Five satellite products deriving beam and global irradiance

Five satellite products derivingbeam and global irradiance validation

on data from 23 ground stations

Pierre IneichenUniversity of Geneva

February 2011

Irradiance map produced by 3Tier

Five satellite products deriving beam and global irradiance against ground data validationP. Ineichen

- 39 -

Five satellite products derivingbeam and global irradiance validation

on data from 23 ground stations

Pierre IneichenUniversity of Geneva

February 2011

Abstract

Models converting satellite images into the different radiation components becomeincreasingly performing and give often better estimation of the solar irradiance availabilitythan ground measurements if the station is not situated in the near vicinity of theapplication.

Five different satellite products deriving both global and beam irradiance are validatedagainst data from 23 ground sites. The main conclusions are:

• the global irradiance is retrieved with a negligible bias and an average standarddeviation around 16% for the best algorithm. For the beam irradiance, the bias isaround several percents, and the standard deviation around 35%,

• the main deviation comes from the knowledge of the aerosol optical depth,

• the high latitude sites give not poorer results than the other sites,

The interannual variability of the irradiance conditions, the lack of independent groundmeasurements such as aerosol data, the difficulty to assess the exact calibration of theground data, and the choice of a specific year to carry out the validation, conduct toresults that give good indications, but from which it is difficult to draw general conclu-sions.


- 1 -

1. Introduction

Models converting satellite images into the different radiation components becomeincreasingly performing and give often better estimation of the solar irradiance availabilitythan ground measurements if the station is not situated in the near vicinity of theapplication (Zelenka, 1999). If the global irradiance can be derived with a good accuracy,it is more difficult for the beam component and the dispersion of the models is higher.

The aim of the present study is to validate and compare five different products derivingthe global and beam irradiance components from meteorological satellite images. It is acomplement to a previous study conducted by the author (Ineichen 2009) on productsfrom Eumetsat Satellite Application Facilities (SAF). These algorithmes are derived byGeoModel in Bratislava (SolarGis), Helioclim Soda (heliosat 3v3), 3Tier company in theUnited States, University of Oldenburg (EnMetSol-Solis and EnMetSol-Dumortier) andthe IrSolAv company in Spain.

2. Ground data

The ground data used in the study are acquired at stations part of networks such asBaseline Solar Radiation Network (BSRN), Commission International de l’Eclairage (CIE),FluxNet network, Swiss Institute of Meteorology (ISM-Anetz) and World Radiation DataCenter (WRDC). Data from 23 ground sites situated mainly on the European continentare used. The work is done on data covering the year 2006.

Beside the global horizontal irradiance Gh, half of the sites acquire the normal beam

irradiance Bn. High precision instruments (WMO 2008) such as Kipp and Zonen CM10

Table I List of the stations, data and parameters availability.

Station Country Climate Dh Bn latitude ° longitude ° altitude m

Cabauw The Netehrlands temperate maritime x 51.970 4.930 2 BSRN - KNMICamborne United Kingdom temperate maritime x 50.220 -5.310 88 BSRN - Met OfficeCarpentras France mediterranean x 44.080 5.060 100 BSRN - Météo FranceDavos Dorf Switzerland semi-continental alpin x 46.810 9.840 1610 WRDC - Met OfficeEl Saler Spain semi arid, warm summer 39.346 -0.319 10 FluxNetGeneva Switzerland semi-continental x 46.199 6.131 420 CIE - UNIGEJungfraujoch Switzerland high alpine x 46.550 7.980 3571 CLIMAP - Météo SuisseLas Majadas Spain semi arid, warm summer 39.942 -5.773 260 FluxNetLerwick United Kingdom cold oceanic x 60.130 -1.180 82 BSRN - Met OfficeLocarno Switzerland warm temperate, humid 46.170 8.780 367 ANETZ - Météo SuisseNantes France oceanic x 47.150 -1.330 30 CIE - CSTBPayerne Switzerland moderate maritime/continental x 46.820 6.950 490 BSRN - Météo SuisseSede Boqer Israel dry steppe 30.867 34.767 457 BSRN - Met OfficeSion Switzerland dry alpine 46.220 7.330 489 ANETZ - Météo SuisseSonnblick Austria temperate alpine x 47.050 12.950 3105 WRDC - ZAMGTamanrasset Algeria hot, dry desert 22.780 5.520 1400 BSRN - Met OfficeThessaloniki Greece mediterranean temperate x 40.630 22.970 60 WRDC - Met OfficeToravere Estonia cold humid x 58.270 26.470 70 BSRN - EMHIVal Alinya Spain warm temperate, humid 42.152 1.449 1770 FluxNetVaulx-en-Velin France semi-continental x 45.780 4.930 170 CIE - ENTPEWien / Hohe Warte Austria continental x 48.250 16.350 203 WRDC - ZAMGYatir Forest Israel hot arid 31.347 35.052 650 FluxNetZürich Switzerland temperate atlantic 47.475 8.530 558 ANETZ - Météo Suisse

ANETZ MeteoSwiss network CUEPE Energy Group, UniGe

BSRN Baseline Surface Radiation Network EHMI The Estonian Meteorological and Hydrological Institute

CIE Commission Internationale pour l'Eclairage ENTPE Ecole Nationale des Mines de Paris

CLIMAP MeteoSwiss Climate Maping application KNMI The Netherlands Institute of Meteorology

CSTB Centre Scientifique et Technique du Bâtiment UNIGE University of Geneva

ZAMG Zentralanstalt für Meteorologie und Geophysik/Geodynamik

operated by


- 2 -

and Eppley PSP pyranometers, and Eppley NIP pyrheliometers, are used to acquire thedata. A stringent calibration, characterization and quality control was applied on all thedata by the person in charge of the measurements, the coherence of the data for allthe stations was verified by the author and is described in the following section. For twosites, the aerosol optical depth aod and the water vapor column w is independentlyacquired and is used as input to clear sky models in order to assess the instrumentscalibration factors.

The climate, latitude, longitude and altitude of the stations are given in Table I.

3. Data quality control

For all the stations, the first quality control consist of an assessment of the acquisitiontime stamp. To point out a possible time shift in the data, the symmetry in solar time ofthe irradiance for very clear days is visually checked. The horizontal global and if available,the normal beam irradiances are plotted versus the sinus of the solar elevation anglefor specific clear days. If the time stamp is correct, the afternoon curve should lay overthe morning curve as visualized on Figure 1a.

If this test is positive, a verification can be done with the help of the global clearnessindex K

t defined as:

)sin(hIG

Ko

ht ⋅

=

where Gh is the horizontal global irradiance, I

o is the solar constant, and h the solar

elevation angle. The clearness index is plotted for the morning and the afternoon datain a separate color. The upper limit, representative of clear sky conditions, should lay

Figure 1a The global horizon-tal and normal beamirradiances are representedversus the sinus of the solarelevation angle for a clear day.

Figure 1b The global clearness index Kt is representedseparately for the morning (green) and the afternoon(yellow) data, versus the solar elevation angle for oneyear in hourly values.Clear sky model data are represented in blue.

0

200

400

600

800

1000

0.0 0.2 0.4 0.6 0.8 1.0

Global and beam irradiances

sin (h)

GenevaJuly 18, 2006

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

Global clearness index Kt

Solar elevation angle h

Geneva 2006 (CH)


- 3 -

over for the morning and the afternoon data as represented on Figure 1b for one yearof data acquired at the site of Geneva for the year 2006. Hourly clear sky conditionvalues are plotted in light blue on the same graph. When these two conditions arefulfilled, the time stamp of the data bank is correct, and the solar geometry can beprecisely calculated. This test is very sensitive and a time shift of only a few minutes willconduct to an visible assymetry. A similar test can be done with the beam clearnessindex K

b defined as:

o

nb I

BK =

but this parameter is less sensitive to a possible time shift.

The coherence test between the two components can be verified with the help of theglobal and beam clearness indices (Ineichen 2010). The hourly beam clearness index isplotted versus the corresponding global index as illustrated on Figure 2 for the site ofCarpentras. On the same graph, the clear sky data evaluated with the Solis clear skymodel (Müller 2004, Ineichen 2008a) are represented for four different values of aerosoloptical depth (aod). The more usual corresponding Linke turbidity coefficient T

Lam2

retrieved from the beam irradiance:

)( 2 MATon

LamcdaeIB ⋅⋅−= δ

and evaluated at air mass AM = 2 is also given on the graph (Linke 1922, Ineichen2008b). δ

cda is the optical depth of a clean and dry atmosphere. An important deviation

from the clear sky lines can indicate calibration uncertainties, beam irradiancemissalignement or soiled sensors.

The absolute sensor calibration can be assessed with the help of a clear sky modelwhen the atmospheric aerosol optical depth and the water vapor column are known.These two parameters are normally retrieved from spectral measurements. When the

Figure 2 The beam clearness index is plottedagainst the global clearness index. On the samegraph, clear sky modelled values arerepresented for 4 different aerosol loads

0

0.2

0.4

0.6

0.8

0 0.2 0.4 0.6 0.8 1

ground measurements aod = 0.01 / Linke = 2.3 aod = 0.1 / Linke = 3.2 aod = 0.2 / Linke = 4.2 aod = 0.5 / Linke = 6.8

Water vapor = cm CarpentrasBeam clearness index Kb

Global clearness index Kt

2 aerosol type: rural

Figure 3 Atmospheric water vapor columnevaluated from the ambiant temperature andrelative humidity against the watre vaporretrieved from spectral measurements.

0

1

2

3

4

5

0 1 2 3 4 5Atmospheric water vapor column retrieved from aeronet spectral measurements

Atmospheric water vapor column retrieved from ground measurements with the Atwater model [cm]

Carpentras (F) 2003 - 2009

pointsslopembdsd R2

=====

1432104%

0.960.750.03


- 4 -

water vapor w is missing, it can be evaluated from the ground ambiant temperature(T

a) and relative humidity (HR) by the use of Atwater model (Atwater 1976) with a

good precision as illustrated on Figure 3 for the site of Carpentras and data acquiredfrom 2003 to 2009.

