Scheirer, Ronald (2001) Solarer Strahlungstransport in der inhomogenen Atmosphäre. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Germany, 123 pp. . Berichte aus dem Institut für Meereskunde an der Christian-Albrechts-Universität Kiel, 322 . DOI 10.3289/ifm_ber_322.

Preview

Text IFM-BER_322.pdf - Reprinted Version Available under License German copyright act UrhG. Download (4MB) | Preview

Abstract

A most profound knowledge about the radiative characteristics of clouds is required for the development of realistic atmospheric circulation models and cloud remote sensing algorithms. At present, cloud fields are treated extremely simplified in both application areas. Cloud radiative flux parameterizations in atmospheric circulation models as well as the correlation between radiance and cloud properties as required for remote sensing algorithm are usually based on the assumption of plane-parallel homogeneous (PPHOM) clouds. Compared to realistically 3D cloud fields, this simplification leads to large systematic errors. In order to quantify these errors a Monte Carlo radiative transfer model has been developed and applied to 3D cloud fields. The latter origin from the non-hydrostatic 3D atmospheric model GESIMA. Absorption and scattering properties of the cloud particles have been calculated by means of Mie-theory for spherical water droplets and a ray-tracing code for non-spherical ice, rain, and snow particles. Line by line calculations have been used to obtain the absorption properties of the relevant atmospheric gases. It is shown that accounting for horizontally inhomogeneous distribution of water vapor does not lead to noticeable improvements in calculating the radiative fluxes compared to using horizontally stratified water vapor fields. Differences in upward reflected and in absorbed solar broadband flux are less than 0.1 % of the total incoming flux. A larger influence is due to the non-stratified structure of cloud hydrometeors. Investigations in the UV-A spectral band for random solar azimuth angle provide errors in domain averaged direct (total) downward solar fluxes up to 49 % (32 %) of the incoming flux for the PPHOM approximation, whereas the use of radiatively independent atmospheric columns (independent column approximation, ICA) yields errors up to 6 % (2 %). Similar investigations for the entire solar spectral range and for fixed solar azimuth show errors in upward reflected flux (absorption) exceeding 17 % (4 %) for the PPHOM approximation and 2 % (1 %) for the ICA. Largest deviations are found for the most inhomogeneous convective cloud fields. Errors due to the ICA are very sensitive to horizontal resolution and solar zenith angle. It is proposed that presently used correction schemes (artificial reduction of cloud optical thickness) for radiative flux parameterizations based on the PPHOM assumption or the ICA should be derived as a function of these parameters.

Actions (login required)

View Item