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IR Loss in SBD Pyranometers and a Correction Methodology

IR Loss in SBD Pyranometers and a Correction Methodology. Chuck Long NOAA ESRL GMD/CIRES. Pyranometer. What is IR Loss?.

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IR Loss in SBD Pyranometers and a Correction Methodology

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  1. IR Loss in SBD Pyranometers and a Correction Methodology Chuck Long NOAA ESRL GMD/CIRES

  2. Pyranometer What is IR Loss? With pyranometers we want to measure the SW irradiance. We equate a given voltage signal from the thermopile detector to the amount of irradiance through a sensitivity factor. The thermopile voltage is generated by the flow of heat from the detector surface through the thermopile to the heat sink (body). For a pyrgeometer, we want to measure the LW, so the dome is silvered to reflect away the SW irradiance. Then the heat flow is from the body out through the detector surface (and through the dome) towards the radiatively colder sky. Pyrgeometers and pyranometers are essentially the same instrument (body, thermopile, etc.) except for the domes: one lets sunlight through and the other doesn’t.

  3. Day Night Pyrgeometer Pyranometer Pyrgeometer Pyranometer What is IR Loss? Pyranometers at night act like insensitive pyrgeometers. They have two domes to help mitigate the flow of heat from the body out through the detector/domes, but the two domes do not completely eliminate the flow. This reverse flow is labeled an “IR loss” from the instrument, and is most noticeable at night when there is no solar input to mask it. But during the day, it is still present. This “reverse flow” acts like a “back pressure” and (usually) causes a low bias in the pyranometer measurements.

  4. Dutton et al. IR Loss Correction for Diffuse SW • Dutton et al. (2001) recommended methodology: • Use co-located pyrgeometer • Develop relationship using night time data • Apply derived relationship during day • Used a detector-only correction Dutton, E. G., J. J. Michalsky, T. Stoffel, B. W. Forgan, J. Hickey, D. W. Nelson, T. L. Alberta, and I. Reda, 2001: Measurement of broadband diffuse solar irradiance using current commercial instrumentation with a correction for thermal offset errors. J. Atmos. and Ocean. Tech., 18(3), 297–314.

  5. Long et al. (2001) • Adapted the Dutton et al. correction to ARM data • Both “Full” and “Detector Only” • “Full” = (both detector and Tc-Td term) • Completely eliminates sub-Rayleigh • Excellent correction of night data, both average and standard deviation • Under corrects for IR loss during day • Hinted at by Dutton et al. (2001)

  6. Correction Eqn. Fitting • Fit Eqns. are forced through (0,0) • For the “Detector-only” correction:  • Cdet = a1 * Fdet • DifCorD = DifOrg - Cdet • For the “Full” correction: • Cfull = b1 * Fdet + b2 *  * (Td4 – Tc4) • DifCorF = DifOrg – Cfull • Use iterative process for both that ignores “outliers” in the fitting • Fit to night (SZA>98O) data

  7. Bi-Modal Behavior • Bi-Modal behavior of night time offset noted in Long et al. (2001) • Long et al. related the 2nd mode to moist conditions • Thus two independent variables in “full” correction formulation, and two modes (moist and dry) each needing their own set of formula coefficients

  8. Nighttime Bi-Modality Moist Dry From Long et al., 2001

  9. Nighttime Bi-Modality Fig. 1a From Dutton et al., 2001

  10. Bi-Modal Detection • Must use available information, and reliably detect modes both day and night • Detector only “Moist” mode • (Tc – Te) < 6.0 K • RH > 80% • Full “Dry” Mode (all else) • (Tc – Te) > 6.0 K • RH < 80% LW =  Te4 Solve for Te

  11. Bi-Modal Fitting and Correction • Fit separately for each mode (dry and moist) at night • Detect mode and apply appropriate set of correction factors during day and night • Results:

  12. Result for Daytime Correction B&W has minimal IR loss Still ~4 Wm-2 residual

  13. Result for Daytime Correction Still ~4 Wm-2 residual

  14. Why no improvement? • First, one would expect that the full correction would be better. • Two independent variables that are correlated with the dependent variable. • Second, one would expect bi-modal fitting to be better. • Two apparently different relationships • Something is missing!

  15. Relationship of Night to Day • As noted in Long et al., 2001, application of a daytime correction using a nighttime derived relationship tends to under correct the daytime IR loss. • Day includes SW input to the pyranometer, but not the pyrgeometer. • Thus, relationship between the two detectors is different.

