ARM TR-027

> Ground Radiation (GNDRAD) Handbook
>
> November 2004
>
> T. Stoffel
>
> Work supported by the U.S. Department of Energy,
>
> Office of Science, Office of Biological and Environmental Research

# Contents {#contents .Heading-Front}

[1. General Overview 1](#general-overview)

[2. Contacts 1](#contacts)

[3. Deployment Locations and History
1](#deployment-locations-and-history)

[4. Near-Real-Time Data Plots 1](#near-real-time-data-plots)

[5. Data Description and Examples 1](#data-description-and-examples)

[6. Data Quality 3](#data-quality)

[7. Instrument Details 4](#instrument-details)

**Tables**

[1. 2](#__RefHeading___Toc87435066)

[2. 4](#__RefHeading___Toc87435067)

[3. 5](#__RefHeading___Toc87435068)

# General Overview

The Ground Radiation (GNDRAD) collection of radiometers provides each
Atmospheric Radiation and Cloud Station (ARCS) with continuous
measurements of broadband shortwave (solar) and longwave (infrared)
irradiances for upwelling components. These 1-minute data are collected
from a network of stations to help determine the total radiative energy
exchange within the Tropical Western Pacific.

# Contacts

## Mentor

Tom Stoffel\
National Renewable Energy Laboratory\
1617 Cole Blvd.\
Golden, CO 80401-3393\
Phone: 303-384-6395\
Fax: 303-384-6391\
Email: [tom_stoffel@nrel.gov](mailto:Tom_Stoffel@nrel.gov)

Richard L. Hart, Scientific Associate\
Argonne National Laboratory\
Bldg. 203\
Argonne IL 60439\
Phone: 708-252-5839\
Fax: 708-252-9792\
Email: <hart@anler.er.anl.gov>

## Instrument Developer

This section is not applicable to this instrument.

# Deployment Locations and History 

The GNDRAD is operational at the North Slope of Alaska and Tropical
Western Pacific sites.

# Near-Real-Time Data Plots

[Barrow, Alaska Quicklooks](http://nanuna.gi.alaska.edu/nsadocs/)

[Atqasuk, Alaska Quicklooks](http://nanuna.gi.alaska.edu/atqdocs/)

# Data Description and Examples

See Near-Real-Time Data Plots, Section 4, and Data Quality Health and
Status, Section 6.1.

## Data File Contents

### Primary Variables and Expected Uncertainty

###  {#section .unnumbered}

The following *broadband* irradiance measurements are available from the
GNDRAD platform:

UPWELLING SHORTWAVE (0.3 TO 3.0 micrometers)

1.  Reflected irradiance measured by an inverted pyranometer with a
    hemispheric field of view.

UPWELLING LONGWAVE (4.0 to 50 micrometers)

2.  Irradiance measured by an inverted pyrgeometer with a hemispheric
    field of view.

#### Definition of Uncertainty

Estimating radiometer measurement uncertainties continues to be a topic
for additional research. Based on experiences with these and similar
instruments for renewable energy applications, the following estimated
measurement uncertainties in Table 1 conservatively apply to SKYRAD and
GNDRAD measurement platforms.

[]{#__RefHeading___Toc87435066 .anchor}Table 1.

