ITALIAN AIR FORCE METEOROLOGICAL SERVICE AND MÉTÉO FRANCE

1. ITALIAN AIR FORCE METEOROLOGICAL SERVICE AND MÉTÉO FRANCE UPDATING AND DEVELOPMENT OF METHODS FOR WORLDWIDE ACCURATE MEASUREMENTS OF SUNSHINE DURATION WMO TECO, Brussels, Belgium, 16-18 October 2012 VuerichEmanuele (IMS)

2. Objective Contents 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 purpose • Procedures and methods • Data analysis and results • Conclusions

3. 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 beingSDPERIOD [h day-1] =ΣSDSUB-PERIODS if I ≥ ITHR = 120 W m-2 , than: • a “threshold accuracy” of 20% is admitted ( I Є[96;144] W m-2 ); • SD uncertainty should be ±0.1 h day-1 • SD resolution should be 0.1 h Are such requirements applicable?

4. 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)

5. Procedures and methods1. Intercomparison references and instruments Carpentras, France (2011) Messrs Morel and Mevel at work • Reference: • Pyrheliometer: Epply NIP • Instruments: • Pyranometer: K&Z CM11 • Network SD sensor: C&S recorder (burn method) • Reference: • Pyrheliometer: K&Z CH1 • Instruments: • Pyranometer: K&Z CM11 • Network SD sensor: CE181 (scanning by optical fiber) Primary reference: absolute TMI pyrheliometer (RRC Carpentras)

6. 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; • SDthr 96, seconds of SD if I ≥ 96 W m-2; • 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) ). If G≥ GTHRthen SDSA = 1 minute, otherwise SDSA = 0 minute

7. 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. • If G≥ GSeuilthen 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: • - Fcrepresenting 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; Carpentras, France (2009) Messers Morel, Olivieri, Didier, Vuerich Fcfactor 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

8. 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 Gand 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.

9. Data analysis and results 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 normaldistribution fits, and by determing pertinent parameters for estimating the dailyachievable 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)

10. SDpyrh 1m versus SDREF (SDpyrh 1s )

11. SDtrh96 and SDtrh144 versus SDREF (SDpyrh 1s )

12. SDSENSOR CE181 (MF) SDSENSOR C&S (IMS)

13. SDSM versus SDREF (SDpyrh 1s )

14. SDSA versus SDREF (SDpyrh 1s )

15. SDMFA versus SDREF (SDpyrh 1s )

16. RESULTS (Carpentras) CIMO Guide target uncertainty * Calculated at 95% of confidence level (normal distribution) , otherwise the interval with the 95% of samples

17. RESULTS (Vignadi Valle) CIMO Guide target uncertainty * Calculated at 95% of confidence level (normal distribution) , otherwise the interval with the 95% of samples

18. 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 intercomparisonis 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?

19. … any successful enterprise starts with a group photo and around table! Italy and France Contacts: vuerich@meteoam.it jean-philippe.morel@meteo.fr

20. Extra data analysis and results • 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.