For some sites, the aod measurements retrieved from independent networks such asaeronet are acquired as soon as direct sun is available; these values are then averagedto give a daily value and used with the Solis clear sky model to evaluate hourly clear skyG

h and B

n values.

Day by day, the highest hourly value is then selected from the measurements andplotted against the day of the year on Figure 4. These points are representative of theclearest daily sky conditions. Based on the aod and water vapor content w of theatmopshere, the corresponding clear sky values are evaluated with the model. As thehighest values for each day is selected, the upper limit for these two series shoud laytogether if the two sets of measurements (irradiance and aod) are coherent. On thesame graph are also represented the daily clear sky indices defined as:

∑∑

=

dayhc

dayh

h G

GK and ∑

∑=

daync

dayn

hb B

BK

These values, if the data are coherent, should have an upper limit near of the unity.

4. The clearness index Kt and sky type classification

As it is the case for the majority of the national networks, the global irradiance is theonly available measured parameter concerning the solar radiation. Even if for half of thestations the beam component is available, the global irradiance and the corresponding

Figure 4 Daily highest value of the global irradiance reported versus the day of the yearfor the station of Carpentras, for the measurements and the corresponding clear sky evaluatedfrom the aod and the Solis model. The daily clear sky index is also represented.

0

200

400

600

800

1000

1200

0 31 61 92 122 153 183 214 244 275 305 336 3660

1

2

3

4

5

6

Clear sky (aeronet)

Ground measurements

Daily clear sky index

Day by day hourly global irradiance maximum value [W/m2]

Day of the year

Daily clear sky index Carpentras 2006

0

200

400

600

800

1000

1200

0 31 61 92 122 153 183 214 244 275 305 336 3660

1

2

3

4

5

6

Clear sky (aeronet)

Ground measurements


Day of the year

Daily clear sky index Carpentras 2006Day by day hourly beam irradiance maximum value [W/m2]


- 5 -

clearness index Kt are key parameters in the field of irradiance modelization. The clearness

index Kt was introduced as a norm (Black 1954) to characterize the insulation condi-

tions at a given point in time when only the global component is known. Unfortunately,this parameter is not independent of the solar elevation angle as it is shown on the leftgraphe of Figure 5 where the clearness index K

t is plotted versus the solar elevation

angle for the site of Carpentras. It can be seen on this Figure that clear sky conditions,determined by the upper limit of the clearness index values, are not equally representedby K

t for the different solar elevation angles.

In order to use the clearness index as a reliable sky condition descriptor, Perez et al.(1990) modified this parameter to make it independent of the solar elevation angle.The formulation is the following:

( )( )( )1.0/4.99.0/4.1exp031.1'

++−⋅=

AMK

K tt

where AM is the optical air mass as defined by Kasten (1980). This modified clearnessindex is represented on Figure 5 (right graph) for the same points than above. It canclearly be seen on this Figure that even if some patterns are still present, the modifiedclearness index is relatively independent from the solar elevation angle. Therefore, it isnow possible to define three zones to characterize three sky types:

clear sky conditions 0.65 < K’t ≤ 1.00

intermediate sky conditions 0.30 < K’t ≤ 0.65

cloudy sky conditions 0.00 < K’t ≤ 0.30

These limits are arbitrary, but are coherent with other classifications, like for examplethe Cloud Free Index saturation (CFIsat) as defined by Dürr (2006):

CFIsat = 100 (CFI - 1) / z

where z and CFI are defined in Dürr (2004). CFIsat is independent from the irradiancemeasurements; it is a function of the downward surface longwave irradiance and thedry bulb temperature. The comparison between the CFIsat and K’

t is illustrated on

Figure 6 (right graph). The red dashed lines represent the limits used in the presentstudy and applied on the modified clearness index. In the CFIsat classification, clear skyconditions are defined by a CFIsat below 0%, and cloudy conditions above 50%. Thecorresponding limits are represented in blue dashed lines. It can be seen on Figure 6 thatthe majority of the points are situated in the intersections of the corresponding threezones.

Another assessement can be done with the cloud cover as illustrated on Figure 6 (leftgraph). Here also, the limits used in the present study are well correlated with the cloud


- 6 -

cover (0% cloud cover for the clear sky conditions and 100% cloud cover for theovercast sky conditions).

5. Satellite derived data

Five different products are validated in the present study. The methodology and theinput parameters are described in the following section. The product from University ofOldenburg (EnMetSol) is evaluated for two different aerosol climatologies.

5.1 SolarGis

The irradiance components are the results of a five steps process: a multi-spectralanalysis classifies the pixels, the lower boundary (LB) evaluation is done for each timeslot, a spatial variability is introduced for the upper boundary (UP) and the cloud indexdefinition, the Solis clears sky model is used as normalization, and a terrain disaggregationis finally applied.

Four MSG spectral channels are used in a classification scheme to distinguish cloudsfrom snow and no-snow cloud-free situations. Prior to the classification, calibrated pixelvalues were transformed to three indices: normalized difference snow index (Ruyter2007), cloud index (Derrien 2005), and temporal variability index. Exploiting the potential

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60 70 80

Clearness index Kt = Gh / Io sin(h)

Solar elevation angle [°]

Measurements (BSRN Carpentras 2006)

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60 70 80

Modified clearness index Kt'



Figure 5 Global and modified clearness index versus the solar elevation angle.

Figure 6 Cloud cover and CFIsat coefficient versus the modified clearness index. The skycondition selection limits are represented in dashed red lines.

0

20

40

60

80

100

0.0 0.2 0.4 0.6 0.8 1.0

Cloud cover [%] Carpentras

Modified clearness index, K t'

overcast intermediate clear

-75%

-50%

-25%

0%

25%

50%

75%

100%

125%

0 0.2 0.4 0.6 0.8 1Modified clearness index Kt'

CFIsat Carpentras



- 7 -

of MSG spectral data for snow classification removed the need of additional ancillarysnow data and allowed using spectral cloud index information in cases of complexconditions such as clouds over high albedo snow areas.

In the original approach by Perez (2002), the identification of surface pseudo-albedo isbased on the use of a lower bound (LB), representing cloudless situations. This approachneglects diurnal variability of LB that is later corrected by statistical approach. Instead ofidentifying one value per day, LB is represented by smooth 2- dimensional surface (inday and time slot dimensions) that reflects diurnal and seasonal changes in LB andreduces probability of no cloudless situation.

Overcast conditions represented in the original Perez model by a fixed Upper Bound(UB) value were updated to account for spatial variability which is important especiallyin the higher latitudes. Calculation of cloud index was extended by incorporation ofsnow classification results.

The broadband simplified version of Solis model (Ineichen 2008a) was implemented. Asinput of this model, the climatology values from the NVAP water vapor database (Randl1996) and Atmospheric Optical Depth data by (Remund 2008) assimilated with Aeronetand Aerocom datasets are used.

Simplified Solis model was also implemented into the global to beam Dirindex algorithmsto calculate Direct Normal Irradiance component (Perez 1992, Ineichen 2008c). Diffuseirradiance for inclined surfaces is calculated by updated Perez model (1987).

Processing chain of the model includes post-processing terrain disaggregation algorithmbased on the approach by Ruiz-Arias (2010). The disaggregation is limited to shadowingeffect only, as it represents most significant local effect of terrain. The algorithm useslocal terrain horizon information with spatial resolution of 100 m. Direct and circumsolardiffuse components of global irradiance were corrected for terrain shadowing.

5.2 Heliosat-2 algorithm

The Helioclim 3 data bank is produced with the Heliosat-2 method that converts obser-vations made by geostationary meteorological satellites into estimates of the globalirradiation at ground level. This version integrates the knowledge gained by variousexploitations of the original Heliosat method and its varieties in a coherent and thoroughway.

It is based upon the same physical principles but the inputs to the method are calibratedradiances, instead of the digital counts output from the sensor. This change opens thepossibilities of using known models of the physical processes in atmospheric optics,thus removing the need for empirically defined parameters and of pyranometricmeasurements to tune them. The ESRA models (ESRA 2000, Rigollier 2000 and 2004)are used for modeling the clear-sky irradiation. The assessment of the ground albedo


- 8 -

and the cloud albedo is based upon explicit formulations of the path radiance and thetransmittance of the atmosphere. The turbidity is based on climatic monthly Linke Turbiditycoefficients data banks.

The Liu and Jordan (1960) model is used to split the global irradiance into the diffuseand beam components.

5.3 3Tier algorithm

Satellite-based time series of reflected sunlight are used to determine a cloud index timeseries for every land surface worldwide. A satellite based daily snow cover dataset isused to aid in distinguishing snow from clouds. In addition, the global horizontal clearskyradiation G

hc is modeled based on the surface elevation of each location, the local time,

and the measure of turbidity in the atmosphere. 3Tier opted to use a satellite-based,monthly time series of aerosol optical depth and water vapor derived from the ModerateResolution Imaging Spectroradiometer (MODIS). This dataset was combined with anotherturbidity dataset that includes both surface and satellite observations to provide a turbiditymeasure that spans the period of our satellite dataset and is complete for all landsurfaces. The cloud index n and the clear sky irradiance G

hc are then combined to model

the global horizontal irradiance Gh. This component of the process is calibrated for each

satellite based on a set of high-quality surface observations. Gh estimates are then

combined with other inputs to evaluate the other irradiance components Dh and B

n.

5.4 EnMetSol

The EnMetSol method is a technique for determining the global radiation at ground bythe use of data from a geostationary satellite (Beyer 1996, Hammer 2003). It is usedin combination with a clear sky model to evaluate the 3 irradiance parameters G

h, D

h

and Bn. The key parameter of the method is the cloud index n, which is estimated from

the satellite measurements and related to the transmissivity of the atmosphere via

Kc = 1 – n

where the transmissivity is expressed by the clear sky index Kc defined as the ratio of

global irradiance Gh and the corresponding clear sky irradiance G

hc:

hc

hc G

GK =

Two sets of data produced with the EnMetSol algorithm will be analyzed, correspondingto two different clear sky irradiance models:

• the model of Dumortier (Fontoynont 1998) with the Remund (2009)MeteonormHR high resolution data base for the turbidity input,


- 9 -

• and the original Solis clear sky model (Mueller 2004) with monthly averages ofAOD (Kinne 2005) and water vapour content (Kalnay 1996) as input parameters.

