#### Surface Radiation Research Branch

J. Michalsky (Editor), G. Andersen, J. Augustine, C. Cornwall, J.
Delamere, P. Disterhoft,

G. Hodges, P. Kiedron, K. Lantz, I. Petropavlovskikh, S. Stierle, C.
Wilson

**Introduction**

GMD\'s Surface Radiation Research Branch (SRRB) was formed in 1993 as a
part of NOAA\'s Air Resources Laboratory. On October 1, 2005, SRRB
merged with the former Climate Monitoring and Diagnostics Laboratory to
form the Global Monitoring Division (GMD) of the new Earth System
Research Laboratory (ESRL) in Boulder, Colorado. On October 1, 2006,
SRRB merged with the Solar and Thermal Atmospheric Radiation (STAR)
group in GMD to form the GMD Radiation (G-RAD) group. Since during most
of the reporting period the STAR and SRRB groups operated independently,
this report will separate the activities. The G-RAD group will report as
one in future GMD summaries.

SRRB conducts research in radiation (solar and infrared) measurements at
the earth\'s surface mainly within the United States. Our primary
emphases are providing quality surface radiation measurements through
the SURFRAD and ISIS networks and, in cooperation with NIST and the
USDA, operating the Central Ultraviolet Calibration Facility (CUCF) for
the calibration of UV-monitoring instruments.

The SURFRAD network consists of seven data collection stations located
in diverse geographic areas of the continental U.S. These stations
collect surface radiation budget data, including incoming and outgoing
shortwave and longwave radiation, incoming UVB and photosynthetically
active radiation, station pressure, 10-meter air temperature, relative
humidity, wind speed and direction, and supplemental measurements using
Total Sky Imagers (TSI) and Multi Filter Rotating Shadowband Radiometers
(MFRSR). Daily data files are collected from all sites just after 0000
UTC each day, and then run through automatic and manual data quality
control programs before being published to the web on the next working
day. The data consist of 3-minute averages of 1-second sampled data,
plus standard deviations for each measurement. The Integrated Surface
Irradiance Study (ISIS) network, made up of nine sites around the
country, collects incoming visible and UVB radiation data. ISIS data
collection and processing follow a parallel path to SURFRAD. The ISIS
network was terminated at the end of 2006 because of funding. The
Central Ultraviolet (UV) Calibration Facility (CUCF) became operational
in 1997 as a joint project between NOAA and NIST. Its mission is to
provide accurate and long-term repeatable calibrations and
characterizations of UV monitoring instruments. CUCF consists of three
main facilities: the calibration laboratory, Table Mountain Test
Facility, and the High Altitude Observatory.

The CUCF has developed dedicated measurement systems for performing
various types of UV calibration services including:

Vertical or horizontal secondary standards of irradiance

Laboratory irradiance calibration

Angular response measurements

### Spectral response measurements

### Slit scattering/stray light measurements for spectroradiometers and spectrographs

### Field calibrations of spectral instruments

### UV spectroradiometer and UV broadband radiometer comparisons

In addition to SURFRAD/ISIS and the CUCF, other efforts at SRRB include
research on the effects of surface UV radiation, improvement of surface
radiation instruments and observing techniques, development of improved
calibration and data quality control techniques, support for other
national and international data collection programs, management of
surface radiation networks, and basic research using the network and
field campaign data.

#### Table Mountain Test Facility

**Buildings and instrumentation support:** The Table Mountain Test
Facility (TMTF) is located on the south edge of a mesa approximately 15
km north of Boulder, Colorado (40.125°N/105.237°W). The primary
structures are a cement block building known as E12B (or the optical
building), a trailer outfitted for office space, and a 9 m x 9 m
elevated deck. E12B provides space for computers, a limited shop area,
as well as storage for support equipment. The main function of the
trailer is to house computers and support equipment for UV spectroscopy.
The deck was designed and built primarily to support instrumentation
used in the monitoring of solar and atmospheric radiation parameters. It
is built around two independent instrument stands that each support 5.5
m long by 1.2 m wide tables. The instrument stands are constructed from
aluminum angle, and the tables, upon which instruments are mounted, are
made of fiberglass decking. This construction design is important
because it isolates the instrument tables from deck vibrations. . A
third smaller freestanding instrument platform is located immediately
south of the deck and can be accessed from the deck. To the east are a
series of eight concrete pads that are used for both temporary and
permanent instrumentation.

**Communication:** Fiber optic cable enters E12B from the north. The
fiber is then split with one run terminating at a Cisco router and the
other run going to the trailer, along with two strands of CAT5e. In
addition to ethernet, analog phone lines are used for data transmission.
There are two active phone lines at the site. One, used for voice
communication, is available in both E12B and the trailer. The other,
used for data transmission, is available only in E12B. This line, in
conjunction with a phone switch, is used to communicate with and
download data from three data acquisition systems.

**Primary Functions**

**Instrument calibration:** The TMTF is in near constant use for the
calibration of a variety of instruments. Typically a reference group of
three instruments is used to calibrate one or more instruments of
similar type. For example, a reference group of three Eppley Precision
Infrared Radiometers (PIRs) are routinely used to field calibrate other
PIRs. The PIR reference triad is calibrated on an annual basis at the
World Radiation Center in Davos, Switzerland. Other reference groups
also undergo annual calibrations.

