparameters was developed. Finally, TIPA and NIPA are combined to form NTIPA. These\\napproaches have proven to be superior to IPA with respect to several aspects. The improvement\\nranges from several percent to 50% if maximum errors of the transmitted and\\nreflected light are considered. Criteria like the distribution of the errors or the vertical\\nprofiles of the RP are also more preferable than their counterparts derived by IPA.'