Solar Radiation model - Diffuse Model Type

[toddsams]

I am exploring the parameters that control the amount of diffuse radiation in the Solar Radiation tools, specifically, the "diffuse model type" and "diffuse_proportion".

The attached charts show comparisons between different values for these parameters. The top chart shows the diffuse radiation with diffuse_proportion set to 0.3 and 0.5. The results show what I would expect with more diffuse radiation with a value of 0.5.

However, the bottom chart that shows the comparison between the two different diffuse model types (Overcast vs. Uniform Sky) does not seem to make sense. For all sites, it shows the Overcast model giving more diffuse radiation than the Uniform Sky model. This difference is even more pronounced at sites near ridgelines.

Why would an overcast sky produce more diffuse radiation near the ridgeline than in a valley bottom?

[ShaunWalbridge]

Todd,

I think your results line up as expected. The two models differ in how they handle diffuse radiation, as described in the 1999 Fu and Rich paper:

In a uniform diffuse model, sometimes referred to as a "uniform overcast sky (UOC)", incoming diffuse radiation is the same from all sky directions. In a standard overcast (SOC) diffuse model, diffuse radiation flux varies with zenith angle.

So, in the uniform model, zenith angle has no role, so valleys will receive similar diffuse radiation to peaks, because as long as the sky map isn't blocked, either case will get the same diffuse radiation. Perhaps the best way to build up an intuition for this is to play with the formulae provided in the Solar Analyst user guide, pages 11-12, to see how the results differ for individual sky sectors.

cheers,
Shaun