+-------------+-------------+-------------+-------------+-------------+
| **Me        | **Abb       | *           | **Typical   | **Estimated |
| asurement** | reviation** | *Radiometer | Res         | Measurement |
|             |             | Model**     | ponsivity** | Uncert      |
|             |             |             |             | ainty^\*^** |
|             |             |             | **(μ        |             |
|             |             |             | V/Wm^-2^)** |             |
+-------------+-------------+-------------+-------------+-------------+
| Direct      | DNI         | NIP         | 8.0         | ±3.0% or    |
| Normal      |             |             |             |             |
|             |             |             |             | 4 Wm^-2^    |
| (beam)      |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Diffuse     | DD          | PSP         | 9.0         | ±6.0% or    |
| Horizontal  |             |             |             |             |
|             |             |             |             | 20 Wm^-2^   |
| (sky)       |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Downwelling | DS          | PSP         | 9.0         | ±6.0% or    |
| Shortwave   |             |             |             |             |
|             |             |             |             | 10 Wm^-2^   |
| (global)    |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Downwelling | DIR         | PIR         | 4.0         | ±2.5% or    |
| Longwave    |             |             |             |             |
|             |             |             |             | 4 Wm^-2^    |
| (a          |             |             |             |             |
| tmospheric) |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Upwelling   | US          | PSP         | 9.0         | ±6.0% or    |
| Shortwave   |             |             |             |             |
|             |             |             |             | 15 Wm^-2^   |
| (reflected) |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Upwelling   | UIR         | PIR         | 4.0         | ±2.5% or    |
| Longwave    |             |             |             |             |
|             |             |             |             | 4 Wm^-2^    |
| (t          |             |             |             |             |
| errestrial) |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| ^\*^Field   |             |             |             |             |
| measurement |             |             |             |             |
| un          |             |             |             |             |
| certainties |             |             |             |             |
| include the |             |             |             |             |
| un          |             |             |             |             |
| certainties |             |             |             |             |
| associated  |             |             |             |             |
| with        |             |             |             |             |
| instrument  |             |             |             |             |
| c           |             |             |             |             |
| alibration, |             |             |             |             |
| in          |             |             |             |             |
| stallation, |             |             |             |             |
| operation   |             |             |             |             |
| and         |             |             |             |             |
| m           |             |             |             |             |
| aintenance. |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+

### Secondary/Underlying Variables

This section is not applicable to this instrument.

### Diagnostic Variables

This section is not applicable to this instrument.

### Data Quality Flags

[GNDRAD Data Object Design
Changes](http://science.arm.gov/tool/dod/showdod.php?Inst=gndrad) for
ARM netCDF file header descriptions.

### Dimension Variables

This section is not applicable to this instrument.

## Annotated Examples

This section is not applicable to this instrument.

## User Notes and Known Problems

This section is not applicable to this instrument.

## Frequently Asked Questions

None.

# Data Quality

## Data Quality Health and Status

The following links go to current data quality health and status
results.

-   [DQ HandS](http://dq.arm.gov/) (Data Quality Health and Status)

-   [NCVweb](http://dq.arm.gov/ncvweb/ncvweb.cgi) for interactive data
    plotting using.

The tables and graphs shown contain the techniques used by ARM\'s data
quality analysts, instrument mentors, and site scientists to monitor and
diagnose data quality.

## Data Reviews by Instrument Mentor

This section is not applicable to this instrument.

## Data Assessments by Site Scientist/Data Quality Office

All DQ Office and most Site Scientist techniques for checking have been
incorporated within [DQ HandS](http://dq.arm.gov/) and can be viewed
there.

## Value-Added Procedures and Quality Measurement Experiments

Many of the scientific needs of the ARM Program are met through the
analysis and processing of existing data products into \"value-added\"
products or VAPs. Despite extensive instrumentation deployed at the ARM
CART sites, there will always be quantities of interest that are either
impractical or impossible to measure directly or routinely. Physical
models using ARM instrument data as inputs are implemented as VAPs and
can help fill some of the unmet measurement needs of the program.
Conversely, ARM produces some VAPs not in order to fill unmet
measurement needs, but instead to improve the quality of existing
measurements. In addition, when more than one measurement is available,
ARM also produces \"best estimate\" VAPs. A special class of VAP called
a Quality Measurement Experiment (QME) does not output geophysical
parameters of scientific interest. Rather, a QME adds value to the input
datastreams by providing for continuous assessment of the quality of the
input data based on internal consistency checks, comparisons between
independent similar measurements, or comparisons between measurement
with modeled results, and so forth. For more information, see the [VAPs
and QMEs](http://www.arm.gov/data/vaps.stm) web page.

# Instrument Details

## Detailed Description

### List of Components

The following radiometers manufactured by The Eppley Laboratory, Inc.,
in Table 2 are used at each GNDRAD.

[]{#__RefHeading___Toc87435067 .anchor}Table 2.

