[GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006 [resolved]

[Dylan Beaudette]

On 12/7/06, Hamish <hamish_nospam at yahoo.com> wrote:
> Dylan Beaudette wrote:
> > > > > Get local Linke values from the SoDa Solar radiation Databases
> > > > >   http://www.soda-is.com/
> > > >
> > > > Tried this, and got unrealistic values for my site.
> > >
> > > how so? Have you calculated good Linke values from your light
> > > meters? (formula for that, please?)
> ..
> > Attached is a plot of estimated Linke turbidity values, compared to
> > the SoDa  derived data. Details on this calculation soon to follow in
> > a new thread.

Hi Hamish,

Looks like I sent an update this morning with an attachment that was too large!

I'll Include it at the bottom of my replies to you last message.


> I assume the physical basis for the Linke turbidity factor is the amount
> of scattering dust, ash, water vapour, turbulence etc. in either the
> column of air directly above the sampling site or in the ray between the
> site and the Sun (which will pass through more atm in winter due to the
> lower angle). Maybe the paper sheds some light on that. So e.g. LA
> county would have very high turbidity numbers.
>
> I would figure in the winter the air to be crisper/clearer due to less
> water vapour, less turbulence, and less upwelling of aerosol laiden warm
> air from the Earth's surface. So lower turbidity numbers in the winter
> as the SoDa data shows. BUT if you live in a place which is prone to
> inversion layers in winter and many locals heat their homes with wood or
> coal, winter air quality may turn out to be much more turbid.
>

Actually have a couple good papers on the defintion for the Linke
turbidity values:

Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity factor
Solar Energy, 1986, 37, 393-396

Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
climatic parameters Solar Energy, 2000, 69, 99-111

Kasten, F. The linke turbidity factor based on improved values of the
integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-244

My site is in a mediterranean, semi-coastal / semi-arid setting where
winter time moisture and summer time dryness are the usual weather
patterns. Summer through fall are usually cloud free, and quite warm.
Also- this site is rather remote, so urban ag-related dust influences
are minimal. These are some of the reasons that I was having second
thoughts about the SoDa values.

>
> > I have calculated Linke turbidity for my site with the equation
>
> T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )
>
> > Unfortunately, there are some aspects to this equation which
> > I do not understand: namely the two constants present.
>
> perhaps they are empirically derived?

Ok- big change in direction. I posted an update to the use of the
above equation earlier today but it was rejected due to a large
attachment. Details below....

> I take it I_0 is pure-clear air intentensity?; h is latitude?; and is G
> measured intensity, or ..?

G = global radiance measured on the ground (beam+diffuse)
I_o = extraterrestrial radiance
h = solar elevation (angle)

> > 1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
>
> I will have to track down a copy of that. Holy cow, for once our library
> is actually subscribed to the e-journal. Downloading the PDF now..
>

... i know how that can be sometimes!

> What software did you use to make that plot? Looks nice.
>

Thanks. It was made in R, a fun command-line based environment for
plotting, analysis, etc. I can post the code used after a small
cleanup.

Ok, here is the text from the message rejected this morning:


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 appropriate approach to calculating Linke turbidity values.

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)

here is a link to a bit of my thesis on this topic: (rather incomplete!)
http://169.237.35.250/~dylan/temp/linke-cucumo2000.png

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