**Inter-comparisons:** Instrument comparisons are performed on an annual
basis as noted above, but large inter-comparison events involving both
national and international participants are hosted as well. For example,
five inter-comparisons of UV spectrometers have been conducted that
included the CUCF's reference UV spectroradiometer. The reference UV
spectrometer at TMTF is routinely used to transfer calibrations to
UV-MFRSRs and broadband UVB radiometers in support of a network operated
by the USDA and SURFRAD.

**NOAA SURFRAD:** One of the seven NOAA surface radiation (SURFRAD)
measurement sites is located at the TMTF. This site was installed in the
summer of 1995 and the first published data are from August 1995. The
SURFRAD instruments measuring incoming radiation are deployed on one of
the instrument stands on the deck. The instruments measuring upward
radiation as well as meteorological parameters are located on a 10-m
tower approximately 150 m to the northwest of the deck.

**USDA UV research:** The USDA UV program uses the Table Mountain
facility for on-going research. They have deployed UV MFRSRs, a UV
Rotating Shadowband Spectroradiometer (RSS) instrument, and the U1000
ultraviolet spectroradiometer. GMD staff members collaborate with the
USDA-supported research staff from the Colorado State University's
Natural Resources Ecology Laboratory in conducting this research.

**UV materials monitoring:** The TMTF has been chosen as one of eleven
sites a NIST-Smithsonian partnership will be using to study the
degradation of materials caused by UV. The Smithsonian has installed a
spectral instrument they will be using for this research, but no
materials racks have been constructed.

**NEUBrew:** The NOAA-EPA UV Brewer spectrometer network is operated out
of Boulder and one of the six sites is at Table Mountain. Table Mountain
is the primary location for the calibration and characterization of the
Brewer instruments used in NEUBrew.

**Instrumentation**

A variety of instrumentation installed in temporary or permanent
configurations operates at the TMTF. Table 1 below is a summary of all
Table Mountain instrumentation.

Table 1. Table Mountain Continuous Monitoring Instruments

+-----------------------+-----------------------+-----------------------+
| Measurement           | Instrument            | Sampling Frequency/   |
|                       |                       |                       |
|                       |                       | Averaging Period      |
+-----------------------+-----------------------+-----------------------+
| *Solar Radiation*     |                       |                       |
+-----------------------+-----------------------+-----------------------+
| > Global Irradiance   | Spectrosun SR-75      | 1 second / 3 minute   |
+-----------------------+-----------------------+-----------------------+
| > Direct Irradiance   | Eppley pyrheliometer  | 1 second / 3 minute   |
+-----------------------+-----------------------+-----------------------+
| > Diffuse Irradiance  | Eppley 8-48           | 1 second / 3 minute   |
|                       | pyranometer           |                       |
+-----------------------+-----------------------+-----------------------+
| > Reflected           | Spectrosun SR-75      | 1 second / 3 minute   |
| > Irradiance          |                       |                       |
+-----------------------+-----------------------+-----------------------+
|                       |                       |                       |
+-----------------------+-----------------------+-----------------------+
| *IR Radiation*        |                       |                       |
+-----------------------+-----------------------+-----------------------+
| > Incoming (Sky)      | Eppley PIR            | 1 second / 3 minute   |
+-----------------------+-----------------------+-----------------------+
| > Outgoing            | Eppley PIR            | 1 second / 3 minute   |
| > (Terrestrial)       |                       |                       |
+-----------------------+-----------------------+-----------------------+
|                       |                       |                       |
+-----------------------+-----------------------+-----------------------+
| *UV*                  |                       |                       |
+-----------------------+-----------------------+-----------------------+
| > Broadband UV        | Yankee UVB-1          | 1 second/ 3 minute    |
|                       |                       |                       |
|                       | Solar Light 501A      |                       |
|                       |                       |                       |
|                       | Solar Light 501B      |                       |
|                       |                       |                       |
|                       | Kipp & Zonen UVS-A-T  |                       |
|                       |                       |                       |
|                       | Kipp & Zonen UVS-B-T  |                       |
|                       |                       |                       |
|                       | EKO                   |                       |
|                       |                       |                       |
|                       | ScinTec               |                       |
+-----------------------+-----------------------+-----------------------+
| > Spectral UV         | SUNY U111             | 1 twenty-min scan per |
|                       |                       | hr                    |
+-----------------------+-----------------------+-----------------------+
| > Spectrograph UV     | Yankee UV-RSS         | 1 measurement per     |
|                       |                       | minute                |
+-----------------------+-----------------------+-----------------------+
| > Narrowband          | Yankee UV-MFRSR       |                       |
+-----------------------+-----------------------+-----------------------+
| > Narrowband          | Smithsonian SR-18     | 1 measurement per     |
|                       |                       | minute                |
+-----------------------+-----------------------+-----------------------+
| *Aerosols*            |                       |                       |
+-----------------------+-----------------------+-----------------------+
| > Aerosol Optical     | Yankee MFRSR          | 15 second / 2 minute  |
| > Depth               |                       |                       |
+-----------------------+-----------------------+-----------------------+
|                       |                       |                       |
+-----------------------+-----------------------+-----------------------+
| *Meteorology*         |                       |                       |
+-----------------------+-----------------------+-----------------------+
| > Wind                | R.M. Young 05103      | 1 second / 3 minute   |
| > Speed/Direction     |                       |                       |
+-----------------------+-----------------------+-----------------------+
| > Atmospheric         | Vaisala               | 1 second / 3 minute   |
| > Pressure            |                       |                       |
+-----------------------+-----------------------+-----------------------+
| > Air Temp @ 10 m     | Vaisala               | 1 second / 3 minute   |
+-----------------------+-----------------------+-----------------------+
| > RH @ 10 m           | Vaisala               | 1 second / 3 minute   |
+-----------------------+-----------------------+-----------------------+
|                       |                       |                       |
+-----------------------+-----------------------+-----------------------+
| *Clouds*              |                       |                       |
+-----------------------+-----------------------+-----------------------+
| > Sky Images          | Yankee TSI-440a       | 1 image per minute    |
+-----------------------+-----------------------+-----------------------+