For the Dumortier clearsky, a diffuse fraction model (Lorenz 2007) is used to calculatethe all sky diffuse horizontal irradiance (via G

h-D

h). A recently developed beam fraction

model (Hammer 2009) is used to calculate the Bn for all sky conditions with the Solis

model.

5.5 IrSolAv

In the IrSolAv irradiance derivation scheme, the cloud index n is derived using themethodology developped by Dagestad and Olseth (Dagestad and Olseth, 2007) withsome modifications in the ground albedo determination. The ground albedo is computedfrom a forward and backward moving window of 14 days taking into account its evolutionduring the day, as function of the co-scattering angle.

The global horizontal irradiance Gh is then evaluated from the cloud index with the

model proposed by Zarzalejo (Zarzalejo et al., 2009); it uses as independent variablesthe cloud index, the 50-percentile of the cloud index for a given place, and the air massAM. The normal beam irradiance B

n is calculated from the global irradiance with the help

of Louche correlation (Louche et al., 1991).

In a second step, the clear sky conditions are indentified with the algorithm proposedby Polo (Polo et al., 2009a; Polo et al., 2009b); for these clear conditions, the irradiancesare evaluated with the ESRA clear sky model (Rigollier 2000), using the aerosol opticaldepth aod taken from Soda, MODIS or from a method proposed by Polo (Polo et al.,2009a) depending on their availability.

6. Comparison and evaluation procedure

In terms of validation, when evaluating satellite derived parameters with the same time

0

200

400

600

800

1000

0 200 400 600 800 1000

Measurements (ISM Zurich 2006)

EnMetSol Solis (Zurich 2006)Hourly horizontal global irradiance [Wh/m2h]

MBD =SD =R2 =

3%28%0.950

0

200

400

600

800

1000

0 200 400 600 800 1000

Measurements (BSRN Payerne 2006)

EnMetSol Dumortier (Payerne 2006)Hourly horizontal global irradiance [Wh/m2h]

MBD =SD =R2 =

4%18%0.978

Figure 8 Scatter plots for the global horizontal irradiance produced by EnMetSol for Zurichand Payerne.


- 10 -

step, the comparison can be done by means of scatter plots; these give a visual

evaluation of the capability of the model to reproduce the measurements. On these

graphs, the diagonal line is representative of an ideal model, and the points should lay

around this line. An illustration is given on Figure 8 for Zurich and Payerne, two sites that

showing different dispersions.

The statistical parameters like the mean bias difference (mbd), the root mean square

difference (rmsd), the standard deviation (sd) and the determination coefficient (R2)

represent a quantification of the model dispersion. These statistical parameters include

dispersions introduced by:

• the retrieval procedure,

• the comparison of point measurements (ground data) with aera

measurements,

• the comparison of the average of four instantaneous measurements

with 60 minutes integrated values.

In the field of solar radiation and natural light, the comparison is often done in term offrequency of occurrence: for the irradiance, it gives an indication of the repartition foreach level of radiation, and for the clearness index, that the level of radiation occures atthe right time during the day. The obtained graph is a line (or a bar chart) representativeof the relative frequency of occurrence of the considered parameter. This is illustratedon Figure 9 for the global irradiance and the corresponding clearness index K

t. On the

same graph, the frequency of occurrence of the ground measurements are representedas grey bars, and the different models in color lines.

A second order statistic, the Kolmogorov-Smirnov test (Espinar 2009), is also appliedto the data. It represents the capability of the model to reproduce the frequency ofoccurrence at each of the irradiance level. In order to avoid a peak at the zero level ofbeam irradiance, these values are excluded form the statistic. A visualisation is given onFigure 10 where the irradiance cumulated frequency of occurrence is represented against

Figure 9 Global irradiance and clearness index Kt relative frequency of occurrence for data

acquired in Vaulx-en-Velin (F). The grey bars are representative of the measurements.

0 200 400 600 800 1000

Measurements (CIE Vaulx-en-Velin 2006)

Geomodel (Vaulx-en-Velin 2006)

Helioclim3 (Vaulx-en-Velin 2006)

3Tier (Vaulx-en-Velin 2006)

IrSolAv (Vaulx-en-Velin 2006)

EnMetSol Solis (Vaulx-en-Velin 2006)

EnMetSol Dumortier (Vaulx-en-Velin 2006)

Global irradiance [W/m2]

Relative frequency of occurrence of the global irradiance Gh

0.0 0.2 0.4 0.6 0.8 1.0

Measurements (CIE Vaulx-en-Velin 2006)

Geomodel (Vaulx-en-Velin 2006)

Helioclim3 (Vaulx-en-Velin 2006)

3Tier (Vaulx-en-Velin 2006)

IrSolAv (Vaulx-en-Velin 2006)

EnMetSol Solis (Vaulx-en-Velin 2006)

EnMetSol Dumortier (Vaulx-en-Velin 2006)

Relative frequency of occurrence of the clearness index Kt

Clearness indexKt = Gh / Ioh


- 11 -

the irradiance for the same site than above. The quantitative value representative of theKolmogorov Smirnov test Integral (KSI) is defined as:

h

G

G hchc dGGFGFKSIh

h∫ ⋅−= max

min

)()( mod

where Fc(G

h) and F

c(G

hmod) are respectively the ground measurements and the

corresponding modelled cumulated frequencies of occurrence.

7. Global irradiance results

To ensure a correct and comparable validation of the different products, the followingmethod was used to merge the products and the ground measurements: for eachgenerated value, the nearest time stamped corresponding ground value is searched inthe data base; this means that the satellite image was taken within the ground integration

-100

0

100

200

300

400

Cab

auw

Cam

borne

Car

pentr

as

Dav

os

El S

aler

Gen

eva

Jun

gfrau

joch

Las

Maja

das

Ler

wick

Loc

arno

Nan

tes

Pay

erne

Sed

e Boq

er S

ion

Son

nblic

k

Tam

anras

set

The

ssalo

niki

Tora

vere

Val

d'Alin

ya

Vau

lx-en

-Veli

n

Wien

Yati

r For

est

Zuri

ch

All s

ites

Global irradiance average SolarGis Heliosat 3v3

3Tier EnMetSol (Solis) EnMetSol (Dumortier) IrSolAv

Global irradiance mean bias deviation [W/m2]

Figure 11 Average global irradiance and absolute mean bias difference

Figure 10 Relative frequency of occurrence for the global and the beam irradiance formeasurements at the site of Vaulx-en-Velin.

0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000

Measurements (CIE Vaulx-en-Velin 2006) Geomodel (Vaulx-en-Velin 2006) Helioclim3 (Vaulx-en-Velin 2006) 3Tier (Vaulx-en-Velin 2006) IrSolAv (Vaulx-en-Velin 2006) EnMetSol Solis (Vaulx-en-Velin 2006) EnMetSol Dumortier (Vaulx-en-Velin 2006)


Global irradiance cumulated frequency of occurrence

0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000

Measurements (CIE Vaulx-en-Velin 2006) Geomodel (Vaulx-en-Velin 2006) Helioclim3 (Vaulx-en-Velin 2006) 3Tier (Vaulx-en-Velin 2006) IrSolAv (Vaulx-en-Velin 2006) EnMetSol Solis (Vaulx-en-Velin 2006) EnMetSol Dumortier (Vaulx-en-Velin 2006)

Normal beam irradiance [W/m2]

Normal beam irradiance cumulated frequency of occurrence


- 12 -

Table

II

Firs

t an

d s

econd o

rder

sta

tist

ics

in a

bso

lute

val

ues

for th

e glo

bal

horizo

nta

l irr

adia

nce

. The

site

s in

gre

y ar

e not ta

ken in

to a

ccount in

the

ove

rall

stat

istics

.

Table

III

Firs

t an

d s

econd o

rder

sta

tist

ics

in r

elat

ive

valu

es for

the

glo

bal

horizo

nta

l irr

adia

nce

. The

site

s in

gre

y ar

e not ta

ken in

to a

ccount in

the

ove

rall

stat

istics

.

Gh [

W/m

2 ]nb

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v


- 13 -

0

20

40

60

Cab

auw

Cam

borne

Carp

entra

s

Dav

os

El S

aler

Gen

eva

Jun

gfrau

joch

Las

Maja

das

Lerw

ick

Loc

arno

Nan

tes

Pay

erne

Sed

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er S

ion

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nblic

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Tam

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vere

Val

d'Alin

ya

Vau

lx-en

-Veli

n

Wien

Yati

r Fore

st

Zuri

ch

All s

ites

SolarGis Heliosat 3v3 3Tier

EnMetSol (Solis) EnMetSol (Dumortier) IrSolAv

Global irradiance relative standard deviation [%]

Figure 12 Absolute standard deviation for the global irradiance. In red, the average Gh.

0

100

200

300

400

Cab

auw

Cam

born

e

Car

pentr

as

Dav

os

El S

aler

Gen

eva

Jun

gfraujo

ch

Las

Maja

das

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ick

Loc

arno

Nan

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er S

ion

Son

nblic

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Tam

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set

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vere

Val

d'Alin

ya

Vau

lx-en

-Veli

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Wien

Yati

r Fore

st

Zuri

ch

All s

ites

Global irradiance average SolarGis Heliosat 3v3


Global irradiance standard deviation [W/m2]

Figure 13 Relative standard deviation for the global irradiance.

0

20

40

60

Cab

auw

Cambo

rne

Carpe

ntras

Dav

os

El S

aler

Gen

eva

Jun

gfrau

joch

Las

Maja

das

Lerw

ick

Loc

arno

Nan

tes

Payer

ne

Sede B

oqer

Sion

Son

nblic

k

Taman

rass

et

The

ssalonik

i

Torav

ere

Val

d'Alin

ya

Vau

lx-en

-Veli

n

Wien

Yati

r Fore

st

Zuric

h

All s

ites



Global irradiance second order statistic: Kolmogorof-Smirnov index KSI

Figure 14 Second order statistics KSI for the global irradiance


- 14 -

period. Then, only hours for which ground and both generated products are present aretaken into account in the validation procedure; so, mainly the ground data availabilityrestrict the number of points in the comparison.