  ----------------- -------------- --------------- ----------------- -------------------
  **Measurement**   **Radiometer   **Mounting      **Typical         **Typical
                    Model**        Arrangement**   Responsivity**\   Calibration
                                                   **(μV/Wm^-2^)**   Uncertainty^\*^**

  Upwelling         PSP            Inverted w/o    9.0               ±3.0% or\
  Shortwave                        ventilation                       10 Wm^-2^

  Upwelling         PIR            Inverted w/o    4.0               ±2% or\
  Longwave                         ventilation                       2 Wm^-2^

  ^\*^Field                                                          
  measurement                                                        
  uncertainties are                                                  
  larger and                                                         
  include the                                                        
  uncertainties                                                      
  associated with                                                    
  instrument                                                         
  calibration,                                                       
  installation,                                                      
  operation and                                                      
  maintenance.                                                       
  ----------------- -------------- --------------- ----------------- -------------------

Additional information is available from
[http://www.eppleylab.com](http://www.eppleylab.com/),
<http://www.nrel.gov/srrl/>, and
[http://rredc.nrel.gov](http://rredc.nrel.gov/).

### System Configuration and Measurement Methods

GNDRAD instruments provide two broadband radiometric measurements:

> US = Upwelling Shortwave (reflected solar) Irradiance\
> UIR = Upwelling Infrared (terrestrial) Irradiance.

The data acquisition system is a Coastal Environmenal Systems Model
ZENO^®^-3200 Datalogger. The data system is inside a weather-resistant
enclosure that is mounted above ground. The system is designed to
operate over the full range of environmental conditions anticipated for
the network of ARCS stations (see
[http://www.coastal.org](http://www.coastal.org/) for detailed
measurement and environmental specifications).

### Specifications

Selected theoretical measurement parameters of the SKYRAD and GNDRAD
radiometers are listed below in Table 3.

[]{#__RefHeading___Toc87435068 .anchor}Table 3.

  ----------------- -------------- -------------- -------------- --------------
  **Measurement**   **Radiometer   **Wavelength   **Minimum      **Maximum
                    Field of       Range\         Irradiance\    Irradiance\
                    View**         (microns)**    (Wm^-2^)**     (Wm^-2^)**

  Direct Normal\    5.7°           0.3 to 3.0     0.0            1100
  (beam) \[DNI\]                                                 

  Diffuse           2π sr          0.3 to 3.0     0.0            600
  Horizontal\                                                    
  (sky) \[DD\]                                                   

  Downwelling       2π sr          0.3 to 3.0     0.0            1400
  Shortwave\                                                     
  (global) \[DS\]                                                

  Downwelling       2π sr          4.0 to 50      50             800
  Longwave\                                                      
  (atmospheric)                                                  
  \[DIR\]                                                        

  Upwelling         2π sr          0.3 to 3.0     0.0            1100
  Shortwave\                                                     
  (reflected)                                                    
  \[US\]                                                         

  Upwelling         2π sr          4.0 to 50      100            800
  Longwave\                                                      
  (terrestrial)                                                  
  \[UIR\]                                                        
  ----------------- -------------- -------------- -------------- --------------

**Field of View and Angular Response**

The SKYRAD pyrheliometer (Eppley Laboratory, Inc., Model NIP) field of
view was designed to meet the World Meteorological Organization's design
requirements circa 1960. Without the scattering effects of a clear
atmosphere, the solar disc would appear to be about 0.5° at the earth's
surface. The NIP geometry allows for solar tracker alignment tolerances
possible at the time of the WMO specification and includes, therefore,
an amount of circumsolar (forward scatter) radiation.

The SKYRAD and GNDRAD pyranometers (Eppley Laboratory, Inc., Model PSP)
have unique angular response characteristics that can be determined and
verified with calibration (see separate section). The angular response
of the pyranometer is a major contributor to the estimated measurement
uncertainty for the various shortwave irradiance elements.

**Spectral Response**

The spectral response of the SKYRAD and GNDRAD radiometers listed above
are nominal values. The precise values vary with instrument model.

