Solar radiative transfer parameterizations for three-dimensional effects in cloudy atmospheres

摘要

This thesis addresses two major problems in the field of radiative transfer (RT) in theearth’s atmosphere. The first problem is linked with the need for significant computationalresources of RT in a three-dimensional (3D) atmospheric model. Although only highlyefficient one-dimensional (1D) RT models are employed for each pixel of the model domainseparately and independently, it is still not possible to utilize these models on a frequentbasis, compared to the rate at which meteorological variables are computed. That meansthat the calculated radiative properties (RP) are held constant for a longer period of time,while the prognostic meteorological variables are updated at a rapid rate. Even thoughthere is no detailed study about the consequences of this disproportion, an attempt wasmade to develop an RT model which permits the fast computation of basic radiative transferproperties which could be used in the future to update this information more frequently.The developed model is based on the application of the radiative transfer perturbationtheory to realistic cloud fields column by column. It turned out that the application,intended to replace the Independent Pixel Approximation (IPA), see below, is possibleand promising within the assumptions and constraints of the utilized methods. It couldbe demonstrated that, depending on the actual case, errors in the pixel transmission andreflection stay bounded to values of up to 10%−15%. In one case the achieved accelerationcould be investigated. It was about a factor of four compared to the direct application ofthe usual forward variant of the model, although no numerical optimization was carriedout.The second problem concerns the realistic treatment of the 3D interactions of clouds andsolar radiation. As implied in the above paragraph, 1D RT models are usually employedcolumn by column which suppresses the exchange of radiation between those columns.Thus, fundamental 3D effects are neglected by this so-called Independent Pixel Approximation(IPA). These comprise not only small scale contributions due to diffuse radiativetransport, but also large scale patterns like geometric effects of the inclined solar illumination.Examples are blurred radiative structures due to radiative smoothing and theshifted location of shadows and bright areas. To parameterize those effects strong effortshave been undertaken during the last couple of years. However, no method has proven tobe completely satisfactory and ready for implementation. To carry this research one stepfurther two approaches have been adopted and extended. The first is the concept of theTilted Independent Pixel Approximation (TIPA). In contrast to the IPA, which ignores thesolar geometry, this method correctly accounts for the slant illumination due to the correcttracking of the direct beam. As a result, the optical parameters in the slant columns arearranged in a more realistic order and the attenuation and the positions of the RP are lesserroneous. To further improve this method a transformation has been developed whichyields 3D resolution of the RP in the original grid. Since the TIPA still does not includeany diffuse radiative exchange as another approach the Nonlocal Independent Pixel Approximation(NIPA) has been explored. This technique uses 1D results and carries outa convolution product to distribute RP across column boundaries. In order to arrive ata fully independent treatment of this method a simplified derivation of the convolutionparameters was developed. Finally, TIPA and NIPA are combined to form NTIPA. Theseapproaches have proven to be superior to IPA with respect to several aspects. The improvementranges from several percent to 50% if maximum errors of the transmitted andreflected light are considered. Criteria like the distribution of the errors or the verticalprofiles of the RP are also more preferable than their counterparts derived by IPA.