**SURFRAD**

The first national [Surf]{.underline}ace [Rad]{.underline}iation Budget
Network (SURFRAD; Augustine et al. 2000) was established in 1993 through
the support of NOAA\'s Office of Global Programs. It was initiated and
operated by NOAA\'s Air Resources Laboratory until the merger with
GMD/ESRL in October 2005. In April 2004, SURFRAD became an official part
of the Global Climate Observing System (GCOS) through its affiliation
with the international Baseline Surface Radiation Network (BSRN). Seven
stations are operating in climatologically diverse regions of the United
States (Figure 1). The first four stations, Bondville, Fort Peck,
Goodwin Creek, and Table Mountain were established in 1995, Penn State
and Desert Rock were added in 1998, and the Sioux Falls station was
installed in 2003 (Figure 2). Station locations were chosen in regions
where the landform and ground cover are nearly homogeneous over an
extended region so that station measurements would be representative of
a larger area for validating satellite retrieval algorithms and the
radiation transfer code of hydrologic, weather, and climate models.

Three platforms house the instruments and support equipment at a SURFRAD
station (see Figure 2). The upward-viewing radiometers rest on a narrow
rectangular platform (0.3 m by 2.7 m) and on an automatic solar tracker.
North of the main platform is a 10-m tower with downward viewing
radiometers and meteorological instruments. The north-south layout
ensures that the tower will never shade the upward-viewing radiometers.
Summed upwelling and

![](media/image1.jpeg){width="5.775in" height="3.7125in"}

#### Figure 1. The national surface radiation budget network---SURFRAD

![](media/image2.jpeg){width="5.73125in" height="4.309027777777778in"}

Figure 2. The SURFRAD station at Sioux Falls, South Dakota. The large
dome in the background is a LandSat antenna operated by the EROS Data
Center.

downwelling components of broadband solar and thermal infrared yield the
surface radiation budget, and thus are the fundamental measurements at a
SURFRAD station. Direct and diffuse solar measurements are the primary
source for downwelling solar, but global solar is also measured for
redundancy. Two instruments on the main platform monitor wavebands of
special interest. One is a broadband UVB radiometer that measures the
amount of harmful ultraviolet radiation (280-320 nm) that penetrates the
ozone layer and reaches the surface. The other monitors the intensity of
the waveband active in photosynthesis (400 to 700 nm). The Multi-Filter
Rotating Shadowband Radiometer (MFRSR) measures both global and diffuse
solar radiation in six narrow bands (10 nm) of the solar spectrum of
which five are used to derive a multispectral aerosol optical depth
product. SURFRAD has adopted the standards for measurement set by the
BSRN (McArthur 1998). These are ±5 W/m2 for broadband solar measure­ments
and ±10 W/m2 for thermal infrared. A world standard for calibration of
broadband thermal infrared instruments has not yet been identified.
However, the three standard pyrgeometers that are used to calibrate
SURFRAD monitoring pyrgeometers are sent annually for calibration to the
World Radiation Center\'s Physikalisch-Meteorologisches Observatorium in
Davos (PMOD), Switzerland. The PMOD blackbody infrared radiation
calibration unit was chosen because it performed well in a round-robin
test that involved a blind comparison of several such devices (Philipona
et al. 1998). After the SURFRAD standard pyrgeometers are calibrated in
the blackbody at PMOD, their calibrations are fine tuned by running them
outdoors against PMOD\'s reference group of four pyrgeometers, which are
referred to as the World Pyrgeometer Standard Group. That group is
traceable to the Absolute Sky-scanning Radiometer (ASR) (Philipona
2001), which was used as a reference in two international pyrgeometer
comparisons, IPASRC-I (Philipona et al. 2001) and IPASRC-II (Marty et
al. 2003).

Calibration of the UVB radiometer is provided by NOAA\'s CUCF. UVB
broadband radiometer standards used there are calibrated regularly
against lamps that are traceable to the National Institute for Standards
and Technology (NIST) in Gaithersburg, Maryland (Lantz et al. 1999).

As a quality assurance measure, radiometers at SURFRAD stations are
exchanged annually. Before field deployment, freshly calibrated
replacement instruments are compared to sets of three standards at the
CUCF\'s field test site at Table Mountain. After the instrument
exchange, the returning instruments are operated alongside the same
standards. This process adheres to basic climate monitoring principles
by providing a pseudo overlap of the newly deployed instruments with
those that they replace, thus documenting systematic differences between
them.

