Results of Baltic Region Pyrheliometer Comparison, BRPC-2012 Thomas Carlund Swedish Meteorological & Hydrological Institute (SMHI) (Currently at PMOD/WRC) IPC-XII Symposium, PMOD/WRC, Davos, Switzerland.

Pyrheliometer comparison Pyranometer comparison RRC Norrköping

BRPC-2012 Pyrheliometer comparison Participants 11 cavity pyrheliometers 7 institutions 5 countries

Measurements Measurement cadence/series as during IPC’s Reference: Mean of 8 of the participating instruments which all had WRR-factors from IPC-XI Data selection criteria:

Measurement conditions AOD limit @500 nm for IPCs

Some example plots FMI, Finland VNIIOFI, Russia SP, Sweden LHMS, Lithuania MGO, Russia PMOD/WRC

BRPC-2012 Pyrheliometer comparison Summary of results Measurements made on 6 days Data from 4 days used in the final evaluation >300 valid data points for most participating instruments All instruments deviated on average less than 1‰ from the reference The standard deviation was less than 1‰ for all instruments except one The average of the ratios of the three PMOD/WRC instruments to the BRPC reference was 0.999867. Conclusion: BRPC-2012 was a successful pyrheliometer comparison

BRPC-2012 Pyrheliometer comparison Final results

Pyranometer comparison BRPC-2012 Pyranometer comparison During BRPC-2012 also a small pyranometer comparison was carried out 10 pyranometers from 7 countries Calibration and ventilation from their home institutions Pyranometer models: CM10, CM11, CMP11, CM21, CMP21, CM22, LP02, Star References: CH1#030347CaF2 and CM21#051527 calibrated against the BRPC reference.

BRPC-2012 Pyranometer comparison Participants

BRPC-2012 Pyranometer comparison Field references CH1s Shaded, ventilated and netIR offset corrected CM21s. (Two 2AP GD tracker in «clock mode») Calibrated against the BRPC-2012 reference

BRPC-2012 Pyranometer comparison Measurement conditions

All except one pyranometer within ±1% from reference! BRPC-2012 Pyranometer comparison Main result Relative deviation from reference for 6-days accumulated global irradiation Reference (Pyrheliometer+shaded pyranometer) All except one pyranometer within ±1% from reference!

BRPC-2012 Pyranometer comparison Results Comparison of 1-minute mean values Examples for instruments which deviated less than 0.6% from the reference for accumulated values:

Comparison of 1-minute mean values Very good agreement

Comparison of 1-minute mean values Good agreement

Comparison of 1-minute mean values Good agreement

Comparison of 1-minute mean values Slightly less agreement

Comparison of 1-minute mean values Less agreement

Comparison of 1-minute mean values Even less agreement

Comparison of 1-minute mean values The least agreement This one deviated 1.4 % from the reference 6-day global irradiation

Results for accumulated irradiation after calibration of all instruments using SMHI routine (component sum) calibration Relative deviation from reference for 6-days accumulated global irradiation 2-3 times lower std dev than with responsivities from users Instruments 5-13: Mean= 0.27 %, Std.dev.= 0.23 % Maybe there is a potential for us at the Met services who are responsible for the the pyranometer calibrations to perform/derive more homogenous results?

Pyranometer daytime thermal offset Result of capping experiment under clear sky Only ventilated instruments above -3 Wm-2

Pyranometer nighttime and daytime thermal offset Linear offset functions determined from nighttime data (black lines) and from capping (blue or green lines), respectively, versus (shaded) thermopile signal (netIR) from a shaded pyrgeometer.

Conclusion Using the netIR offset correction determined by capping sometimes improves the measurements at solar elevations >15° to 20 °, but not always.

Results for accumulated irradiation after calibration of all instruments using netIR thermal offset correctionand SMHI routine (component sum) calibration Relative deviation from reference for 6-days accumulated global irradiation Instruments 5-13: Mean= 0.64 %, Std.dev.= 0.25 %

Conclusions With one exception the accumulated results of the participating global pyranometers were within ±1 % from the reference global irradiation over 6 days during BRPC-2012. Based on the BRPC-2012 pyranometer comparison results, it is anticipated that pyranometer measurements from several European countries between 45°-60°N agree fairly well for annual irradiation values and for monthly irradiation values from the summer half year. The precision should be within 2 %. Total uncertainties are higher. Global radiation measurements using pyrheliometer and shaded pyranometer show the best agreement with the reference For global radiation measurements ventilated pyranometers of the same type as the shaded reference pyranometers show the best agreement with the reference Instruments should be compared to the same or a better type of instrument. The thermal offset is still an uncertain source of uncertainty It would be interesting to perform a longer pyranometer comparison which also includes more different instrument models. Any volunteers?

Thanks for your attention! More detailed info can be found on the web at https://www.wmo.int/pages/prog/www/IMOP/publications-IOM-series.html , look for IOM 112. Or the direct link: http://library.wmo.int/pmb_ged/iom_112.pdf

Ratio of Responsivity from BRPC and used Responsivity varied between 0 Ratio of Responsivity from BRPC and used Responsivity varied between 0.985 – 1.013 with a mean of 0.999 (Instr. 5-14) Least scatter: Ventilated pyranometer with netIR offset correction. Using netIR offset correction determined from capping did not improve the the calibration results