ITALIAN AIR FORCE METEOROLOGICAL SERVICE ITALIAN AIR FORCE METEOROLOGICAL SERVICE AND AND

MÉTÉO FRANCEMÉTÉO FRANCE

UPDATING AND DEVELOPMENT OF METHODS UPDATING AND DEVELOPMENT OF METHODS FOR WORLDWIDE ACCURATE MEASUREMENTS FOR WORLDWIDE ACCURATE MEASUREMENTS

OF SUNSHINE DURATIONOF SUNSHINE DURATION

WMO TECO, Brussels, Belgium, 16-18 October 2012Vuerich Emanuele (IMS)

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Contents Motivation and purpose Procedures and methods Data analysis and results Conclusions

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Objective To provide the scientific community with the results of

an all-seasons bilateral inter-comparison on sunshine duration (SD) measurement methods that has been

recently conducted by the Italian Meteorological Service (IMS-AF) and Météo France (MF) during the

period 2010-2012

Motivation and purpose1. Motivation

Need for cost-effective solutions with suitable accuracy for the climatology and solar energy applications;a transition from no-more-recommended SD methods to methods with improved achievable accuracy;a standardized calibration method for SD measurements is not yet available and agreed in the community need to verify the applicability of the CIMO Guide requirements:CIMO Guide being SDPERIOD [h day-1] =ΣSDSUB-PERIODS if I ≥ ITHR = 120 W m-2 , than:

1. a “threshold accuracy” of 20% is admitted ( I Є [96;144] W m-2 );2. SD uncertainty should be ±0.1 h day-1

3. SD resolution should be 0.1 h

Are such requirements applicable?

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Motivation and purpose 2. Intercomparison objectives

IMS and MF started a bilateral intercomparison that has been carried out in two different climatological

locations (Vigna di Valle, Italy and Carpentras, France) to achieve the following objectives:

• Primary. Evaluation of the daily achievable uncertainty of SD measurements: pyranometric methods (global

irradiance),1-min-averaged direct solar irradiance and network detectors.

• Secondary. Global use of a pyranometric method for SD by using BSRN (Baseline Surface Radiation Network)

data (for time reason this secondary argument is not treated here: see Vuerich et al. – TECO2012 and Morel et al. - BSRN meeting,

Postdam, 1-3 August 2012) 4

Procedures and methods1. Intercomparison references and instruments

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Carpentras, France (2011) Messrs Morel and Mevel at work

Reference:Pyrheliometer: Epply NIP

Instruments:Pyranometer: K&Z CM11Network SD sensor: C&S recorder (burn method)

Reference:Pyrheliometer: K&Z CH1

Instruments:Pyranometer: K&Z CM11Network SD sensor: CE181 (scanning by optical fiber)

Primary reference: absolute TMI pyrheliometer (RRC Carpentras)

Procedures and methods2. SD measurements methods (1/3)

Reference method (SDREF ): pyrheliometric method based on 1 second direct irradiance (I) measurements (to reduce the uncertainty due to questionable sunshine minutes in case of using 1 minute averages of I)SD using 1 min avg of I and applying the “threshold accuracy”

Three SD measurements by the pyrheliometer:(1) SDpyrh 1m, minutes of SD if the 1 min average of I ≥ 120 W m-2;(2)SDthr 96, seconds of SD if I ≥ 96 W m-2;(3)SDthr 144, seconds of SD if I ≥ 144 W m-2;

Pyranometric methods by using global irradiance, G: Step Algorithm (SA): minutes of SD using 1-min averaged G

compared with a “rough” threshold (GTHR = 0.4 Go, with Go = I0 sin(h) ).

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If G ≥ GTHR then SDSA = 1 minute, otherwise SDSA = 0 minute

Procedures and methods2. SD measurements methods (2/3)

Carpentras method (Olivieri,1998) or Météo-France Algorithm (MFA): minutes of SD through the measurement of 1-min averaged global irradiance (G) compared with an accurate threshold value.

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Carpentras, France (2009)Messers Morel, Olivieri, Didier,

Vuerich

If G ≥ GSeuil then SDMFA = 1 minute, otherwise SDMFA = 0 minute

where:- GSeuil = Fc 1080 (sin(h))1.25

(model)- h ≥ 3° (data filtering)

-Fc = A + Bcos(2πd/365) with:

- Fc representing a fraction of global irradiance in clear sky in mean conditions of

atmospheric turbidity;- h being the elevation angle of the sun in

degrees;- d being the day number of the annual

sequence; Fc factor depends on the climatic conditions of the location and A,B coefficients can be empirically calculated through a long term comparison with SD measurements by means of a pyrheliometer. For Vigna di Valle and Carpentras: A=0.73 and B=0.06

Procedures and methods2. SD measurements methods (3/3)

Slob and Monna Algorithm (SM): algorithm to calculate daily SD from the sum of 10 minutes SD which implies the use of 10 minutes average of G and the use of its maximum and minimum values during the 10 min interval. The procedure to apply the algorithm is documented in the Annex of Chapter 8 of the CIMO Guide (WMO, 2008) and by Hinssen and Knap (2007).