SURFRAD stations are polled automatically once per day. Calibration
factors are automatically applied to the measured voltages to produce
the daily data files for distribution. Raw data files are retained to
facilitate reprocessing. Before being released, all SURFRAD data are
subjected to automated data quality control, and also plotted for visual
inspection, as required by the BSRN. Daily quality controlled data files
for each station are distributed the next working day by anonymous FTP
and on the worldwide web at (http://www.srrb.noaa.gov/).

A number of ancillary measurements and products support the basic
SURFRAD measurements and subsequent research involving SURFRAD data. For
example, each station has a Total Sky Imager that produces hemispheric
images of the sky and an estimate of fractional sky cover at one-minute
intervals. For radiative transfer modeling applications, rawinsonde
soundings are interpolated to the SURFRAD station locations at 0000 and
1200 UTC every day from the national radiosonde network. The clear-sky
identification method of Long and

Ackerman (2000) is applied to SURFRAD data periodically, and the
resulting products are available from an FTP site. Last, an aerosol
optical depth algorithm and product has been developed for SURFRAD that
uniquely utilizes SURFRAD\'s clear-sky identification product (Augustine
et al. 2003), and introduces several features that minimize the error in
the aerosol optical depth retrieval (Augustine et al. 2005).

Owing to the quality, utility, and continuity of its datasets, SURFRAD
has garnered a diverse and extensive clientele that includes the NOAA
National Environmental Satellite and Data Information Service (NESDIS),
the National Centers for Environmental Prediction (NCEP), the National
Aeronautics and Space Administration (NASA), particularly NASA\'s EOS
program, the Department of Defense (DOD), the United States Geological
Survey (USGS), the European Centre for Medium Range Weather Forecasting
(ECMWF), several levels of academia, state governments, and private
industry.

#### The Central Ultraviolet Calibration Facility

Members of the United States Global Change Research Program established
the Central Ultraviolet Calibration Facility (CUCF) in the early 1990\'s
through a Memorandum of Understanding (MOU). The MOU was signed by
supporting agencies, which include the National Institute of Standards
and Technology (NIST), NOAA, the United States Department of Agriculture
(USDA), the Environmental Protection Agency (EPA), the National Science
Foundation (NSF), the Smithsonian, and NASA. Construction and minimal
operation began in 1994. The facility became fully operational in early
1997. Its sole purpose is to transfer standards and
calibration/characterization procedures from the NIST and disseminate
them to cooperating agencies.

The CUCF consists of three facilities: the calibration laboratory, the
Table Mountain Test Facility (TMTF), and the High Altitude Observatory
(HAO). The calibration laboratory is located in NOAA's David Skaggs
Research Center in Boulder, Colorado. Characterizations and calibrations
are performed on many types of solar UV monitoring instruments. The
three main types of instruments that the CUCF calibrates regularly are
the broadband radiometer, the narrow-band filter radiometer, and the
high spectral resolution spectroradiometer. Each instrument is
characterized for its angular response, spectral response, and stability
using a reference lamp prior to its absolute calibration. The
spectroradiometers and narrow-band instruments are then calibrated
against a NIST-traceable standard of irradiance either in the laboratory
or in the field with the CUCF's portable field calibrator. Broadband
instruments are indirectly calibrated against a reference
spectroradiometer by comparing to a triad of reference broadband
radiometers, all of which are located at the TMTF.

The TMTF is located 15 km north of Boulder, Colorado, on the Table
Mountain plateau. The site is multi-purpose. It serves as a field
calibration site for broadband UV radiometers, a NOAA SURFRAD site, a
USDA monitoring site, a NOAA-EPA Brewer network monitoring site, and it
is host to UV spectroradiometer and filter instrument intercomparisons.

The High Altitude Observatory (HAO) is located 25 km west of the TMTF at
an elevation of 2896 m above sea level, approximately 1207 meters higher
than the TMTF. The altitude difference serves two purposes. First, it
serves as a clean high-altitude site for the NEUBrew network, and
second, because of the low aerosol content air and unimpeded view to the
eastern horizon, it is a prime location for Langley-method calibrations
of narrow band instruments.

The CUCF is currently calibrating more than 80 instruments per year for
agencies performing solar UV and atmospheric aerosol research.

#### The NOAA-EPA UV Brewer (NEUBrew) Network

The Surface Radiation Research Branch of NOAA's Global Monitoring
Division has entered into an interagency agreement with the EPA's Office
of Air and Radiation to operate a six-site solar UV and ozone-monitoring
network.

Each network site will be equipped with an automated Brewer Mark IV
spectrophotometer. The Brewer spectrophotometer, shown in Figure 3 was
initially developed to measure total column ozone through direct-sun
measurements. Measurement capabilities that have been included in the
Brewer Mark IV include solar UV with an extended scanning range from
286.5 to 363 nm, total column NO~2~ and SO~2~ trace gases, and ozone
profile using the Umkehr method.

![](media/image3.jpeg){width="4.622222222222222in"
height="3.4770833333333333in"}

**Figure 3. The Brewer Mark IV spectrophotometer**

The sites for the new Brewer network were chosen according to their
typical air quality, ranging from clean, to moderately polluted, and to
heavily polluted. Existing useful ancillary instrumentation located at
the sites also factored into the site selection. The sites are listed in
Table 2 along with the Brewer instrument serial numbers and their
latitude, longitude and altitude. Some of the collocated instrumentation
at each site is listed in Table 3. This is not a comprehensive list of
all instrumentation at each site, but includes instruments that
complement the research goals of the network.

