Radiative Transfer

Three-dimensional radiative transfer problems

Except for very few occations when the sky is not covered by clouds, a one-dimensional description of the atmosphere is usually not appropriate. Although several three-dimensional questions can still be solved using one-dimensional models (e.g., the independent pixel approach for broken clouds), a three-dimensional model opens the doors for a variety of interesting and important investigations, like the effects of realistic clouds compared to one-dimensional cloud layers, the radiation over a partly snow-covered surface, or the interpretation of measurements in mountainous terrain.

Model development

I am probably not old enough to talk about my work of a lifetime. But if asked, it would certainly answer that's the development of the freely available libRadtran (library for Radiative transfer) package and the three-dimensional Monte Carlo model MYSTIC for investigations of realistic clouds, inhomogeneous surface albedo, and topography. libRadtran originated from the uvspec model by Arve Kylling. In 1996 we joined forces and have been developing libRadtran since then. libRadtran is a very flexible too which has been used for numerous purposes, as is documented by the large list of publications which used libRadtran. As one example of the beautiful things one can do, see a simulation of a sky radiance distribution, or the simulation of the backscatter glory for remote sensing purposes by [Mayer et al., 2004] and [Mayer and Emde, 2007].

Measurement and modeling of spectral UV irradiance at Garmisch-Partenkirchen

In my PhD thesis I combined experimental and theoretical methods to investigate the transfer of ultraviolet radiation through the Earth's atmosphere. After showing that a radiative transfer model can be used simulate the measurements with about ± 5-10% accuracy, the model was applied as a tool to quantify the influence of aerosols, surface albedo, and clouds on surface UV irradiance.

Satellite-based calculation of global spectral irradiance

We use observations of total ozone, cloudiness, and snow cover from different satellite instruments to calculate global distributions of spectral irradiance. Comparisons between different approaches and with measurements show good agreement.

Actinic flux and photolysis frequencies

The photolyses of nitrogen dioxide, NO₂ -> NO + O(³P), and ozone, O₃ -> O₂ + O(¹D), have been shown to be the most important reactions controlling tropospheric chemistry. Accurate knowledge of the actinic flux which controls these reaction rates is therefore essential for all tropospheric chemistry models. As chemistry is not confined to the surface, the actinic flux needs to be calculated as a function of altitude. This is a much greater challenge than calculating surface radiation quanitities, especially when satellite-based observations of clouds are used as input. One of the main aims of these studies is the evaluation of the simple parameterizations used by current chemistry models.