The results of the validation are given on Table II and Table III respectively in absoluteand relative values. The corresponding graphs are given on Figure 11 to 14.

The first established fact is that the sites of Davos, Jungfraujoch and Sonnblick givemuch higher differences than the other stations. This is due to the snow cover during allor part of the year, which is not taken into account by all the models, and is difficult toevaluate precisely. This is clearly visible on frequency of occurrence plotted fir the clearnessindex as shown on Figure 15 for the site of Sonnblick. These site are not part of theoverall statistics. The site of Nantes is also represented on Figure 15 for comparisonpurpose.

The overall average mean bias deviation is very low for all the products, and except forheliosat 3, the sign of the mean bias deviation is in general the same for all the algorithms.It is interesting to note that for high latitude sites (Cabauw, Camborne, Lerwick andToravere), the bias is even lower than for the middle latitude sites. The site of Val d’Alyniain Spain shows slightly variable bias depending on the product, this can be explained by

0.0 0.2 0.4 0.6 0.8 1.0

Measurements (WRDC Sonnblick 2006)

Geomodel (Sonnblick 2006)

Helioclim3 (Sonnblick 2006)

3Tier (Sonnblick 2006)

IrSolAv (Sonnblick 2006)

EnMetSol Solis (Sonnblick 2006)

EnMetSol Dumortier (Sonnblick 2006)



0.0 0.2 0.4 0.6 0.8 1.0

Measurements (CIE Nantes 2006)

Geomodel (Nantes 2006)

Helioclim3 (Nantes 2006)

3Tier (Nantes 2006)

IrSolAv (Nantes 2006)

EnMetSol Solis (Nantes 2006) EnMetSol Dumortier (Nantes 2006)



Figure 15 Frequency of occurence of the clearness index for the site of Sonneblick (highaltitude and snow) and for Nantes.

0.0 0.2 0.4 0.6 0.8 1.0

Measurements (FluxNet Val d'Alinya 2006)

Geomodel (Val d'Alinya 2006)

Helioclim3 (Val d'Alinya 2006)

3Tier (Val d'Alinya 2006)

IrSolAv (Val d'Alinya 2006)

EnMetSol Solis (Val d'Alinya 2006)

EnMetSol Dumortier (Val d'Alinya 2006)



Figure 16 Frequency of occurence of global irradiance and the clearness index for the site ofVal d’Alynia.

0 200 400 600 800 1000

Measurements (FluxNet Val d'Alinya 2006)

Geomodel (Val d'Alinya 2006)

Helioclim3 (Val d'Alinya 2006)

3Tier (Val d'Alinya 2006)

IrSolAv (Val d'Alinya 2006)

EnMetSol Solis (Val d'Alinya 2006)

EnMetSol Dumortier (Val d'Alinya 2006)


Relative frequency of occurrence of the global irradiance Gh


- 15 -

the high altitude of the site (1770m) and the mountainous environment. The producedirradiance varie from one algorithm to the other; the corresponding graph is given onFigure 16 for the global irradiance and the clearness index. This can be due to the clearsky model used for the normalisation in conjunction with their input parameter(atmospheric water vapor content, aerosol load and linke turbidity).

In term of absolute standard deviation (due to the negligible bias, the root mean squaredifference and the standard deviation are equivalent), except for the high altitude sta-tions (Davos, Jungfraujoch, Sonnblick and Val d’Alynia), all the site show the same orderof magnitude, including the high latitude sites. Due to the high irradiance level for Car-pentras, Sede Boqer, Tamanrasset and Yatir forest, the sites show good relative stan-dard deviations.

The choice of a clear sky model is a key point in the satellite irradiance derivation, it willhave a direct influence on the output. The measured and modelled global clearness

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80


EnMetSol Dumortier (Carpentras 2006)

Solar elevation


0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80


EnMetSol Solis (Carpentras 2006)

Solar elevation


0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80


3Tier (Carpentras 2006)

Solar elevation


0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80


SolarGis (Carpentras 2006)

Solar elevation


0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80


Helioclim3 (Carpentras 2006)

Solar elevation


Figure 17 The global clearness index Kt represented against the solar elevation angle for the

site of Carpentras. In yellow, the measurements and in blue the different products.

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80


IrSolAv (Carpentras 2006)

Solar elevation



- 16 -

index is plotted against the solar elevation for all the products and the site of Carpentrason Figure 17. On these graphs, the clear sky conditions are represented by the upperboundary; it is interesting to point out that even if there is a great gap between themeasurements and the modelled values for Helioclim, the overall performance is of thesame order of magnitude than for the other products. It can also be noted here that forhalf of the products, the highest and the lowest modelled values of K

t are never reached.

To better understand the bias, a dependence analysis is done with the solar elevationangle and the water vapor column. To illustrate the results, two examples are given onFigure 18 where the bias is represented against the considered parameter, and a best fitis traced to underline the tendency. If the main pattern of all the models is to underestimatethe global irradiance for low water vapor column values and overestimate it for high w,the bias pattern is variable from one model to the other with the solar elevation angle.

The study was also done with the turbidity, but this parameter was not available exceptfor the sites of Carpentras and Sede Boqer, and on a daily basis. As for the water vaporcontent, the general tendency is a positive slope on the bias with the daily aerosoloptical depth. An illustration for EnMetSol is given on Figure 19.

The irradiance for clear sky conditions is directly derived from the clear sky model. For

Figure 18 Global irradiance mean bias difference against the solar elevation angle (left) andthe atmospheric water vapor content for two products and the site of Carpentras.

-200

-150

-100

-50

0

50

100

150

200

0 20 40 60 80


MBD =SD =

2%16%


Global irradiance bias [Wh/m2h]

-200

-150

-100

-50

0

50

100

150

200

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0


MBD =SD =

4%12%

Atmospheric water vapor column [cm]


-200

-150

-100

-50

0

50

100

150

200

0.0 0.1 0.2 0.3 0.4 0.5

EnMetSol Solis (Sede Boqer 2006)

MBD =SD =

-3%9%

Daily aerosol optical depth


-200

-150

-100

-50

0

50

100

150

200

0.0 0.1 0.2 0.3 0.4 0.5


MBD =SD =

2%12%



Figure 19 Global irradiance mean bias difference against the daily aerosol load of theatmosphere for the two sites of Sede Boqer and Carpentras.


- 17 -

all sky condition, the clear sky model is combined with the cloud index. It is thereforeinteresting to differentiate the sky conditions following the rules defined in section 4.Table IV gives the overall results obtained for the three sky types. Without surprise, theclear sky conditions show the lowest standard deviation. All the algorithms have thesame tendency to underestimate for clear conditions, and overestimate the globalirradiance for intermediate and overcast conditions. Going more into details, the sametendency is also visible for all the sites. This is illustrated on Figure 20 for a cloudy(Lerwick) and a sunny (Carpentras) site. All the Figures and the complete Tables aregiven in the Annex.

The second order statistic given by the Kolmogorov-Smirnov index is a combination of

0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000

Measurements (ISM Sion 2006) Geomodel (Sion 2006) Helioclim3 (Sion 2006) 3Tier (Sion 2006) IrSolAv (Sion 2006) EnMetSol Solis (Sion 2006) EnMetSol Dumortier (Sion 2006)



0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000

Measurements (CIE Nantes 2006) Geomodel (Nantes 2006) Helioclim3 (Nantes 2006) 3Tier (Nantes 2006) IrSolAv (Nantes 2006) EnMetSol Solis (Nantes 2006) EnMetSol Dumortier (Nantes 2006)



Figure 21 Clearness index cumulated frequecy of occurrence for two sites with comparablestandard deviation: Sion and Nantes.

-200

-150

-100

-50

0

50

100

150

200

0 0.2 0.4 0.6 0.8 1

3Tier (Lerwick 2006)

MBD =SD =

4%35%




-200

-150

-100

-50

0

50

100

150

200

0 0.2 0.4 0.6 0.8 1

3Tier (Carpentras 2006)

MBD =SD =

-1%14%




Figure 20 Global irradiance mean bias difference against the modified clearness index for thetwo sites of Lerwick and Carpentras.

sky type Gh nb mbd sd mbd sd mbd sd mbd sd mbd sd mbd sd

-9 43 -23 49 -18 57 -16 41 -16 43 -25 66

-2% 9% -5% 10% -4% 11% -3% 8% -3% 9% -7% 17%

12 67 22 75 12 80 20 60 19 61 19 95

5% 27% 9% 30% 5% 32% 8% 24% 8% 25% 11% 53%

18 49 39 65 42 62 25 48 24 48 36 79

22% 60% 48% 80% 51% 75% 30% 58% 29% 59% 63% 138%

clear 498 35824

24096249intermediate

1591782overcast

EnMetSol (Dumortier)

IrSolAvSolarGis Heliosat 3v3 3TierEnMetSol

(Solis)

Table IV First order statistics in absolute and relative values for the global horizontal irradianceand for the three sky conditions. The absolute values are in [W/m2].


- 18 -

the bias, the standard deviation (dispersion) and the cumulated frequency of occurence.It is more sensitive and highlights smaller deviations than the first order indicators. Thiscan be seen for example for the sites of Sion or Sede Boqer, where the standarddeviation is similar to the other sites, but the clearness index frequency of occurenceshows descreapancies with the corresponding measurements. The cumulated frequencyof occurrence for Sion and Nantes are given on Figure 21. The deviation from themeasurements in Sion is visible for all the models at low and high irradiance levels; it canbe due to snow in the Rhône Valley.

In conclusion, except for the four high altitude sites (potentially with snow), the averagebias is around 1% (4 [W/m2]), positive for all the models. The best product derives theglobal irradiance with a standard deviation of 16% (55 [W/m2]).

Some models never reach the measured highest and lowest clearness index. The majorityof the products show a bias dependance with the solar elevation angle, the water vaporcolumn and when available, with the aerosol optical depth. In term of sky type, thegeneral pattern is to underestimate the global irradiance for clear conditions, and tooverestimate it for all the other conditions. Not all models take into account the snow,but even when included in the algorithm, the global irradiance for the concerned sites isnot satisfactory.