The first site was deployed in August 2006 in Houston, in time for the
2006 Texas Air Quality study, and all six sites were operational by
mid-November 2006. The Brewer at Houston (Figure 4) is located on the
roof of an 18-story building on the University of Houston campus.

**Table 2. The location information for each of the NOAA-EPA UV Brewer
monitoring sites**

  -------------- -------------- -------------- -------------- --------------
  Location       Brewer         Latitude       Longitude      Altitude

  Boulder,       93-101         N 40.125       W 105.237      1689 m ASL
  CO-Table                                                    
  Mountain                                                    

  Niwot Ridge,   96-146         N 40.032       W 105.533      2896 m ASL
  CO                                                          

  Fort Peck, MT  97-147         N 48.310       W 105.100      634 m ASL

  Houston, TX    97-154         N 29.718       W 95.341       80 m ASL

  Bondville, IL  96-141         N 40.040       W 88.360       213 m ASL

  Raleigh, NC    96-144         N 35.728       W 78.680       124 m ASL
  -------------- -------------- -------------- -------------- --------------

Table 3. Some of the collocated instrumentation at each of the UV
monitoring sites is listed here.

  --------------- --------- --------- --------- ----------- --------- ---------
                  Boulder   Houston   Niwot     Bondville   Ft Peck   Raleigh
                                      Ridge                           

  SURFRAD site~1~ X                             X           X         

  USDA site~2~    X         X                   X           X         X

  Surface Flux~3~                               X           X         

  TSI~4~                    X                                         

  CIMEL~5~                  X                                         

  Atmospheric               X                                         
  Chemistry~6~                                                        

  UV Broadband~7~                     X                               

  PSP~8~                              X                               

  UV-MFRSR~9~                                                         

  Vis-MFRSR~10~                       X                               
  --------------- --------- --------- --------- ----------- --------- ---------

1\. SURFRAD sites consist of a pyranometer, pyrgeometer (upwelling,
downwelling, and diffuse), a normal incidence pyrheliometer (NIP), a
total-sky imager (TSI) broadband UV radiometer (UVB-1), a LI-COR PAR
sensor, vis-MFRSR, a barometer, and a meteorological tower with
temperature, relative humidity, and wind speed/direction

2\. USDA sites consist of a UV-MFRSR, a vis-MFRSR, a UV broadband
radiometer (Yankee UVB-1), a temperature and relative humidity probe and
a LI-COR pyranometer sensor.

3\. Surface heat flux measurements

4\. Yankee Total Sky Imager

5\. CIMEL automatic sun-tracking photometer

6\. Surface trace gas measurements

7\. Yankee UVB-1 and a Yankee UVA radiometer

8\. Eppley Precision Solar Pyranometer

9\. UV multi filter rotating shadowband radiometer

10\. Visible multi filter rotating shadowband radiometer

Network data, site information, and pictures are available via ftp
through a webpage interface.

![](media/image4.jpeg){width="5.983333333333333in" height="4.4875in"}

Figure 4. Brewer 96-146 is located on the top of a dormitory tower at
the University of Houston.

#### CUCF Research

#### 

#### UV-MFRSR Out-of-Band Rejection Studies

Recent work has involved identifying the source of an error in the
measured solar irradiance in 5 out of 7 channels from the UV
Multi-Filter Rotating Shadowband Radiometers (UV-MFRSRs) after a
manufacturing up-grade \[Lantz et al., 2005\]. Comparison of solar
irradiance at the Table Mountain Test Facility (TMTF) between the
UV-MFRSR measurements and the U111 spectroradiometer measurements
weighted with the filter functions from the UV-MFRSR channels showed
poor agreement for channels 3-7. The average error for 47 radiometers
was 0%, 0%, 8.8% 22.1%, 12.1%, 11% and 4% for channels 300 nm, 305nm,
311 nm, 317 nm, 325 nm, 332 nm and 368 nm, respectively (Table 4).
Detectors for channels 3 -- 7 were recently changed from GaP to Si
photodetectors because the GaP detectors were failing in the field. The
CUCF measured out-of-band rejection for these channels and determined
that these newer detectors were susceptible to out-of-band light when a
calibration lamp was used as a source. In contrast, the UV-MFRSRs showed
minimal light leakage with the sun as a source (Table 5). This suggested
that the out-of-band rejection problem was associated with the 1000W
tungsten quartz halogen lamps used in the laboratory calibrations (i.e.,
spectral irradiance from the lamps is distributed differently from the
sun). UV-MFRSR instruments were re-calibrated and tested for out-of-band
light leakage, and corrections for the out-of-band light were determined
for all seven channels of each of the 47 radiometers. This study was
conducted over a two-year period.

Several UV-MFRSRs were placed in the field at the TMTF, and the spectral
solar irradiance from the various channels, corrected for the
calibration error, was compared to the filter-weighted solar irradiance
measurements from the U111 precision spectroradiometer. The out-of-band
corrections to the absolute response of the two UV-MFRSRs at the 2003
Intercomparison resulted in very good comparisons with the
filter-weighted U111 spectroradiometer (Figure 5). Subsequent
comparisons between the UV-MFRSR and the U111 spectroradiometer
measurements show that after out-of-band rejection corrections, the
UV-MFRSR measurements are approximately 3-5% too high across the seven
channels. Investigations into the cause of the remaining discrepancy, as
well as the failure of the original GaP photodetectors are on going.
Results from these studies were published and presented at the Remote
Sensing of Atmospheres, Society for Optical Engineering (SPIE), Bruges,
Belgium, September, 2005 \[Lantz et al., 2005\].

