[GRASS-user] r.sun and linke turbidity values

[Dylan Beaudette]

Hi,

Some updates on my findings with respect to obtaining optimal Linke
turbidity values via local pyranometer measurements.

Chatting with a local atmospheric scientist, I learned that it is
possible (with a loss of some precision of course) to dissaggregate
beam radiance from global radiance (as measured by pyranometer), for
use in an equation [1] for the linke turbidity factor. Compared to a
previous paper I mentioned on grass-dev [2], the equation presented in
[1] is a much more generalized- and therefore globally appropriate
estimator.

The key points to using the equation from [1] are:

- historical pyranometer data from a weather station, with coordinates!
- r.sun mode 1 to calculate solar elevation at this station, for each
day at some time. extraterrestrial radiance is also calculated in this
step
- dissagregation of the beam component from global radiance with the
assumption that diffuse radiance is approx 10%-20% that of the beam
radiance
- conversion of dissaggregated beam radiance to beam radiance on the
normal (B/sin alpha)

I have attached a PDF of a more legible summary.

Here is a link of linke turbidity values, as calculated by the method
described above, along with a comparison to the SoDa-derived values.
There is considerable difference between the two sources!

http://169.237.35.250/~dylan/temp/cucumo-estimated_daily_linke_values.png

With the equation from [1] it is possible to estimate the linke
turbidity value for each day (using the 12pm calculation as
representative for an entire day), and use these estimates in r.sun
mode 2. here is a summary of the incorporation of estimated linke
values into r.sun - as compared to the weather station data:

http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png


I'll post some updates as they come. If I have anything wrong please
don't hesitate to point it out!

Cheers,

Dylan


1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method
for the estimation of the Linke turbidity factor Renewable Energy,
2000, 19, 249-258

2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
Israel International Journal of Climatology, 2001, 21, 1561 - 1576
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