SD network detectors: SD sensors respectively operated by the Italian Met Service (IMS) and Météo-France (MF) radiation networks.

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Data analysis and results

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The data have been analyzed by:

- scatter plots of SDx versus SDREF (where x is the measuring principle used) with linear fits,- dispersion plots of SD daily differences (SDx – SDREF) with normal distribution fits,

and by determing pertinent parameters for estimating the daily achievable uncertainty of each method and the applicability of CIMO guide requirements

BIAS (trueness)St.dev (precision, repeat)R2 (goodness of fitting)

R2 (goodness of fitting)Skewness (symmetry)

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SDpyrh 1m

versus

SDREF

(SDpyrh

1s )

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SDtrh96 and SDtrh144

versus

SDREF

(SDpyrh 1s )

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SDSENSOR

CE181(MF)

SDSENSOR

C&S(IMS)

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SDSM

versus

SDREF

(SDpyrh

1s )

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SDSA

versus

SDREF

(SDpyrh

1s )

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SDMFA

versus

SDREF

(SDpyrh

1s )

RESULTS (Carpentras)

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METHODUncertainty*

[hday-1]Npts [%] in range {-0,1 ; +0,1}

[h/d]SDREF (1s) ± 0.06 /

SD pyrh1 min ± 0.27 76.9%Threshold accuracy

± 0.42 Not applicable

CE181 sensor ± 0.23 84.6%MFA

(best G method)± 0.73

37.5%

SA [-1.37; +1.71] 21.5%Slob&Monna

(Worse G method)

[-1.41; +2.07] 17,4%

* Calculated at 95% of confidence level (normal distribution) , otherwise the interval with the 95% of samples

CIMO Guidetarget

uncertainty

RESULTS (Vigna di Valle)

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METHODUncertainty*

[hday-1]Npts [%] in range {-0,1 ; +0,1}

[h/d]SDREF (1s) ± 0.06 /

SD pyrh1 min ± 0.29 74.8%Threshold accuracy

± 0.45 Not applicable

C&S recorder [-2.35; +1.69] 15.5%MFA

(Best G method)± 0.88

31.7%

SA [-1.36; +1.68] 18.2%Slob&Monna

(Worse G method)

[-1.45; +2.43] 13,8%

* Calculated at 95% of confidence level (normal distribution) , otherwise the interval with the 95% of samples

CIMO Guidetarget

uncertainty

Summary and conclusions

The SDREF must be calculated by 1s I instead of 1-min averaged I;

The achievable uncertainties of pyranometric methods are not comparable with the CIMO Guide target uncertainty and the threshold tolerance must be reviewed;The CIMO Guide requirement is not an appropriate target for routine operational measurements and should be reviewed;The CIMO Guide should be reviewed for taking into account additional algorithms for estimating SD (such as MFA) and for updating the typical achievable uncertainty of those methods;An IOM report on this bilateral intercomparison is expected very soon for extensively providing the results and the achievements.ET on II is investigating the possibility to organize a large and all-seasons intercomparison for all classes of radiation instruments in Europe with the cooperation of PMOD/WRC and sponsorship of interested countries. Is the time to seriously think about that?

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… any successful enterprise starts with a group photo and around table!

Italy and FranceContacts:

vuerich@meteoam.itjean-philippe.morel@meteo.fr

Extra data analysis and results

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Improvement and global use of MFA by BSRN data:- Concept: MFA algorithm was also extended to nine BSRN

stations by using 1-min average global and direct irradiances for at least 4 consecutive years.

- Purpose: determination of the best set of A and B coefficient that minimize the total relative error of SD over a long period of time (years) and a method for an universal application of the MFA for estimating SD from global irradiance at all latitudes.

- Technique: consists in an empirical method that permits to select the A,B from the plot of the cumulative difference between the SD from MFA and SD from 1 min average of direct irradiance (assumed as reference because available from BSRN data).

- Results: recently presented at the BSRN meeting in Postdam, 1-3 August 2012.

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CABAUW (NL) CUMULATED (SD-Sdref) 2006-2009

A=0.77 B=0.05 and 0.06

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-20

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-12

-8

-4

0

4

8

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0 365 730 1095 1460years

ho

urs

B=0,05

B=0,06

2007 200920082006 2010

CARPENTRAS (France) CUMULATED (SD-Sdref) 2007-2010

A=0.77 B=0.05

-24

-20

-16

-12

-8

-4

0

4

8

12

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0 365 730 1095 1460years

ho

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2008 201020092007 2011

BSRN STATION

Latitude A B Err %

MOMOTE -2 0,67 0 -0,1TAMANRASSET 22 0,77 0 -0,2

TATENO 36 0,73 0,05 -0,2BOULDER 40 0,67 0,06 0,1

CARPENTRAS 44 0,71 0,05 0,1PAYERNE 47 0,75 0,06 0,4PALAISEAU 48 0,75 0,04 0,2CABAUW 52 0,77 0,06 0,2TORAVERE 58 0,74 0,06 0,4