Table 4. Average percent contribution of out-of-band light to the
measured lamp signal during calibration with a FEL-type
quartz-tungsten-halogen lamp from 40 filter radiometers

  --------------- -------------- -------------- -------------- -------------- -------------- -------------- --------------
                  ***300-nm***   ***305-nm***   ***311-nm***   ***317-nm***   ***325-nm***   ***332-nm***   ***368-nm***

  ***Average***   *0.0%*         *0.0%*         *8.8%*         *22.1%*        *12.1%*        *11.2%*        *4%*

  ***Maximum***   *0.1%*         *0.1%*         *28.4%*        *35.7%*        *28.5%*        *23.1%*        *10.5%*

  ***Minimum***   *0.0%*         *0.0%*         *4.2%*         *6.6%*         *3.9%*         *4.9%*         *1.1%*
  --------------- -------------- -------------- -------------- -------------- -------------- -------------- --------------

Table 5. Average % contribution of out-of-band light to the measured
solar signal (47 radiometers)

  --------------- -------------- -------------- -------------- -------------- -------------- -------------- --------------
                  ***300-nm***   ***305-nm***   ***311-nm***   ***317-nm***   ***325-nm***   ***332-nm***   ***368-nm***

  ***Average***   *0.1%*         *0.0%*         *0.2%*         *0.4%*         *0.1%*         *0.1%*         *0.1%*

  ***Maximum***   *0.0%*         *0.3%*         *0.5%*         *1.3%*         *0.3%*         *0.4%*         *0.5%*

  ***Minimum***   *0.0%*         *0.0%*         *0.0%*         *0.0%*         *0.0%*         *0.0%*         *0.0%*
  --------------- -------------- -------------- -------------- -------------- -------------- -------------- --------------

#### The 2003 North American UV Spectroradiometer and Filter Radiometer Intercomparison

One mission of the CUCF is to hold UV instrument intercomparisons at the
Table Mountain Test Facility. The UV instrument intercomparisons at the
TMTF included U.S. agencies with UV monitoring and research programs and
international guests. The CUCF, in collaboration with the Optics
Division at NIST, has held five UV spectroradiometer intercomparisons
since 1994. The CUCF has been responsible for organizing the UV
spectroradiometer intercomparisons, data collection, formatting,
analysis, and publishing of the results \[Thompson et al., 1997, Early
et al., 1998a, 1998b, Lantz et al., 2002\]. The analysis

![](media/image5.jpeg){width="6.5in" height="4.832638888888889in"}Figure
5. Comparison of solar irradiance measurements from 7 channels of the
UV-MFRSR to the U111 spectroradiometer solar irradiance measurements
weighted with the 7 UV-MFRSR filter functions before the correction of
the UV-MFRSR responsivity for out-of-band light (top 7 panels) and after
correction for out-of-band-light (bottom 7 panels) for instrument
USDA_290.

includes comparing UV irradiance from several different instrument types
such as scanning spectroradiometers, spectrographs, and narrow-band
filter radiometers. The analysis included comparing the measured
irradiance from the different instruments and providing the results of
those comparisons on the next day to the participants for immediate
feedback on instrument performance. A review of four intercomparisons
and what was learned to improve UV Monitoring in the U.S. was presented
at the 2000 Ozone Symposium, Sapporo, Japan \[Lantz et al., 2000\].

The latest NOAA North American UV Radiometer Intercomparison was held
from 13 to 21 June 2003. The primary purpose of this intercomparison was
to assess the basic level of agreement of ultraviolet measuring
instruments used by UV Monitoring Networks in North America and several
international agencies. Participating agencies included NOAA\'s Air
Resources and Climate Monitoring and Diagnostics Laboratories, the
Natural Resources Ecology Laboratory (NREL), which manages the United
States Department of Agriculture\'s (USDA) UV Monitoring Program,
Biospherical Instruments (BSI) representing the National Science
Foundation\'s UV Polar Programs (NSF), the University of Georgia\'s
National UV Monitoring

Center managing the Environmental Protection Agency\'s (EPA) UV
Programs, the National Institute of Water & Atmospheric Research of New
Zealand (NIWA), the University of ![](media/image6.jpeg){width="6.5in"
height="4.253472222222222in"}Hanover\'s Institute of Meteorology and
Climatology (IMUK), the Smithsonian Environmental Research Center
(SERC), and the Atmospheric Sciences Research Center (ASRC) at the State
University of New York. In addition, several observers were present from
NASA\'s Goddard Space Flight Center.

