Yesterday was a nice clear day, so I was able to quantify the extent of the direct sun blockage on the radiometers at Thunderhead. In the morning, the sun cleared the mountains at 1424 UTC (07:24 local time). In the afternoon, the lodge and lift housing block the sun starting at 13 degrees elevation, currently at 2303 UTC (16:03 local), then the sun becomes unblocked at 7 degrees elevation, currently at 2337 UTC (16:37 local).
Given the givens, that’s not too bad. As I mentioned before, any radiative closure efforts are bound to avoid times of low sun elevations. So, looks like we’re good all around at Thunderhead for solar elevations greater than 15 degrees. — Chuck
3 responses to “How Low Can We Go?”
A nice example of the “compromises” one must make in a complex terrain deployment while still getting representative measurements of the local environment.
Chuck,
could you blow up the diffuse a bit? It appears to me that it is increasing in the shadows, I would have expected the diffuse to equal the total since there is no direct in the shadows.
Rick,
You’re paying attention! The light blue line labeled “SWdif” in the plot is actually an approximation based on the SPN-1 diffuse-to-total ratio applied to the unshaded pyranometer (PSP)measurements. However, I have not yet normalized the SPN-1 to the unshaded pyranometer for this high elevation deployment yet (needed to account for the differing cosine and specrtal responses between the two differing radiometer designs). So the light blue line is based on the low altitude normalization, and is a bit off for this plot. The actual measured unshaded pyranometer total (SWdn), SPN-1 total (SPNdn) and SPN-1 diffuse all agree and are on top of one another in the plot.
I probably shouldn’t have included the estimated light blue line in this plot yet, given it is not normalized. But it is a “working” plot for daily quality screening of the data…