8. Beam irradiance results

The same methodology is used for the validation of the beam irradiance component,except that the slots with no direct irradiance are excluded from the validation in orderto avoid a bias in the overall statistic and a peak at zero irradiance in the frequency ofoccurrence.

Figure 22 Average beam irradiance and absolute mean bias difference

-100

0

100

200

300

400

Cab

auw

Cam

borne

Car

pentr

as

Dav

os

El S

aler

Gen

eva

Jun

gfrau

joch

Las

Maja

das

Ler

wick

Loc

arno

Nan

tes

Pay

erne

Sed

e Boq

er S

ion

Son

nblic

k

Tam

anras

set

The

ssalo

niki

Tora

vere

Val

d'Alin

ya

Vau

lx-en

-Veli

n

Wien

Yati

r For

est

Zuri

ch

All s

ites

Normal beam average SolarGis Heliosat 3v3


Normal beam irradiance mean bias deviation [W/m2]


- 19 -

Table

VFi

rst

and s

econd o

rder

sta

tist

ics

in a

bso

lute

val

ues

for

the

norm

al b

eam

irr

adia

nce

. The

site

s in

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.

Table

VI

Firs

t an

d s

econd o

rder

sta

tist

ics

in r

elat

ive

valu

es f

or

the

norm

al b

eam

irr

adia

nce

. The

site

s in

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.

B n [W

/m2 ]

nbR

2m

bd%

sd%

KSI

R2

mbd

%sd

%KS

IR

2m

bd%

sd%

KSI

R2

mbd

%sd

%KS

IR

2m

bd%

sd%

KSI

R2

mbd

%sd

%KS

I

Cab

auw

220

4095

0.93

9-6

4228

0.87

715

5928

0.86

811

6214

0.93

1-4

4547

0.93

34

4439

Cam

born

e23

134

710.

942

342

280.

885

2958

260.

896

1056

200.

937

344

460.

940

1242

31 C

arpe

ntra

s44

839

930.

936

-426

450.

856

238

370.

905

-232

300.

924

-129

390.

923

729

200.

729

-153

28 D

avos

314

4200

0.86

411

6094

0.74

833

7910

80.

707

1186

113

0.73

5-3

682

276

0.75

7-7

7819

00.

663

492

153

El S

aler

Gen

eva

307

3622

0.91

82

4141

0.88

74

4739

0.86

910

5242

0.93

33

3735

0.93

715

3614

0.85

31

5434

Jun

gfra

ujoc

h18

533

110.

541

154

194

158

0.32

717

620

121

60.

461

167

206

174

0.31

9-4

818

957

80.

333

419

748

50.

399

6919

736

5 L

as M

ajad

as L

erw

ick

157

3119

0.89

06

7030

0.78

250

102

420.

764

3710

747

0.90

2-2

6741

0.90

68

6527

Loc

arno

Nan

tes

284

4199

0.94

3-1

134

370.

838

455

320.

890

-746

250.

945

-533

270.

944

-934

330.

851

-354

11 P

ayer

ne26

440

050.

889

653

320.

841

663

730.

832

1967

120.

887

954

480.

893

2254

300.

821

767

68 S

ede

Boq

er63

827

460.

887

-718

560.

763

-13

2610

10.

760

-826

630.

835

-11

2284

0.82

6-7

2362

0.70

0-7

2960

Sio

n S

onnb

lick

257

3933

0.70

866

107

320.

496

105

130

800.

487

6714

158

0.45

5-4

812

536

50.

450

013

524

60.

524

6514

085

Tam

anra

sset

545

4293

0.92

7-5

2561

0.75

617

4382

0.81

12

3862

0.81

95

3760

0.84

313

3571

The

ssal

onik

i T

orav

ere

286

3784

0.92

9-1

139

360.

807

464

470.

792

566

320.

890

-15

4984

0.89

7-5

4765

Val

d'A

linya

Vau

lx-e

n-V

elin

316

4060

0.93

9-2

3334

0.88

86

4330

0.89

310

4454

0.94

63

3123

0.95

04

3011

0.87

80

4621

Wie

n25

742

030.

900

-148

920.

880

653

129

0.85

810

5884

0.90

7-6

4714

30.

909

1146

116

0.83

41

6214

0 Y

atir

Fore

st Z

uric

h

All

site

s32

645

590

n/a

-435

n/a

n/a

850

n/a

n/a

549

n/a

n/a

-139

n/a

n/a

638

n/a

n/a

-154

n/a

EnM

etSo

l (So

lis)

IrSol

Av

EnM

etSo

l (D

umor

tier)

Sola

rGis

Hel

iosa

t 3v3

3Tie

r

B n [W

/m2 ]

nbR

2m

bdsd

KSI

R2

mbd

sdKS

IR

2m

bdsd

KSI

R2

mbd

sdKS

IR

2m

bdsd

KSI

R2

mbd

sdKS

I

Cab

auw

220

4095

0.93

9-1

393

280.

877

3213

028

0.86

824

136

140.

931

-810

047

0.93

310

9739

Cam

born

e23

134

710.

942

696

280.

885

6813

426

0.89

622

129

200.

937

710

146

0.94

027

9831

Car

pent

ras

448

3993

0.93

6-1

911

845

0.85

68

172

370.

905

-914

130

0.92

4-3

129

390.

923

3212

820

0.72

9-6

237

28 D

avos

314

4200

0.86

434

187

940.

748

104

247

108

0.70

735

271

113

0.73

5-1

1525

727

60.

757

-23

244

190

0.66

312

291

153

El S

aler

Gen

eva

307

3622

0.91

85

125

410.

887

1314

539

0.86

932

161

420.

933

1011

335

0.93

746

112

140.

853

316

534

Jun

gfra

ujoc

h18

533

110.

541

285

360

158

0.32

732

637

321

60.

461

310

381

174

0.31

9-8

835

057

80.

333

736

548

50.

399

127

365

365

Las

Maj

adas

Ler

wic

k15

731

190.

890

911

030

0.78

278

160

420.

764

5816

847

0.90

2-4

104

410.

906

1310

227

Loc

arno

Nan

tes

284

4199

0.94

3-3

195

370.

838

1215

732

0.89

0-1

913

225

0.94

5-1

494

270.

944

-27

9633

0.85

1-9

154

11 P

ayer

ne26

440

050.

889

1514

032

0.84

116

166

730.

832

4917

812

0.88

724

142

480.

893

5814

230

0.82

119

177

68 S

ede

Boq

er63

827

460.

887

-46

115

560.

763

-86

168

101

0.76

0-4

916

663

0.83

5-6

813

884

0.82

6-4

314

462

0.70

0-4

318

760

Sio

n S

onnb

lick

257

3933

0.70

817

127

632

0.49

627

033

480

0.48

717

336

358

0.45

5-1

2332

036

50.

450

034

624

60.

524

166

359

85 T

aman

rass

et54

542

930.

927

-27

138

610.

756

9423

582

0.81

114

208

620.

819

2720

460

0.84

368

191

71 T

hess

alon

iki

Tor

aver

e28

637

840.

929

-30

112

360.

807

1318

347

0.79

215

188

320.

890

-43

140

840.

897

-15

133

65 V

al d

'Alin

ya V

aulx

-en-

Vel

in31

640

600.

939

-610

334

0.88

819

137

300.

893

3014

054

0.94

611

9623

0.95

012

9411

0.87

80

144

21 W

ien

257

4203

0.90

0-3

124

920.

880

1513

512

90.

858

2615

084

0.90

7-1

512

114

30.

909

2711

911

60.

834

315

814

0 Y

atir

Fore

st Z

uric

h

All

site

s32

645

590

n/a

-11

115

n/a

n/a

2516

3n/

an/

a17

160

n/a

n/a

-512

8n/

an/

a19

125

n/a

n/a

-317

6n/

a

EnM

etSo

l (D

umor

tier)

Sola

rGis

Hel

iosa

t 3v3

3Tie

rEn

Met

Sol (

Solis

)IrS

olA

v


- 20 -

The first and second statistics are given on Table V and VI, and on Figures 22 to 24. Itcan be seen on Figure 22 that the absolute bias for the beam component is variablefrom one site to the other, and depends on the product: for example, it is positive inPayerne (15-60 W/m2) and highly negative in Sede Boqer (-70 to -110 W/m2). ForTamanrasset, it varies from -27 [W/m2] (Geomodel) to +68 [W/m2] (EnMetSol) and+94 [W/m2] (Heliosat), and for Lerwick, from -4 to +78 [W/m2].

The standard deviation varies from 20% to 100%; it is highly variable in absolute andrelative values. The site of Sede Boqer show the best values, but as it is a standarddeviation, it doesn’t take into account the its high mean bias. These deviation areillustrated on Figure 25, where on the left graph, the beam clearness index K

b is

represented versus the solar elevation angle. It can be seen that the modelled clear skyupper limit never reaches the corresponding measurements. On the right graph, the

0

20

40

60

80

Cab

auw

Cam

borne

Car

pentr

as

Dav

os

El S

aler

Gen

eva

Jun

gfrau

joch

Las

Maja

das

Lerw

ick

Loc

arno

Nan

tes

Pay

erne

Sed

e Boq

er S

ion

Son

nblic

k

Tam

anra

sset

The

ssalo

niki

Tor

aver

e

Val

d'Alin

ya

Vau

lx-en

-Veli

n

Wien

Yati

r For

est

Zur

ich

All s

ites



Normal beam irradiance second order statistic: Kolmogorof-Smirnov index KSI

0

20

40

60

80

Cab

auw

Cam

borne

Car

pentr

as

Dav

os

El S

aler

Gen

eva

Jun

gfrau

joch

Las

Maja

das

Lerw

ick

Loc

arno

Nan

tes

Pay

erne

Sed

e Boq

er S

ion

Son

nblic

k

Tam

anra

sset

The

ssalo

niki

Tor

aver

e

Val

d'Alin

ya

Vau

lx-en

-Veli

n

Wien

Yati

r Fore

st

Zur

ich

All s

ites



Normal beam irradiance relative standard deviation [%]

Figure 23 Relative standard deviation for the global irradiance.