  16. Day vs Night Detector Flux Dry Day 40% Slope Difference Dry Night

  17. Apply Enhanced Correction • The day-night detector relationship difference does not affect the Tc-Td relationship. • Difference is related to the detector term only. • There is no detector relationship difference for the detector-only moist mode • For both the Full and Detector Only corrections, apply an enhancement factor during daylight. • 1.4 for Det Only, dry mode only • 2.0 for Det part of full, both modes

  18. No Enhancement Undercorrects

  19. Enhanced Daytime Correction Properly corrects

  20. Enhanced Daytime Correction

  21. Enhanced Daytime Correction

  22. SURFRAD, Desert Rock

  23. SURFRAD, Penn State

  24. Adjustment Factor • Values: • 1.4 for Det Only, dry mode only • 2.0 for Det part of full, both modes • Application: • Fully applied for (SZA < 80O) • Not applied at night (SZA > 90O) • Interpolated for 80O < SZA < 90O X = 0.4 Det X = 1.0 Full X

  25. Correction Eqn. Application • For the “Detector-only” correction:  • AdjDet = 1.0 for SZA > 90O, 1.4 for SZA < 80O, Interpolated between • Cdet = a1 * Fdet* AdjDet • DifCorD = DifOrg - Cdet • For the “Full” correction: • AdjFull = 1.0 for SZA > 90O, 2.0 for SZA < 80O, Interpolated between • Cfull = b1 * Fdet* AdjFull + b2 *  * (Td4 – Tc4) • DifCorF = DifOrg – Cfull

  26. What about Unshaded (Global) Measurements? • Unshaded SBD pyranometers exhibit night time offsets, too • The night offsets are about the same magnitude as shaded (duh) • Are the daylight offsets the same? Greater? Less? • Can we use the same methodology to correct?

  27. Ad Hoc IR Loss Group

  28. SGP E13 19990821-20010221 How BorCals used to be (shaded PSP in reference)

  29. SGP E13 19990821-20010221 What BorCals became (shaded B&W in reference)

  30. SGP E13 19990821-20010221 What BorCals are now (IR loss accounted for in PSP being calibrated)

  31. Consensus • Ultimately decided that yes, the global IR loss was about the same magnitude as shaded IR loss • Can use same methodology to correct • Include an IR loss correction for BorCals • (using black body calibrator assessment of pyranometer IR sensitivity)

  32. Ventilation and IR Loss • The IR Loss is caused through heat transfer from the instrument case (heat sink) out through the detector to the colder sky • The ARM NSA Pyranometer IR Loss Study (extended into the NSA Evaluation of Heated Ventilators in the Arctic) 2006-2009 includes testing of 12V DC fans • With an unexpected result!

  33. Radiometers • Fielded total of 8 PSPs (two shaded), 2 B&Ws (1 shaded), 1 PIR, 1 NIP • Various configurations of fans and heater coils

  34. NSA Radiometer IOP

  35. Night Offsets No heaters, Neg offsets With heaters, +/- offsets AC fan

  36. Night Offsets, AC Fans

  37. Night Offsets, DC Fans DC fans, less Offset, less scatter DC fan

  38. Night Offsets, DC Fans

  39. Oregon State Vignola Test DC fan Oct 27-Nov 4, 2007 DC 3 nights, AC 2 nights Same PSP Swapped into different ventilator AC fan

  40. ARM Results • as Reported by Mark Kutchenreiter • 2015 ASR Science Team Meeting • March 16, 2015 • “ARM Radiometer Ventilator DC Fan Upgrades Ventilation Configuration Evaluations” • Mark Kutchenreiter, Ibrahim Reda, Manajit Sengupta, Mike Dooraghi, Aron Habte, Afshin Andreas

  41. InitialTesting of Candidate 12VDC Fans • Six 12VDC fans with 50-60cfm rating were evaluated at NREL, with original model AC reference fans during January and February 2014. • PSP night time offsets vs. netIR from NREL collocated PIR demonstrated offset improvements of ~ 2.5 - 3.0 W/m2. • All of the DC fans tested provided higher flows and generally equivalent IR loss reduction. • Delta Electronics Model FFB0812VH-T500 was selected based on design characteristics. DC fans AC fans

  42. Results of DC Fan Deployment at Sites • Transition to the higher flow 12 volt DC fans in ventilators consistently resulted in the reduction of PSP thermal offset responses. • Average change in nighttime thermal offset for PSPs from the 18 sites was from -7.0 w/m2 prior to the fan change to -2.3 w/m2 after the fan change.

  43. Results of DC Fan Deployment at Sites Shaded B&Ws • Thermal offset magnitudes were always significantly less for the 8-48s as compared to the PSPs. • Scatter of the nightly thermal offsets for some sites was reduced; SGP C1 BRSshown. • The average change in 8-48 nighttime offsets for the 18 sites was from -0.7 W/m2 prior to performing the fan change to -0.3 W/m2 after the fan change.

  44. Thanks! Summary • IR loss is a reverse flow of heat generating an unwanted negative voltage in pyranometer thermopiles • IR Loss occurs in unshaded SBD pyranometers about the same as for shaded ones • An enhanced methodology has been developed to better correct for IR loss esp. during daylight hours. • Use of 12V DC ventilator fans has been shown to significantly reduce night offsets (IR loss) and noise compared to 120V AC fans

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