The results show an improvement in the comparison of synchronized UV
solar irradiance scans from the scanning spectroradiometers and
spectrograph for noontime measurements from a

Figure 6: UV solar irradiance as a function of wavelength at 17:00 UTC
from the 2003 North American spectroradiometer intercomparison.

standard deviation of 10%, when using the participant's calibration
methods from previous years, to 3% for the current study (see Figure 6).
In addition, agreement between scanning spectroradiometers, including
the spectrograph, as a function of solar zenith angle has improved from
previous years (relative standard deviation of 2-4% between sza 20-80°).
Better comparison of UV solar irradiance from earlier intercomparisons
can be attributed to improvements in the calibration methods and
development of field calibrators, improvements in processing and
corrections applied to the data, and improvements in instrumentation,
e.g. angular response and stray light rejection. The data analysis and
plots were archived on the CUCF web site. Two papers on the results of
the 2003 intercomparison are currently in draft form
(ftp://ftp.srrb.noaa.gov/pub/data/CUCF/intercomparison_2003). Final
results were presented at the International Optical Society (SPIE)
annual meeting in Denver, Colorado, in August 2004 and at the Climate
Monitoring and Diagnostics Laboratory's Annual Meeting in April 2005.

**Investigations of UV erythemally weighted irradiance**

SRRB scientists have collaborated with colleagues on investigations of
surface UV solar irradiance measurements. Recent work with Dr. Jeff
Hicke from Colorado State University consists of a ten-year analysis of
UVB broadband data of the USDA UV Monitoring Network \[Hicke et al.,
2004; 2006\]. These studies evaluate changes in UV during specific
seasons to provide information on UV effects on agriculture. Changes in
UV radiation during specific times of the year, for example, during
initial plant development or during the latter part of the growing
season, can have different impacts on the health of the plant species.
Evaluating long-term changes in UV solar irradiance is challenging due
to the uncertainties in the measurements and requires detailed and
frequent instrument calibrations and characterizations as provided by
the CUCF. Dr. Lantz recently worked with scientists at New Zealand's
National Institute for Water and Atmospheres (NIWA) on the study of
geographical differences in UV radiation \[McKenzie et al., 2006\].
Previous studies highlighted differences in UV radiation at similar
latitudes in the northern and southern hemisphere represented by Germany
and New Zealand \[Seckmeyer et al., 1995\]. The current work extended to
a larger UV monitoring network in the United States (USDA UV Monitoring
Network) and was compared with the New Zealand sites. The current work
again clearly identified 41% higher erythemally-weighted irradiance in
the southern hemisphere compared to similar latitudes in the United
States. Differences are attributed to hemispherical differences in total
ozone, cloud cover, and tropospheric pollution.

Several ground-based ultraviolet (UV) monitoring networks exist in the
United States, and each employs different instrumentation. Two of these
UV networks are the U.S. Environmental Protection Agency's (EPA's)
Brewer Spectrophotometer Network and the U.S. Department of
Agriculture's (USDA's) UV-B monitoring network, (52 sites), most of
which are located in the mainland United States. The Brewer
spectrophotometer records full sky spectra from 287 to 363 nm at 0.55-nm
resolution, whereas the USDA instrument is a broadband device that
measures broadband erythemally-weighted UV data. To date, limited
comparisons of data collected from these networks have been analyzed for
comparative and quality assurance (QA) purposes. The data used were
taken from sites where instruments from each program are co-located,
namely, Big Bend National Park, Texas, and Everglades National Park,
Florida. To reduce the contribution of errors in the Brewer-based
instruments, the raw data is corrected for stray light rejection, the
angular response of the full sky diffuser, the temperature dependence of
the instruments, and the temporal variation. This reduces the estimated
errors of the absolute irradiance values of each Brewer spectral
measurement to approximately +/-5%. The estimated uncertainty of the
USDA instruments is approximately +/-6% with a systematic bias of (-13
to 5% depending on the total ozone) and is comprised of (1) standard
lamp measurement errors, (2) spectral response differences, and (3) the
angular response differences. Comparisons were performed between the
Brewer spectrally integrated and erythemally-weighted UV irradiance
measurements and the data collected by the broadband erythemal UV meters
at co-located sites between 1997 through to 2002. The data presented in
this paper show that the USDA broadband erythemal detector and the
UGA/EPA Brewer spectrophotometer produce daily erythemal irradiance data
whose ratios are in agreement within the estimated absolute
uncertainties of the two instruments. The differences between the
collected data were smallest at the Everglades site (\~3.3%) and largest
at the Big Bend site (\~8.4%). About 3% of the difference at Big Bend
can be accounted for by the altitude differential between the two
instruments. However, as shown by the correlation coefficients for the
Big Bend data, even when the instruments are relatively close to each
other (\~40 km), the instruments can have a different sky view \[Kimlin,
et al, 2005\].

**Stability of the Lamp Transfer System**

The National Institute of Standards and Technology (NIST) employs the
1000W FEL-type quartz-tungsten-halogen (QTH) lamp as its transfer device
for the spectral irradiance scale. The NIST-calibrated 1000W FEL-type
QTH lamp is a primary standard. The primary lamps are expensive, have a
limited useful lifetime, and require careful handling to maintain
stability. NOAA's Central UV Calibration Facility (CUCF) currently owns
six of the NIST primary standards. NOAA's CUCF uses standard lamps to
calibrate more than 70 solar UV measuring instruments per year. Large
spectroradiometer instruments are calibrated *in situ* always using
three lamps during each calibration event. For multi-filter instruments,
which are calibrated in the laboratory, at least two lamps are used per
instrument. Additionally, broadband instruments are operated in front of
calibrated standard lamps to track their long-term stability. A workload
of this amount equates to many hours of operation on multiple standard
lamps. The prohibitive cost of the calibrated lamps purchased directly
from the NIST precludes their use for routine calibrations. To minimize
costs and extend the working lifetime of the lamps, it is customary to
transfer the NIST scale to secondary lamps and then use these to
transfer to tertiary (working standard) lamps. In an effort to reduce
overall measurement uncertainties, the CUCF uses the secondary standards
as its working standards and does not transfer to tertiary standards.