Figure 24 Second order statistics KSI for the global irradiance


- 21 -

frequency of occurrence is represented: the clear sky peak is to low for all the products.Part of the discreapancy can be an calibration issue. In fact, when comparing the BSRNbeam ground measurements with the clear sky irradiance evaluated with solis fromaeronet aod measurements, a 4% difference is visible as illustrated on Figure 26 (left:uncorrected, right: corrected by 4%). Also, inspecting BSRN data from Sede Boqer, acommon 41 days sequence was found in data from 2005 and 2006. Comparison withsatellite evaluated data conducted to eliminate this sequence from the present valida-tion as it was much better reproduced with 2005 data. The 4% correction is allreadyincluded in the above results.

The relative standard deviation is comparable for all the sites, except for Lerwick, the

0.0 0.2 0.4 0.6 0.8 1.0

Measurements (BSRN Sede Boqer 2006)

Geomodel (Sede Boqer 2006) Helioclim3 (Sede Boqer 2006) 3Tier (Sede Boqer 2006)

IrSolAv (Sede Boqer 2006) EnMetSol Solis (Sede Boqer 2006) EnMetSol Dumortier (Sede Boqer 2006)

Relative frequency of occurrence of the beam clearness index Kb

Beam clearness indexKb = Bn / Io

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80

Measurements (BSRN Sede Boqer 2006)

EnMetSol Dumortier (Sede Boqer 2006)

Solar elevation

Beam clearness index Kb = Bn / Io

Figure 25 Beam clearness index versus the solar elevation angle, and the corresponding rela-tive frequency of occurrence for the site of Sede Boqer.

0

200

400

600

800

1000

1200

0 31 61 92 122 153 183 214 244 275 305 336 3660

1

2

3

4

5

6

Clear sky (aeronet)

Ground measurements


Day of the year

Daily clear sky index Sede Boqer 2006Day by day hourly beam irradiance maximum value [W/m2]

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200

Normal beam irradiance evaluated with Solis and aeronet data [W/m2] Sede Boqer (2006)

Ground measurements normal beam irradiance [W/m2]

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200

Normal beam irradiance evaluated with Solis and aeronet data [W/m2] Sede Boqer (2006)

Ground measurements normal beam irradiance [W/m2]

Ground measurements corrected by 4%

0

200

400

600

800

1000

1200

0 31 61 92 122 153 183 214 244 275 305 336 3660

1

2

3

4

5

6

Clear sky (aeronet)

Ground measurements


Day of the year

Daily clear sky index Sede Boqer 2006Day by day hourly beam irradiance maximum value [W/m2]

Ground measurements corrected by 4%

Figure 26 Sede Boqer beam irradiance before and after correction. On top, beam irradianceand clear sky index; bottom, daily measurements and évaluated data.


- 22 -

highest site in term of latitude, and where the average beam irradiance is very low. Asfor the global component, the results are good for the high latitude sites, and, due tothe snow, not significative for the high altitude sites.

The same parameter dependance than for the global irradiance is conducted on thebeam component. The results are given on Figures 27 and 28, where the same sitesand models as for the global component are represented; they show very similar pat-tern (see Figures 18 and 19). This is a consequence of the method used to derive thebeam irradiance: all the algorithm derive in a first step the global component, and split itinto beam and diffuse with the help of a diffuse fraction model (Dirindex (Perez 1992)for SolarGis, Liu and Jordan (Liu 1960) for Helioclim, and in-house models for the otherproducts). This means that the conclusion drawn for the global are also valid for the

-300

-200

-100

0

100

200

300

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0


MBD =SD =

4%12%

Atmospheric water vapor column [cm]

Beam irradiance bias [Wh/m2h]

-300

-200

-100

0

100

200

300

0 20 40 60 80


MBD =SD =

2%16%



-300

-200

-100

0

100

200

300

0.0 0.1 0.2 0.3 0.4 0.5


MBD =SD =

2%12%



-300

-200

-100

0

100

200

300

0.0 0.1 0.2 0.3 0.4 0.5

EnMetSol Solis (Sede Boqer 2006)

MBD =SD =

-3%9%



Figure 27 Normal beam irradiance mean bias difference against the solar elevation angle(left) and the atmospheric water vapor content (right) for the site of Carpentras.

Figure 28 Normal beam irradiance mean bias difference against the daily aerosol load of theatmosphere for the two sites of Sede Boqer and Carpentras.

Table VII First order statistics in absolute and relative values for the normal beam irradiance andfor the three sky conditions. The absolute values are in [W/m2].

sky type Bn nb mbd sd mbd sd mbd sd mbd sd mbd sd mbd sd

-66 121 -61 163 -45 161 -66 138 -24 137 -83 179

-10% 19% -10% 26% -7% 26% -10% 22% -4% 22% -13% 29%

40 110 119 146 81 163 58 108 72 114 73 171

28% 78% 84% 103% 57% 115% 41% 76% 51% 81% 52% 120%

14 45 51 70 38 93 18 50 18 52 39 89

302% 983% 1117% 1522% 831% 2022% 394% 1084% 393% 1129% 845% 1927%overcast

intermediate 142 15173

IrSolAv

clear 627 20352


5 10065

EnMetSol (Solis)

EnMetSol (Dumortier)


- 23 -

beam irradiance, even if the effects are more important due to the chained models.

The validation results for the three categories of the sky type are given on Table VIIwhere the results for overcast conditions are included, even if they are not significative.Here again, comparable conclusions can be drawn, the bias is negative for clear condi-tions and positive for intermediate skies.

The above results are also visible on the second order statistic KSI, as illustrated onFigure 24 and 29, and given in Table V and VI. The fact that the beam is measured orretrieved from the global and the diffuse is not the main issue on the difference in thecumulated frequency of occurrence curves, it is more a site effect.

In conclusion, the beam irradiance is retrieved from the satellite images with an averagestandard deviation of 35% to 54% depending on the model. In term or irradiance value,it rages from 90 to 200 [W/m2] according to the model and the site. The mean biasdifference is slightly higher than for the global component, negative for clear conditionsand positive for intermediate skies.

As the beam component is derived from the global, the dependances with the differentparameters are similar but sharper than for the global irradiance.

9. Conclusions

The first conclusion is that the quality control is a key point in any model validation. Evenif the data are highly qualified by the organisation in charge of the acquisition, uncertaintiescan remain in the data and influence the validation. The best case is when independentdata such as aerosol optical depth are available.

The main conclusions of the present study are represented by the first order statistics:

• for latitude from 20° to 60°, altitude from sea level to 1800 m and various climate,the global irradiance is retrieved with a negligible bias and an average standard

0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000

Measurements (CIE Nantes 2006) Geomodel (Nantes 2006) Helioclim3 (Nantes 2006) 3Tier (Nantes 2006) IrSolAv (Nantes 2006) EnMetSol Solis (Nantes 2006) EnMetSol Dumortier (Nantes 2006)



0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000

Measurements (BSRN Payerne 2006) Geomodel (Payerne 2006) Helioclim3 (Payerne 2006) 3Tier (Payerne 2006) IrSolAv (Payerne 2006) EnMetSol Solis (Payerne 2006) EnMetSol Dumortier (Payerne 2006)



Figure 29 Beam clearness index cumulated frequecy of occurrence for Nantes where thebeam is retrieved from the diffuse, and Payerne where the beam componant is acquired.


- 24 -

deviation around 16% for the best algorithm. For the beam irradiance, the bias isaround several percents, and the standard deviation around 35%,

• as expected, the main dependance comes from the knowledge of the aerosoloptical depth. A lower dependance with the atmospheric water vapor column andthe solar elevation angle is pointed out,

• even if the snow cover is taken into account in the algorithm, the sites situated inhigh altitude such as Junfraujoch and Sonnblick give bad results and are not takeinto account in the overall statistic.

• the high latitude sites such as Cabauw, Camborne or Toravere give not poorerresults than the other sites, only Lerwick, the highest site in latitude (60°N) presentsmore difficulties. It has also very low levels of irradiance,

• for the majority of the sites, SolarGis and EnMetSol give the best statistics forboth of the components.

Neverteless, the interannual variability of the irradiance conditions (it has be shown in aprevious study that even for the same site and algorithm, the results can vary from oneyear to the other, Ineichen 2010a), the lack of independent ground measurements suchas aerosol data, the difficulty to assess the exact calibration of the ground data, and thechoice of a specific year to carry out the validation, conduct to results that give goodindications, but from which it is difficult to draw general conclusions.

10. Acknowledgements

This comparative study could be performed thanks to the data provided by the FluxNetcommunity (http://www.fluxdata.org), the Baseline Surface Radiation Network, theAeronet network, the CSTB of Nantes and ENTPE in Vaulx-en-Velin, the World RadiationData Center, and the Swiss Institute of Meteorology.

The derived products where kindly provided by Geomodel in Slovakia, 3Tier in Seattle,the Helioclim data bank in France, University of Oldenburg in Germany, and IrSolAvcompany in Spain.


- 25 -

11. Nomenclature and abbreviations

Io

solar constant in W/m2

Bn

direct normal incidence irradiance in W/m2

Gh

surface solar global irradiance on a horizontal plane in W/m2

Ghc

clear sky surface solar irradiance in W/m2

Dh

surface solar diffuse irradiance on a horizontal plane in W/m2

Kt

global or surface solar irradiance clearness indexK

bbeam clearness index

Kc

clear sky indexK

hdaily global clear sky index

Khb

daily beam clear sky indexn cloud index

Fc(G

h) cumulated frequecy of occurrence of the global irradiance

h solar elevation or altitude angle in degrees (°)AM optical air mass

RTM radiative transfer modelT

LLinke turbidity

aod aerosol optical depth, or atmospheric aerosol load

Ta

surface (ambient) temperatureHR relative humidityw atmospheric total water vapour column

mbd mean bias differencermsd root mean square differencesd standard deviationKSI Kolmogorov-Smirnov index, second order statistic


- 26 -

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- 30 -


- 31 -

Annexe: figures in the following pages:

For the clear, intermediate and overcast sky conditions:

• First and second order statistics in absolute values for the global hori-zontal irradiance. The sites in grey are not taken into account in theoverall statistics.

• First and second order statistics in relative values for the global horizon-tal irradiance. The sites in grey are not taken into account in the overallstatistics.