**Thermal Offsets**

Shortwave radiometers should produce a null signal in the absence of
solar radiation. However, all commercially available pyranometers based
on thermoelectric transducers, exhibit a non-zero output signal in the
absence of solar radiation. These signals are believed to be a result of
thermal gradients within the pyranometer.

The Eppley Laboratory, Inc., Model PSP is a thermoelectric, single-black
detector design with well-established performance characteristics
**\[Zerlaut, 19??\].** As shown in **Figure 4**, the two Schott Glass
hemispheres protect the thermopile detector from the weather. These
inner and outer domes are used to also reduce the thermal exchange
between the detector, or thermopile "hot" junction, and the environment.
The body of the instrument is a relatively massive bronze casting that
is used to control the "cold" or reference junction of the thermopile.
Recent studies of this and other models of pyranometers suggest thermal
offsets are producing as much as 30 Wm^-2^ during clear-sky, nighttime
conditions. Similarly, daytime measurements of downwelling diffuse
shortwave irradiance must be corrected for thermal offsets. ARM will use
a correction method based on correlations with the net infrared and
observed diffuse irradiances **\[Dutton, et al, 2001\]**.

## Theory of Operation

**Shortwave Irradiance**

Three pyranometers are positioned to measure the hemispheric irradiance
fields:

> DS Unshaded, ventilated, and mounted in horizontal orientation\
> US Inverted, unventilated and mounted 10m above the ground level\
> DD Shaded, ventilated, and mounted on automatic solar tracker in
> horizontal orientation.

A single pyrheliometer is mounted on an automatic solar tracker and
aligned to the sun's disc.

The following relationships are expected from the SKYRAD and GNDRAD
measurement systems for the various broadband shortwave irradiance
elements.

![](media/image1.png){width="1.71875in" height="0.9166666666666666in"}

where,

> DS = Downwelling Hemispheric Shortwave (Global) Irradiance\
> DNI = Direct Normal Shortwave (Beam) Irradiance\
> Z = Solar Zenith Angle (sunrise/sunset = 90°)\
> DD = Downwelling Diffuse Shortwave (sky) Irradiance\
> US = Upwelling Diffuse Shortwave (reflected) Irradiance\
> ![](media/image2.png){width="0.19791666666666666in"
> height="0.17708333333333334in"}= Surface Shortwave Albedo (typically
> 0.2 for vegetation, 0.8 for fresh snow)\
> ETR = Extraterrestrial (exo-atmospheric) Radiation on horizontal
> surface\
> ETRN = Extraterrestrial Radiation Normal (beam) to the sun (1366 ±5
> Wm^-2^ **\[REF??\]**).

**Longwave Irradiance**

Elements of the pyrgeometer measurements are used to compute the
infrared:

![](media/image3.png){width="2.2916666666666665in"
height="0.24861111111111112in"}

where,

> IR = Infrared Irradiance (W/sq m)\
> T~tp~ = PIR thermopile voltage (mV)\
> T~c~ = PIR case temperature (K)\
> T~d~ = PIR dome temperature (K)\
> C~1~ = PIR Calibration Factor (W/sq m per mV)\
> C~2~ = PIR Dome Correction Factor (4.0 = default for all PIRs)\
> ![](media/image4.png){width="0.13472222222222222in"
> height="0.11388888888888889in"}= Stephan-Boltzman Constant = 5.67E-08
> W/m^2^ K^4^.

## Calibration

### Theory

The calibration of all ARM *shortwave* radiometers (pyrheliometers and
pyranometers) is traceable to the World Radiometric Reference (WRR)
maintained by the World Radiation Center (WRC) for the World
Meteorological Organization. Calibrations are performed at the SGP
Radiometer Calibration Facility (RCF-see separate entry) using the
Broadband Outdoor Radiometer CALibration (BORCAL) methods developed at
NREL. A group of three electrically self-calibrating absolute cavity
radiometers is maintained by NREL as the reference standards for the
U.S. Department of Energy (DOE) and ARM. WRR calibration traceability of
these reference standards is maintained for all DOE programs by
participation in the International Pyrheliometer Comparisons (IPC) held
every five years at the WRC. IPC-IX was completed in October 2000.

