SGP BORCAL Overview September 2005 Craig Webb, Ibrahim Reda, & Tom Stoffel U.S. DOE Atmospheric Radiation Measurement (ARM) Program

SGP BORCAL OverviewSeptember 2005

Craig Webb, Ibrahim Reda, & Tom StoffelU.S. DOE Atmospheric Radiation Measurement (ARM) Program

OUTLINE

Introduction

• ARM Program Needs

Broadband Outdoor Radiometer Calibrations

• Methodology

Radiometer Calibration & Characterization

• Functionality

Quality Assurance of the BORCAL Process

• Control Radiometer History

Summary

ARMARMAtmospheric Radiation MeasurementAtmospheric Radiation Measurement

Improve Global Climate ModelingImprove Global Climate Modeling


Advance Climate Change Research

by Providing

Accurate Measurements

Most ambitious meteorological measurement program since the International Geophysical Year of

1957.


ARM Climate Research Facilities


Alaska

Oklahoma

NauruARM Mobile Facility

Total of 29 Stations

250+ Radiometers


In-Situ Cloud Measurements

ProteusAltus UAV

“Digital” CM-22 & CG-4


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 groups Subgroups

Radiometry

Radiometer Calibration FacilityLamont, Oklahoma USA

ARM Radiometer Calibration Facility

100 Pyranometers

30 Pyrheliometers

In each of 2 BORCAL Events per Year

Production RadiometersNIPs, PSPs, 8-48s

WorkingStandards

EPLABTransfer Standards

Field RadiometersSIRS/SKYRAD/GNDRAD

BORCALWorking

Standards

NREL Pyrheliometer ComparisonsTransfer Standard Group

GoldenAnnually

WMO Regional CentersNorth America: NOAA/CMDL

NREL, EPLAB, AES,& Others

WRRInternational Pyrheliometer Comparisons

Davos5-year interval

ARM Radiometers: ShortwaveCalibration Traceability

ARM Radiometers: LongwaveCalibration Traceability

ProductionPrecision

Infrared RadiometerModel PIR

ProductionBlackbodyChamber

EPLABR&D, Production

Reference Standards

SIRS/SKYRAD/GNDRADPyrgeometers

OutdoorCharacterizations

Transfer StandardsModel

PIR & CG4

NREL/ARMPyrgeometer

BlackbodyCalibration System

NRELMetrology LaboratoryReference Standards

National Institute of Standards & TechnologyPrimary References

Electrical, Temperature, MassDimensional, etc.

Broadband

Outdoor

Radiometer

CALibration

Based on component summation…

BORCAL is the Process

Component Summation Technique

Direct Beam(Cavity Radiometer)

* Cos(Z)

=+

Reference Diffuse(Shaded Pyranometer)

GlobalReference

But there is NO Reference for Diffuse!

Eppley Model PSP & 8-48 Pyranometers

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

Time (mm:ss)00:00 35:00

Signal

(mV)

0

10Direct

Unshade

Shade

Modified Shade / Unshade Calibration

RCC is the Software Mechanism

Radiometer

Calibration &

Characterization

Used to control the BORCAL process with Lab Windows and Access programs…

RCC Schematic

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.

Responsivities4. 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

Event ConfigurationConfiguration is accomplished through a

hierarchical data base selection process for instruments and system parameters.

Configuration is summarized in the top-level windowInformation is drawn from the underlying Access data tables

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.

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

Responsivity CalculationsDuring 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

BORCAL ResultsNormal Incident Pyrheliometer (NIP)

BORCAL ResultsPyranometer

± 4%

± 0.9% for 1º bin

Rs (45º)

8.10 µV/Wm-2

± 4%

( )∑=

=

•=46

0/)(

i

i

i

iPMAM zCosazRs

Calibration Certificates

Traceability and Certification Calibration ConditionsCalibration Results

Suggested Methods of Applying Results

45-55 Degree Responsivity

Composite Responsivity

Calibration History

Latitude-Optimized

Responsivity

Responsivity Function

9-Degree Responsivities

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

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

~ 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

Pyranometer Calibration Uncertainty

Terms

Uavg = Mean of base uncertainties (%)

Ustd = Standard deviation, base uncertainties

RSmax = Highest responsivity (all data)

RSmin = Lowest responsivity (all data)

RS = Mean responsivity @ 45°

Intermediate Calculations

Urad = [ Uavg2 + (2 • Ustd )

2 ]1/2

E+ = 100 • (RSmax – RS) / RS

E– = 100 • (RS – RSmin) / RS

±Uncertainties (%)U95+ = +(Urad + E+)U95– = –(Urad + E–)

Calculated from 30-second data points

Pyrheliometer Calibration Uncertainty

Terms

Uavg = Mean of base uncertainties (%)

Ustd = Standard deviation, base uncertainties

RSmax = Highest responsivity (all data)

RSmin = Lowest responsivity (all data)

RS = Mean responsivity (all data)

Intermediate Calculations

Range = 100 • (RSmax – RSmin) / RS

Uncertainty (%)U95 = [ Uavg

2 + (2 • Ustd) 2 + (Range/2) 2

]1/2

Calculated from 30-second data points

Database Maintenance Tools

Data Editing FormsData Access

Data Plotting Tools

Calibration Results

SystemData Tables

•Instrument Inventory•Customer•Calibration Results•System Configuration•Calibration Facility•Data Acquisition

Data Exporting and Distribution

Responsivity Data Export for Transferring

Calibration Results

Custom Exports with Selectable Parameters and

Output FormatDedicated Export Format

(AIM Database, Calibration Stickers)

ARM Instrument Management (AIM) System

Confirming the Process

11-year History of Control NIP Calibrations

BORCAL Event1995 2005

8.70

8.40

Rs

(uV

/Wm

-2) 2.0%


11-year History: Control PSP Calibrations (Set 1)


9.10

7.60

Rs

(uV

/Wm

-2) 5.0%


11-year History: Control PSP Calibrations (Set 2)


8.90

7.80

Rs

(uV

/Wm

-2) 5.6%

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.

Thanks!

Questionsfor Craig or Reda?

More information available fromhttp://www.arm.gov

Documents

SGP BORCAL Overview September 2005 Craig Webb, Ibrahim Reda, & Tom Stoffel U.S. DOE Atmospheric Radiation Measurement (ARM) Program