High Turbidity Clear Sky Model:
Validation on Data from South Africa
Dr Pierre Ineichen
University of Geneva – Institute of Environmental Sciences
Why to focus on the clear sky?
Low turbidity High turbidity
In the field of solar radiation modeling (tilt, satellite, splitting, etc.), the highest possible radiation reaching the ground for a specific site is used as a normalization function.
It has to be known with the highest possible accuracy in order to obtain bankable data sets.
It depends mainly on the atmospheric turbidity and its water vapor content.
Should be validated on trustable, high precision, long term ground measurements data sets.
The model, for worldwide online satellite irradiance evaluation, due to the temporal and spatial resolution, it should use as low as possible calculation time.
Models
CPCR2, input: b, w (aod700 < 0.65 rural aerosols)
REST2, input: b, w (aod700 < 1.7 rural aerosols)
Bird, input: aod, w (aod700 < 0.27)
ESRA, input: Linke turbidity TL2 (aod700 < 0.44)
Solis 2008, input: aod , w (aod700 < 0.45, w > 0.2 cm)
-> Solis 2017, new limits for the model Worldwide data sets
10% with aod700 > 0.45 (aod550 > 0.6, rural aerosols)
9% with water vapor column w < 0.2 cm
Limitations of the state of the art clear sky models
Solis clear sky model
Basics of Solis model
Lambert-Beer attenuation equation LibRadTran RTM spectral calculations
use of modified L-B equation due to band calculations, where to is the vertical optical depth and n evaluated at air mass M = 2)
at high aod, Io has to be enhanced and is common for the 3 components
final model derived from RTM calculations at 2 solar elevations
) (
exp t
I M
I
bn osin ) (
exp
' h
I
I
bn ot
bb
h h I
I
gh o gg) sin
( sin exp
'
t
sin ) (
exp
' h
I
I
dh
o t
ddSimplified analytical version of the Solis model
New high turbidity Solis model
LibRadTran RTM spectral calculations for 2300 combinations of aod, w and altitude for each of the 4 aerosol types: rural, urban, maritime and tropospheric
Integration of the spectral results to obtain broadband components
Best fit on the model parameters to obtain an analytical formulation of the model
Limitations of the new model:
aod700 0 -> 7 w 0.01 -> 10cm
altitude 0 -> 7000m
Ground data for the validation
Ground station acquiring simultaneously the irradiance components and the turbidity are very scarce. We used:
four “high” turbidity BSRN sites,
two south African Sauran sites,
Geneva as reference
ground measurements for Ta and HR -> w
aeronet for the aod (except for Geneva where the aod is retrofitted from Bn with CPCR2)
stringent quality control: acquisition time, physical limits, component coherence (closure, Kt/Kb, diffuse fraction), etc.)
Validation against ground data
Comparison with data from Geneva, relatively low
turbidity, Solis 2017 is slightly better than Solis 2008
Comparison with data from Durban, aod550 up to 0.6, w up to 4.5 cm
Frequency distribution of the bias is near of normal -> the first order statistics are representative
Validation against ground data
Gh bias dependence with the aod, and Bn seasonal
dependence of the model
Results for the site of Stellenbosch
Overall validation results
The South African sites present negligible bias and low
uncertainties
Better results for the global due to the higher sensitivity of the beam component to aod and w
Higher standard deviation for Ilorin that can be a result of highly variable meteorological conditions
For Xianghe, the higher
uncertainties can be due to the low quality of the data
Excel Solis tool:
http://www.adpi.ch/Solis2017/Solis2017-tool.xlsx (password: Solis2017)