The calibration of all ARM *longwave* radiometers (pyrgeometers) are
traceable to temperature standards maintained by the National Institute
of Standards and Technology (NIST). ARM pyrgeometers will be calibrated
in the new Pyrgeometer Blackbody Calibration System developed by The
Eppley Laboratory, Inc. for the ARM Program and/or by outdoor
comparisons with transfer standards. Standard operating procedures are
currently under development at NREL for later use at the RCF to provide
routine pyrgeometer calibrations.

**Shortwave Radiometer Calibrations**

The *Component Summation Method* used for all BORCAL events is a
modified version of the shading method described in the American Society
for Testing and Materials (ASTM) Standard E913-82, \"Standard Method for
Calibration of Reference Pyranometers with Axis Vertical by the Shading
Method.\"

The direct normal (beam) irradiance is measured with one or two absolute
cavity radiometers, and the diffuse (sky) irradiance is measured with
one or two pyranometers shaded with a tracking disk. The output voltage
of these standards and the radiometers under test are measured at
approximate 30-second intervals throughout each day during clear-sky
conditions (no clouds near the solar disk).

The responsivity for a pyrheliometer is calculated for each data point
by dividing the value of the instrument\'s output signal (microvolts) by
the mean value of the absolute cavity radiometer(s) output (W/sq m).

The responsivity for a pyranometer is calculated by dividing the value
of the instrument\'s output signal (microvolts) by the computed
reference global horizontal irradiance (W/sq m). The computed reference
irradiance is the sum of the diffuse radiation and the vertical
component of the direct beam irradiance (cavity radiometer irradiance
multiplied by the cosine of the solar zenith angle at the time of
measurement). This assumes that the pyranometer has a perfect angular
response. The solar zenith angle calculations are corrected for
atmospheric refraction effects at the calibration facility.

During a calibration event, one or more pyranometers (The Eppley
Laboratory, Inc., Model PSP) and pyrheliometers (The Eppley Laboratory,
Inc., Model NIP) are used as control standards to monitor the
calibration process. The results from these instruments are reviewed to
monitor the calibration process and compare the results of different
BORCAL events.

**Uncertainty Calculations**

The uncertainty of a measurement result is a parameter that
characterizes the spread of the values that could be reasonably
attributed to the measurement. The evaluation of the uncertainty in the
derived BORCAL responsivity of each instrument is based on the
guidelines of the International Standards Organization \"Guide to the
Expression of Uncertainty in Measurements\" \[9\], similar guides
prepared by and available from national standardizing laboratories
\[8,10\] and previous NREL experience \[11\].

The mean responsivity (Rs) is multiplied by a \"base uncertainty\"
(1.3%, or 0.013, derived below) and added to 1/2 the RANGE of
responsivities over any region (a bin, 45/55 bin, or composite) to
provide the uncertainty in terms of μV/W/sq m. Dividing this value by
the mean Rs and multiplying by 100 provides the percent uncertainty. The
rationale for this approach is described below.

Sources of uncertainty are identified in the reference instrumentation,
the process of data collection and computation, and derived from the
data empirically after processing. The sources are classified as \"type
A\" (random), and \"type B\" (non-random). Type A sources are
attributable to processes assumed to have a known distribution,
typically normal or gaussian. Type B (sometimes referred to as
systematic or bias errors) have unknown or non-gaussian distributions.
The later require robust, non-statistically based estimates of their
magnitude. See the references for detailed discussions of estimating
Type B uncertainty sources.

Our uncertainty analysis of the reference instrumentation and data
processing techniques results in a \"base\" uncertainty equal to ± 1.3%
in each individual responsivity computation. The sources of uncertainty
in these processes include uncertainty in:

-   the World Radiometric Reference (WRR) \[12\] (0.3%)

-   transfer of WRR to NREL Working Standards (0.2%)

-   data logger accuracy (0.2%)

-   incidence angle computations (0.64%).

