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SGP BORCAL Overview September 2005 Craig Webb, Ibrahim Reda, & Tom Stoffel U.S. DOE Atmospheric Radiation Measurement (ARM) Program
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OUTLINE Introduction ARM Program Needs Broadband Outdoor Radiometer Calibrations Methodology Radiometer Calibration & Characterization Functionality Quality Assurance of the BORCAL Process Control Radiometer History Summary
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Improve Global Climate Modeling
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Advance Climate Change Research by Providing Accurate Measurements Most ambitious meteorological measurement program since the International Geophysical Year of 1957.
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ARM Climate Research Facilities
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Alaska Oklahoma Nauru ARM Mobile Facility Total of 29 Stations 250 + Radiometers
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In-Situ Cloud Measurements ProteusAltus UAV “Digital” CM-22 & CG-4
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Instantaneous Radiative Flux Cloud Parameterization and Modeling Cloud Properties Aerosol Properties Shortwave 3D Rad Tran Longwave / Water Single Column 2D/3D Models Cloud Properties Aerosol Working groupsSubgroups Radiometry
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Radiometer Calibration Facility Lamont, Oklahoma USA
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ARM Radiometer Calibration Facility 100 Pyranometers 30 Pyrheliometers In each of 2 BORCAL Events per Year
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ARM Radiometers: Shortwave Calibration Traceability
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ARM Radiometers: Longwave Calibration Traceability
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Broadband Outdoor Radiometer CALibration Based on component summation… BORCAL is the Process
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Component Summation Technique Direct Beam (Cavity Radiometer) * Cos(Z) = + Reference Diffuse (Shaded Pyranometer) Global Reference
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But there is NO Reference for Diffuse! Eppley Model PSP & 8-48 Pyranometers
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Shade / Unshade Calibration Standard Procedure: Rs= US - S Dir * Cos(Z) where, Rs = Pyranometer Responsivity (uV/Wm -2 ) US = Unshaded signal (uV) S = Shaded signal (uV) Dir = Direct Normal (Wm -2 ) Z = Solar Zenith angle Shade / Unshade cycles based on pyranometer time response
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Time (mm:ss) 00:0035:00 Signal (mV) 0 10 Direct Unshade Shade Modified Shade / Unshade Calibration
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RCC is the Software Mechanism Radiometer Calibration & Characterization Used to control the BORCAL process with Lab Windows and Access programs…
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RCC Schematic
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Responsivities 1.Rs (45-55) Traditional - Pyranometers Mid-latitude USA (following Ed Flowers & Don Nelson/NOAA) 2.Rs (Composite) Traditional - Pyrheliometers (results depend on amount of data collected) 3.Rs (9 deg intervals) Discrete Characterization - Improve information available to the user.
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Responsivities 4. Rs (45 +/-0.3) Improved trend analyses by more repeatable results 5. Rs (2 deg interval) Improved Characterization 6.Rs (Z) Polynomial fit to AM & PM responses 7.Rs (Lat) Single Rs optimized for measurement station location
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Event Configuration Configuration is accomplished through a hierarchical data base selection process for instruments and system parameters. Configuration is summarized in the top-level window Information is drawn from the underlying Access data tables
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Configuration Verification Configuration verification is conducted by a two-person team. For each instrument, the configuration is verified using a prescribed process of visual inspection and data logging. Step 1: The instrument ID is visually verified and relayed to the computer operator. Verification Path: Visual System Configuration Data Logging Step 2: The instrument ID located in the RCC system and logging commenced. Step 4: The instrument location and cabling is visually verified and compared that in the RCC configuration. Step 3: The instrument is shaded for several seconds. The computer operator confirms the telltale drop in instrument voltage. Step 5: The instrument is marked as verified.
