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

[Hamish]

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.

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.


> 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?

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

> 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..


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


Hamish