• First and second order statistics in absolute values for the normal beamirradiance. The sites in grey are not taken into account in the overallstatistics.

• First and second order statistics in relative values for the normal beamirradiance. The sites in grey are not taken into account in the overallstatistics.


- 32 -

Cle

ar s

ky c

onditio

ns:

first

and s

econd o

rder

sta

tist

ics

in a

bso

lute

val

ues

for

the

glo

bal

horizo

nta

l irra

dia

nce

. The

site

s in

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.

Cle

ar s

ky c

onditio

ns:

first

and s

econd o

rder

sta

tist

ics

in rel

ativ

e va

lues

for th

e glo

bal

horizo

nta

l irr

adia

nce

. The

site

sin

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.

GH

I [w

/m2]

nbR

2m

bd%

sd%

R2

mbd

%sd

%R

2m

bd%

sd%

R2

mbd

%sd

%R

2m

bd%

sd%

R2

mbd

%sd

%

Cab

auw

420

1374

0.98

0-4

120.

983

-812

0.97

6-6

130.

986

-510

0.98

5-6

10 C

ambo

rne

459

1362

0.98

5-5

100.

986

-710

0.97

6-8

130.

990

-68

0.99

0-6

8 C

arpe

ntra

s48

125

220.

993

-17

0.99

1-3

90.

988

-39

0.99

4-2

60.

993

07

0.92

1-7

22 D

avos

517

1775

0.95

4-7

150.

970

-912

0.91

5-1

021

0.94

8-2

016

0.93

5-1

418

0.83

2-1

629

El S

aler

549

2027

0.98

0-2

90.

987

-49

0.97

2-5

110.

985

-18

0.98

5-3

80.

978

-710

Gen

eva

508

1464

0.98

30

90.

983

-510

0.96

6-2

130.

987

08

0.98

62

90.

957

-515

Jun

gfra

ujoc

h60

015

580.

930

-618

0.93

2-1

716

0.83

1-1

128

0.68

5-5

033

0.67

5-4

434

0.76

8-3

130

Las

Maj

adas

539

2022

0.98

82

80.

990

08

0.98

32

100.

991

37

0.99

21

70.

982

-19

Ler

wic

k40

287

40.

970

-10

140.

972

-11

150.

950

-11

190.

977

-10

130.

976

-11

13 L

ocar

no51

218

130.

982

-19

0.98

5-5

100.

968

-212

0.98

4-1

80.

980

-29

0.97

2-8

11 N

ante

s45

615

420.

982

-411

0.97

8-8

120.

972

-713

0.98

8-4

90.

987

-79

0.96

4-6

15 P

ayer

ne50

415

390.

977

-211

0.97

2-9

130.

964

-414

0.98

0-3

100.

979

-111

0.95

8-7

15 S

ede

Boq

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222

850.

989

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0.98

8-7

60.

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0.98

9-4

60.

988

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0.97

9-7

8 S

ion

535

1851

0.95

7-7

130.

977

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0.94

1-9

160.

979

-69

0.97

2-7

110.

846

-11

25 S

onnb

lick

481

1837

0.92

1-7

210.

946

-817

0.87

0-1

128

0.79

0-3

831

0.77

4-2

934

0.84

7-1

029

Tam

anra

sset

621

3154

0.99

4-2

60.

990

18

0.98

50

90.

992

-26

0.99

2-2

6 T

hess

alon

iki

536

2025

0.98

9-1

70.

988

-68

0.97

1-6

120.

978

-510

0.97

7-6

100.

973

-511

Tor

aver

e40

015

450.

985

-410

0.96

8-8

170.

968

-714

0.98

3-6

110.

979

-612

Val

d'A

linya

578

1532

0.98

5-3

80.

985

010

0.97

9-2

100.

985

-89

0.98

6-5

80.

966

213

Vau

lx-e

n-V

elin

514

1468

0.98

50

80.

984

-49

0.97

30

110.

987

18

0.98

7-1

80.

973

-311

Wie

n46

015

510.

981

-310

0.98

3-7

110.

972

-513

0.98

6-5

90.

986

-39

0.95

6-6

16 Y

atir

Fore

st62

625

230.

993

-25

0.99

3-5

60.

987

-47

0.99

4-6

50.

994

-55

0.98

2-4

8 Z

uric

h46

513

510.

975

-213

0.97

3-7

150.

963

-316

0.97

2-4

130.

971

-115

0.95

6-6

16

All

site

s49

835

824

-29

-510

-411

-38

-39

-717

3Tie

rEn

Met

Sol (

Dum

ortie

r)So

larG

isH

elio

sat 3

v3En

Met

Sol (

Solis

)IrS

olA

v

Gh [

w/m

2]nb

R2

mbd

sdR

2m

bdsd

R2

mbd

sdR

2m

bdsd

R2

mbd

sdR

2m

bdsd

Cab

auw

420

1374

0.98

0-1

750

0.98

3-3

350

0.97

6-2

455

0.98

6-2

342

0.98

5-2

544

Cam

born

e45

913

620.

985

-25

460.

986

-34

470.

976

-36

570.

990

-27

360.

990

-28

36 C

arpe

ntra

s48

125

220.

993

-532

0.99

1-1

544

0.98

8-1

642

0.99

4-8

310.

993

034

0.92

1-3

410

7 D

avos

517

1775

0.95

4-3

976

0.97

0-4

764

0.91

5-5

310

90.

948

-101

810.

935

-70

920.

832

-82

148

El S

aler

549

2027

0.98

0-1

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0.98

7-2

448

0.97

2-2

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978

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0.97

0-4

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0.97

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0.95

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ion

535

1851

0.95

7-3

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nblic

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990.

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830.

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134

0.79

0-1

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400.

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2-2

259

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6-2

241

0.98

6-1

242

0.95

6-2

973

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ir Fo

rest

626

2523

0.99

3-1

233

0.99

3-3

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All

site

s49

835

824

-943

-23

49-1

857

-16

41-1

643

-25

66

3Tie

rEn

Met

Sol (

Solis

)So

larG

isH

elio

sat 3

v3En

Met

Sol (

Dum

ortie

r)IrS

olA

vGlobal irradiance

Clear sky conditions: 0.65 < K’t ≤ 1.00


- 33 -

Beam irradiance

Clear sky conditions 0.65 < K’t ≤ 1.00

Bn [

w/m

2]nb

R2

mbd

%sd

%R

2m

bd%

sd%

R2

mbd

%sd

%R

2m

bd%

sd%

R2

mbd

%sd

%R

2m

bd%

sd%

Cab

auw

530

1335

0.88

4-1

221

0.82

9-1

026

0.82

0-7

270.

867

-15

220.

865

-622

Cam

born

e55

611

950.

894

-920

0.82

6-2

250.

824

-626

0.88

1-1

121

0.88

8-3

20 C

arpe

ntra

s65

225

220.

901

-10

150.

827

-11

220.

870

-918

0.86

7-8

180.

868

-118

0.68

4-1

431

Dav

os67

617

750.

742

-730

0.52

5-8

390.

399

-12

490.

423

-45

430.

463

-22

450.

393

-18

52 E

l Sal

er G

enev

a63

114

640.

805

-820

0.77

5-1

322

0.75

0-3

250.

815

-720

0.83

43

190.

724

-14

26 J

ungf

rauj

och

381

1558

0.39

395

117

0.32

554

117

0.44

370

116

0.28

6-6

711

70.

294

-34

121

0.34

312

119

Las

Maj

adas

Ler

wic

k49

179

00.

839

-14

280.

783

-635

0.71

2-6

390.

849

-20

270.

858

-10

27 L

ocar

no N

ante

s59

515

420.

875

-14

180.

781

-15

250.

794

-13

240.

868

-11

180.

861

-15

180.

748

-14

28 P

ayer

ne58

515

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Yat

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rest

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ich

All

site

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Met

Sol (

Solis

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Met

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sat 3

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Bn [

w/m

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Fore

st Z

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h

All

site

s62

720

352

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121

-61

163

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161

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EnM

etSo

l (So

lis)

EnM

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l (D

umor

tier)

Sola

rGis

Hel

iosa

t 3v3

3Tie

rIrS

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v

Cle

ar s

ky c

onditio

ns:

first

and s

econd o

rder

sta

tist

ics

in a

bso

lute

val

ues

for

the

norm

al b

eam

irra

dia

nce

. The

site

sin

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.

Cle

ar s

ky c

onditio

ns:

first

and s

econd o

rder

sta

tist

ics

in rel

ativ

e va

lues

for th

e norm

al b

eam

irra

dia

nce

. The

site

s in

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.


- 34 -

Global irradiance

Intermediate sky conditions 0.30 < K’t ≤ 0.65

Inte

rmed

iate

sky

conditio

ns:

first

and s

econd o

rder

sta

tist

ics

in a

bso

lute

val

ues

for th

e glo

bal

horizo

nta

l irr

adia

nce

.The

site

s in

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.

Inte

rmed

iate

sky

conditio

ns:

first

and s

econd o

rder

sta

tist

ics

in r

elat

ive

valu

es for

the

glo

bal

horizo

nta

l irr

adia

nce

.The

site

s in

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.

Gh [

W/m

2 ]nb

R2

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2m

bd%

sd%

R2

mbd

%sd

%R

2m

bd%

sd%

R2

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Cab

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1578

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249

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2755

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868

1995

Jun

gfra

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h27

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414

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ante

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All

site

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1267

2275

1280

2060

1961

1995

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Met

Sol (

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larG

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sat 3

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Met

Sol (

Dum

ortie

r)IrS

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v


- 35 -

Beam irradiance

Intermediate sky conditions 0.30 < K’t ≤ 0.65

Bn [

W/m

2 ]nb

R2

mbd

%sd

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2m

bd%

sd%

R2

mbd

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2m

bd%

sd%

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Cab

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122

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577

5612

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3173

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104

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759

144

115

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pent

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Yat

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All

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173

2878

8410

357

115

4176

5181

5212

0

3Tie

rEn

Met

Sol (

Solis

)En

Met

Sol (

Dum

ortie

r)So

larG

isH

elio

sat 3

v3IrS

olA

v

Bn [

W/m

2 ]nb

R2

mbd

sdR

2m

bdsd

R2

mbd

sdR

2m

bdsd

R2

mbd

sdR

2m

bdsd

Cab

auw

122

1568

0.76

414

900.