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Data Acquisition Operator logging GPS or manual time maintenance Atmospheric stability monitor Real-time graphing of responsivity, irradiance, or voltage Real-time error trapping and diagnosis Detailed reference irradiance data Real-time access to data acquisition and quality control
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Responsivity Calculations During the calibration event, responsivity is calculated at 30-second intervals as: RS = Thermopile Voltage / Reference Irradiance Direct Beam Reference Irradiance Measured by the absolute cavity radiometer. Diffuse Reference Irradiance Measured by two Eppley 8-48 pyranometers Global Reference Irradiance Calculated from Direct Beam and Diffuse Reference Glo = Dir Cos(zenith) + Dif Pyrheliometers Shaded Pyranometers* Pyranometers *Experimental Reference Irradiance
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BORCAL Results Normal Incident Pyrheliometer (NIP)
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BORCAL Results Pyranometer ± 4% ± 0.9% for 1º bin Rs (45º) 8.10 µV/Wm -2 ± 4%
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Calibration Certificates Traceability and Certification Calibration ConditionsCalibration Results
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Suggested Methods of Applying Results 45-55 Degree Responsivity Composite Responsivity Calibration History Latitude- Optimized Responsivity Responsivity Function 9-Degree Responsivities
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Instrument Responsivity Pyranometer Responsivity Mean of all 30-second RS at 45° ±0.3° Responsivity (RS) is Calculated from the 30-second Individual Instrument Responsivities Shaded Pyranometer Responsivity* Mean of all 30-second RS Pyrheliometer Responsivity Mean of all 30-second RS at 45° ±0.3° *Experimental
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Additional Pyranometer Characterizations 2-degree Bins Mean of all 30-second RS ±0.3° at 2° zenith angle increments Responsivity Function: RS(z) Polynomial in cos(z), fitted to all available 2-degree responsivities 45-55 Degree Mean of responsivities calculated from RS(z) between 45° - 55° 9-degree Bins Mean of responsivities calculated from RS(z) over 9° wide intervals Composite Cosine weighted from z = 0° to 90° of RS(z) Latitude Optimized Latitude limited, calculated from RS(z) and latitude
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~ 0.25% - 2.5% ~ 0.06% ~ 0.4% WRR Transfer of Direct Beam Irradiance Sources of Calibration Uncertainty Data Logger Zenith Angle Calculations (< 75°) Diffuse Sky Irradiance (w.r.t. reference global) Base Uncertainty for each data point as Root Sum Square of Sources of Uncertainty (with respect to reference irradiance) Pyrheliometers ~ 0.5% Pyranometers ~ 0.8% – 3.0% ~ 0.12% Excludes zenith angle and diffuse irradiance uncertainties Includes zenith angle and diffuse irradiance uncertainties
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Pyranometer Calibration Uncertainty Terms U avg = Mean of base uncertainties (%) U std = Standard deviation, base uncertainties RS max = Highest responsivity (all data) RS min = Lowest responsivity (all data) RS= Mean responsivity @ 45° Intermediate Calculations U rad = [ U avg 2 + (2 U std ) 2 ] 1/2 E + = 100 (RS max – RS) / RS E – = 100 (RS – RS min ) / RS ±Uncertainties (%) U 95+ = +(U rad + E + ) U 95– = –(U rad + E – ) Calculated from 30-second data points
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Pyrheliometer Calibration Uncertainty Terms U avg = Mean of base uncertainties (%) U std = Standard deviation, base uncertainties RS max = Highest responsivity (all data) RS min = Lowest responsivity (all data) RS= Mean responsivity (all data) Intermediate Calculations Range= 100 (RS max – RS min ) / RS Uncertainty (%) U 95 = [ U avg 2 + (2 U std ) 2 + (Range/2) 2 ] 1/2 Calculated from 30-second data points
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Database Maintenance Tools Data Editing Forms Data Access Data Plotting Tools Calibration Results System Data Tables Instrument Inventory Customer Calibration Results System Configuration Calibration Facility Data Acquisition
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Data Exporting and Distribution Responsivity Data Export for Transferring Calibration Results Custom Exports with Selectable Parameters and Output Format Dedicated Export Format (AIM Database, Calibration Stickers)
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ARM Instrument Management (AIM) System
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Confirming the Process 11-year History of Control NIP Calibrations BORCAL Event1995 2005 8.70 8.40 Rs (uV/Wm -2 ) 2.0%
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Confirming the Process 11-year History: Control PSP Calibrations (Set 1) BORCAL Event1995 2005 9.10 7.60 Rs (uV/Wm -2 ) 5.0%
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Confirming the Process 11-year History: Control PSP Calibrations (Set 2) BORCAL Event1995 2005 8.90 7.80 Rs (uV/Wm -2 ) 5.6%
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Summary The Atmospheric Radiation Measurement (ARM) Program needs accurate broadband irradiance data from three climatic regions. NREL has developed RCC software to semi- automate the BORCAL process for hundreds of shortwave radiometer calibrations each year. The AIM Database contains BORCAL and deployment information for all ARM radiometers (http://www.nrel.gov/aim) Control Radiometer calibration history shows long- term performance of BORCAL process and radiometer responsivities.
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Thanks! Questions for Craig or Reda? More information available from http://www.arm.gov
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