Parameterisations of cloud cover in simple radiation models. As used in the class of Hydrology at the University of Trento

1. Riccardo Rigon
IlSole,F.Lelong,2008,ValdiSella
Solar Radiation
Considering Clouds

2. R. Rigon
Transmitted direct
radiation at the surface
before clouds correction
Transmitted direct
radiation at the surface
after clouds correction
Hydrologists (and not only them) treat the
influence of clouds separately
It is assumed that the effects of
clouds is an attenuation of the
transmitted solar radiation
2
Considering Clouds

3. R. Rigon
Correction coefficient for
diffuse radiation
Hydrologists (and not only them) treat the
influence of clouds separately
An analogous formulation holds for
diffuse radiation:
3
Considering Clouds

4. R. Rigon
Measured total radiation
at the ground station i
Estimation of the reduction coefficients
(decomposition model)
These reduction coefficients can be
determined when we have ground
measurements of total radiation,
diffuse plus direct:
4
Considering Clouds

5. R. Rigon
reduction coefficient for
diffuse radiation
Estimation of the reduction coefficients
(decomposition model)
These assumption that is often
made is that, the diffuse solar
radiation measured at the station is
proportional to the total radiation:
5
Considering Clouds

6. R. Rigon
Estimation of the reduction coefficients
(decomposition model)
Therefore when substituting this
diffuse radiation expression in the
total radiation equation of previous
slides, it results at stations:
6
Considering Clouds

7. R. Rigon
Estimation of the reduction coefficients
(decomposition model)
And, for the direct radiation, at
stations:
7
Considering Clouds

8. R. Rigon
The key factor is the to determine the above coefficient, on which the
procedure followed so far has moved all the unknown.
Its estimation pass through various parameterizations:
Among the most known:
•Erbs et al., 1982
•Reindl et al. 1990
•Boland et al. 2001
please find the details in Formetta et al., 2012
8
Considering Clouds

9. R. Rigon
One more issue
With the help of the parameterizations above, the correction facotrs are
determined for the stations. Which are a few points in a rugged terrain.
How do you solve the problem to transport it everywhere ?
9
Considering Clouds

10. R. Rigon
We need to use
some interpolation
technique
Like Kriging* or the Inverse distance weighting method** which is not
the matter of the present slides.
* Goovaerts, 1997
**Shepard, 1968
10
Considering Clouds