680

9712

60.

577

6814

80.

772

3889

0.75

445

97 C

ambo

rne

104

1293

0.81

654

930.

759

150

120

0.73

877

130

0.81

566

890.

810

7710

1 C

arpe

ntra

s14

398

60.

722

7412

20.

729

179

141

0.67

992

152

0.78

210

911

10.

779

136

123

0.41

915

924

0 D

avos

7615

710.

561

111

161

0.56

624

516

20.

557

149

191

0.68

824

900.

675

8112

70.

474

124

197

El S

aler

Gen

eva

153

1208

0.72

560

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0.73

510

612

80.

615

8817

90.

791

6610

40.

805

9311

50.

651

8515

5 J

ungf

rauj

och

1412

200.

269

232

243

0.11

746

125

60.

203

366

322

0.25

064

110

0.27

013

818

50.

160

215

251

Las

Maj

adas

Ler

wic

k82

1230

0.67

352

105

0.57

016

116

20.

513

124

175

0.69

240

870.

662

5110

1 L

ocar

no N

ante

s16

315

950.

798

091

0.64

688

152

0.68

422

134

0.84

027

830.

836

1482

0.64

646

154

Pay

erne

111

1398

0.67

566

123

0.62

098

138

0.54

410

817

80.

711

7611

00.

717

104

124

0.53

189

158

Sed

e Bo

qer

272

450

0.65

937

142

0.35

510

200

0.52

040

185

0.54

121

160

0.51

531

170

0.31

578

222

Sio

n S

onnb

lick

3215

700.

401

216

250

0.40

241

924

20.

395

308

293

0.45

157

112

0.49

014

818

90.

391

262

290

Tam

anra

sset

124

915

0.72

111

512

20.

578

312

212

0.64

118

419

50.

606

213

179

0.63

724

717

3 T

hess

alon

iki

Tor

aver

e13

113

680.

733

310

80.

726

106

148

0.41

175

193

0.63

321

125

0.69

438

122

Val

d'A

linya

Vau

lx-e

n-V

elin

206

1592

0.80

227

110

0.70

479

139

0.69

964

159

0.85

245

950.

860

4495

0.65

345

154

Wie

n15

215

700.

748

2411

40.

748

7412

70.

674

6114

90.

804

2696

0.79

257

109

0.63

351

155

Yat

ir Fo

rest

Zur

ich

All

site

s14

215

173

4011

011

914

681

163

5810

872

114

7317

1

EnM

etSo

l (So

lis)

EnM

etSo

l (D

umor

tier)

Sola

rGis

Hel

iosa

t 3v3

3Tie

rIrS

olA

v

Inte

rmed

iate

sky

conditio

ns:

first

and s

econd o

rder

sta

tist

ics

in a

bso

lute

val

ues

for

the

norm

al b

eam

irr

adia

nce

.The

site

s in

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.

Inte

rmed

iate

sky

conditio

ns:

first

and s

econd o

rder

sta

tist

ics

in rel

ativ

e va

lues

for th

e norm

al b

eam

irra

dia

nce

. The

site

s in

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.


- 36 -

Global irradiance

Overcast sky conditions 0.00 < K’t ≤ 0.30

Gh [

W/m

2 ]nb

R2

mbd

%sd

%R

2m

bd%

sd%

R2

mbd

%sd

%R

2m

bd%

sd%

R2

mbd

%sd

%R

2m

bd%

sd%

Cab

auw

7912

010.

879

1241

0.85

231

600.

820

4856

0.90

723

390.

904

2140

Cam

born

e91

1179

0.86

69

430.

897

4365

0.84

834

490.

898

2241

0.90

022

40 C

arpe

ntra

s77

485

0.80

326

650.

906

6768

0.74

841

650.

879

4654

0.88

145

550.

673

9213

0 D

avos

9985

40.

704

2472

0.89

512

910

90.

775

5167

0.79

814

530.

791

2557

0.48

551

107

El S

aler

8649

00.

697

3179

0.80

348

740.

627

6610

60.

789

4971

0.78

947

710.

594

4789

Gen

eva

8995

00.

829

2452

0.89

326

490.

803

4359

0.87

029

450.

871

3046

0.65

338

84 J

ungf

rauj

och

105

533

0.63

273

123

0.90

018

414

70.

668

141

122

0.61

125

810.

594

4493

0.46

797

170

Las

Maj

adas

100

451

0.60

627

770.

821

8978

0.56

162

960.

749

5672

0.75

252

710.

647

4993

Ler

wic

k85

1125

0.86

831

490.

873

5383

0.79

256

640.

834

2048

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919

48 L

ocar

no64

1172

0.85

055

750.

848

111

119

0.78

299

107

0.87

856

670.

877

5466

0.72

772

114

Nan

tes

9610

630.

856

338

0.81

746

780.

837

2645

0.90

719

340.

909

1432

0.65

027

71 P

ayer

ne88

1068

0.81

315

510.

831

1658

0.76

842

630.

881

2543

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426

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654

3983

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2862

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2096

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118

0.80

047

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840

3561

0.84

531

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555

5610

9 S

onnb

lick

138

526

0.73

552

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747

139

117

0.70

577

108

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723

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686

4494

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875

129

Tam

anra

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114

224

0.67

829

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865

120

107

0.71

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980.

754

7913

10.

754

7813

1 T

hess

alon

iki

9161

40.

751

1865

0.75

63

680.

673

4687

0.77

426

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772

2562

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243

98 T

orav

ere

7987

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2862

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063

960.

668

6071

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500.

825

1950

Val

d'A

linya

9733

00.

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5894

0.80

319

214

80.

603

105

131

0.79

175

880.

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8393

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911

515

1 V

aulx

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2847

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124

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627

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Wie

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450.

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1240

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637

2882

Yat

ir Fo

rest

123

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ich

9112

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760

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740

5572

0.76

929

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770

3169

0.61

942

93

All

site

s82

1591

722

6048

8051

7530

5829

5963

138

3Tie

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Met

Sol (

Dum

ortie

r)So

larG

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elio

sat 3

v3En

Met

Sol (

Solis

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olA

v

Gh [

W/m

2 ]nb

R2

mbd

sdR

2m

bdsd

R2

mbd

sdR

2m

bdsd

R2

mbd

sdR

2m

bdsd

Cab

auw

7912

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932

0.85

225

480.

820

3844

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718

310.

904

1731

Cam

born

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1179

0.86

68

390.

897

3959

0.84

831

450.

898

2037

0.90

020

37 C

arpe

ntra

s77

485

0.80

320

500.

906

5252

0.74

832

500.

879

3542

0.88

135

420.

673

7110

0 D

avos

9985

40.

704

2472

0.89

512

810

80.

775

5166

0.79

814

530.

791

2557

0.48

550

107

El S

aler

8649

00.

697

2768

0.80

341

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627

5791

0.78

942

610.

789

4060

0.59

440

76 G

enev

a89

950

0.82

921

460.

893

2344

0.80

339

530.

870

2640

0.87

127

410.

653

3475

Jun

gfra

ujoc

h10

553

30.

632

7712

90.

900

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155

0.66

814

812

90.

611

2785

0.59

447

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467

102

179

Las

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adas

100

451

0.60

627

770.

821

8978

0.56

162

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749

5672

0.75

252

700.

647

4993

Ler

wic

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1125

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827

410.

873

4671

0.79

248

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834

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41 L

ocar

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7176

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263

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878

3643

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735

430.

727

4673

Nan

tes

9610

630.

856

337

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745

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837

2543

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718

320.

909

1431

0.65

026

68 P

ayer

ne88

1068

0.81

313

450.

831

1451

0.76

837

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881

2238

0.88

423

380.

654

3473

Sed

e Bo

qer

118

830.

579

1786

0.78

753

890.

437

8914

80.

697

3373

0.69

827

710.

443

2411

4 S

ion

8199

20.

690

2267

0.86

911

495

0.80

038

570.

840

2849

0.84

526

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555

4688

Son

nblic

k13

852

60.

735

7113

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747

192

161

0.70

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514

80.

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3211

30.

686

6112

90.

578

103

178

Tam

anra

sset

114

224

0.67

833

870.

865

137

122

0.71

784

112

0.75

490

150

0.75

490

149

The

ssal

onik

i91

614

0.75

116

590.

756

361

0.67

342

780.

774

2456

0.77

223

560.

612

3989

Tor

aver

e79

871

0.83

022

490.

870

5075

0.66

847

560.

818

1540

0.82

515

39 V

al d

'Alin

ya97

330

0.76

256

910.

803

185

143

0.60

310

112

60.

791

7385

0.79

080

900.

609

111

146

Vau

lx-e

n-V

elin

8610

630.

826

2045

0.85

435

530.

789

4055

0.86

924

400.

871

2040

0.62

734

77 W

ien

9010

820.

844

740

0.84

86

480.

811

2745

0.87

810

360.

877

1236

0.63

725

73 Y

atir

Fore

st12

319

30.

714

3793

0.85

881

880.

624

7510

50.

736

4377

0.73

940

770.

574

4110

9 Z

uric

h91

1281

0.76

023

580.

785

3074

0.74

050

660.

769

2660

0.77

029

630.

619

3985

All

site

s82

1591

718

4939

6542

6225

4824

4836

79

3Tie

rEn

Met

Sol (

Solis

)So

larG

isH

elio

sat 3

v3En

Met

Sol (

Dum

ortie

r)IrS

olA

v

Ove

rcas

t sk

y co

nditio

ns:

first

and s

econd o

rder

sta

tist

ics

in a

bso

lute

val

ues

for th

e glo

bal

horizo

nta

l irr

adia

nce

. The

site

s in

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.

Ove

rcas

t sk

y co

nditio

ns:

first

and s

econd o

rder

sta

tist

ics

in r

elat

ive

valu

es for

the

glo

bal

horizo

nta

l irr

adia

nce

. The

site

s in

gre

y ar

e not

take

n into

acc

ount

in t

he

ove

rall

stat

istics

.

Documents

Five satellite products deriving beam and global irradiance