These are all considered to be 2 sigma, random, Type A sources of
uncertainty which have been added together to produce an overall 1.3%
\"base\" uncertainty.

The approach applied here is described in Paragraph 4.6 of Section 4
(\"Type B Evaluation of Standard Uncertainty\") in National Institute of
Standards and Technology Technical Note 1297 \[10\]. This procedure
requires estimating the lower (a-) and upper (a+) limits for the value
(a) in question such that the \"probability that the value lies in the
interval from (a-) to (a+) is for all practical purposes 100%\"\[10\].
Section 2.0 Statistical Tables above describes the procedure for
computing the responsivity, Rs. Responsivities and uncertainty within
the bins of solar zenith angles are provided to assist the user in
understanding the limitations of the instruments, particularly with
respect to angular response.

*-Cautionary Note --*

*This approach asserts that the \"probability that the value lies in the
interval (a-) to (a+) is for all practical purposes 100%\" \[10\] ONLY
for the empirically derived values during the BORCAL event. Only a
LIMITED portion of the angular response of the pyranometers is mapped
out as the result of the solar zenith and azimuth angles available
during the calibration period. However, since the range of incidence
angles encountered generally includes a significant part of those
possible throughout the year, for an instrument mounted horizontally,
the uncertainty quoted for the composite responsivity (if computed) is
probably reasonable for general applications.*

**Applying the Responsivities**

To derive irradiance in Watts per square meter from a radiometer\'s
output voltage, divide the voltage (in microvolts) by the instrument\'s
responsivity (μV/W/sq m).

Example:

Output voltage: 0.00624 V (6240 μV)\
Responsivity: 7.34 μV/W/sq m ± 2.7%\
Irradiance: 6240 / 7.34 = 850.1 W/sq m ± 2.7%.

Thus, with 99% certainty, the true irradiance lies between 827.1 and
873.1 W/sq m, given the cautionary note above in the Uncertainty
Calculations section.

### Procedures

This section is not applicable to this instrument.

### History

This section is not applicable to this instrument.

## Operation and Maintenance

### User Manual

This section is not applicable to this instrument.

### Routine and Corrective Maintenance Documentation

This section is not applicable to this instrument.

### Software Documentation

ARM netCDF file header descriptions may be found at
[GNDRAD](http://science.arm.gov/tool/dod/showdod.php?Inst=gndrad) Data
Object Design Changes.

### Additional Documentation

In order to provide research-quality irradiance measurements, the
radiometers are re-calibrated annually. The existing radiometer
inventory allows for 100% spares, reducing the down time for station
operation due to calibration. Maintenance of the SKYRAD and GNDRAD
equipment is performed during maintenance visits to the ARCS facilities.
Detailed information is available from the Tropical Western Pacific
(TWP) web site (<http://www.dmf.arm.gov/index.html>).

## Glossary

Broadband Irradiance\
Pyranometer\
Pyrgeometer\
Pyrheliometer

Also see the [ARM Glossary](http://www.arm.gov/about/glossary.stm).

## Acronyms

CART: Cloud and Radiation Testbed\
NIP: normal incidence pyrheliometer\
PIR: precision infrared radiometer\
PSP: precision spectral pyranometer\
SIRS: Solar Infrared Station

Also see the [ARM Acronyms and
Abbreviations](http://www.arm.gov/about/acronyms.stm).

## Citable References

Iqbal, M. 1983. *An Introduction to Solar Radiation*. Academic Press,
New York, New York. ISBN 0-12-373750-8 (ISBN 0-12-373752-4 for
paperback), pp. 390.

Maxwell, E.L, and D.R. Myers. 1992. Daily Estimates of Aerosol Optical
Depth for Solar Radiation Models. *Solar \'92, Proceedings of 1992
Annual Conference of the American Solar Energy Society*, p.323. Cocoa
Beach, Florida, June 15-18, 1992.

Reda, I. 1999. *Improving the Shade/Unshade Method to Calculate the
Responsivities of Solar Pyranometer*s. NREL/TR-26483 (June 1999).