Regional climate research

Application of 3D Solar Radiative Transfer to Mountains

Conventional radiative transfer schemes used in weather and climate models do not allow for 3D interaction of photons with topography. To study this interaction, we developed a 3D radiative transfer model simulating solar fluxes over mountain surfaces precisely given distributions of atmospheric scatterers and absorbers.  We applied it at 1-km resolution to a heterogenous 100 km X 100 km mountainous surface in northern Washington state and examined diurnal, seasonal, and geographical variability of solar fluxes under clear skies.  As an illustration of the region's topography and the model's capabilities, this figure shows the direct solar beam (in W/m²) at spring equinox at 9 times of day.

The model quantifies not only the direct solar beam, but also diffuse, terrain-reflected, and coupling (i.e. photons reflected and scattered more than once) fluxes. This figure shows the domain-averaged contributions of each of these components to the total solar flux as a function of time of day and year. Domain-averaged direct and diffuse fluxes together comprise over 96% of the flux year round, with diffuse fluxes’ relative importance varying inversely with that of direct radiation. Direct fluxes generally account for at least 80% of the total.  However, the domain-averaged diffuse flux proportion increases to nearly 40% at high zenith angles, and approaches 100% in particular landscapes within the domain where neighboring slopes obscure the surface from the sun. Domain-averaged terrain-reflected and coupling components each account for less than 1% throughout much of the year.  However, together they comprise ~3% when surface albedo increases during winter on a domain-averaged basis, and their contribution can be as large as 10% in deep valleys throughout the year. 

We also studied controls on geographical variations in flux components. The controls on diffuse radiation are particularly critical, as this component is large and utilized most efficiently by terrestrial ecosystems.  The sky view factor, a conventional predictor of diffuse fluxes proportional to the fraction of sky visible to the target point, is surprisingly weakly correlated with them. This may be seen in scatterplots of simulated diffuse radiation vs sky view factor at summer solstice. The poor performance of the sky view factor as a predictor of geographical variability in diffuse fluxes poses a parameterization problem for diffuse radiation in complex terrain.

Finally, we assessed errors in solar radiation in General Circulation Models with smoothed topography by comparing results with the mountainous surface to identical calculations for a flat surface with the same mean elevation. The differences range from 5-20 W/m², and arise because the atmosphere absorbs a different amount of sunshine when underlying topography is smoothed.

Download the publication (Chen et al. 2006) describing these results in more detail.

Yong Chen, Alex Hall, and Kuo-Nan Liou, make up the team that performed this research.