The CUCF creates its own secondary lamps using a method similar to the
procedure developed by NIST. Careful seasoning and screening of the
secondary lamps is essential to achieve the maximum benefit out of this
transfer and the operation of the expensive primary standards. The
process of lamp seasoning and screening techniques used by the CUCF is
tedious and includes visual inspection of the lamp envelope, filament,
and lead wires during each step. The process helps to identify lamps
that are suitable for use as secondary standards of irradiance. The
screening process does not guarantee that lamps will be stable
standards, but only that they do not exhibit defects or erratic
operational behavior during the screening process. Only half of the most
recent batch of lamps screened by the CUCF passed onto the calibration
portion of the process. The lower than normal results were due mostly to
imperfections with the quartz envelope or improper mounting of the FEL
in the medium bi-post base \-- not lamp instability. In a recent paper
\[Disterhoft, 2005\], the temporal stability of the lamp irradiance,
lamp current and voltage, anomalous emission and absorption lines, and
some of the problems that the CUCF has found with lamps are discussed.

In contrast to the view that standard lamps are fragile and their
calibration quickly drifts, empirical evidence shows that some well
chosen lamps are very robust and long lived. Many of the secondary
standards created by the CUCF have provided hundreds of hours of stable
operation with little or no change in their irradiance calibration. This
type of behavior has prevailed in spite of rough handling from
continually being transported to and from field sites. At this time
quartz-tungsten-halogen lamps are inexpensive, most easily transported,
have the lowest uncertainty, and are the most robust means of
transferring the spectral irradiance scale. Their relatively smooth
continuum is easily transferred onto other standards and compared
internationally. Even though new lamps are exhibiting persistent
anomalous emission lines, the spectral irradiance of the lamp can be
corrected. Additionally, the emission lines provide a means for exact
wavelength registration of the tungsten spectrum.

Several corrections have been made to the irradiance scale of the QTH
lamps provided by the CUCF. There have been several step changes that
have occurred to the CUCF irradiance scale either due to changes in the
CUCF irradiance scale transfer system or to changes to the scale of the
vertical primary lamps that NIST provides. The first correction resulted
from the rebuild of the CUCF transfer scale system necessitated by a
move to a new building in March 1999. The first rebuilt system in the
new building (DSRC) had a 3-4% bias in the transfer scale system. That
bias was removed in a second rebuild in June 2001. Therefore, the
irradiance scale of all horizontal lamps generated from March 1999 to
June 2001 needed to be corrected for the 3-4% bias. The second
correction was caused by an upgrade to the NIST calibration system for
generating vertical primary lamps that resulted in reduced
uncertainties, a tighter irradiance scale, and an approximate increase
of 1.2% in the lamp irradiance. Code was generated to apply the
corrections to each lamp and include a statement in the header lines of
the lamp irradiance files. An extensive correction of calibration files
that depend on the irradiance scale was completed in 2004. All
corrections were applied to data in the CUCF archive including the U111
spectroradiometer responsivities, EPA Brewer responsivities, UV-MFRSR
responsivities, and subsequent effects on the UVB broadband instrument
calibration.

#### The SURFRAD Aerosol Optical Depth Algorithm

The new multi-spectral Aerosol Optical Depth (AOD) analysis method
recently developed for NOAA\'s Surface Radiation Budget network
(SURFRAD) includes several unique features that minimize error and
facilitate the calibration and AOD calculation procedures. The once
painstaking task of identifying calibration Langley plots is uniquely
automated by cross-referencing the Multi-Filter Rotating Shadowband
Radiometer (MFRSR) measurements with the SURFRAD clear-sky
identification product described in Long and Ackerman (2000). Use of the
clear-sky product ensures that only the most pristine cloud-free
conditions are used for Langley calibrations. All channel-specific
Langley calibrations computed within two-month period segments are
normalized to unit solar distance and then averaged to derive mean
channel calibrations (denoted by individual square symbols in Figure 7)
and errors that represent that period. Identifying mean channel-specific
calibrations that represent several weeks and grouping them over
two-year periods in time series allows for the resolution of, and
removal of, the temperature dependence of the MFRSR (Figure 7), thus
further reducing error in the ultimate product. A standard \"propagation
of error\" method (Michalsky et al. 2001) is applied to compute the
error associated with the mean Langley calibration. The rejection of
outlier calibrations using standard statistical methods further
minimizes that error.

Daily AOD files are produced for each station. The first step in the AOD
calculation is the retrieval of channel-specific calibrations for the
day being processed from the best-fit periodic equations that represent
calibration drift and its intra-annual variation (Figure 7). Channel
calibrations for specific days are then corrected to the earth-sun
distance appropriate to the day being processed. The retrieved
calibrations are combined with MFRSR measurements on that day to compute
daily time series of *total optical depth* for each channel. *Aerosol
optical depth* is computed by subtracting the contributions of molecular
scattering and ozone absorption, computed specifically for each channel,
from the total optical depth. Ozone absorption coefficients are chosen
based on the central wavelengths specific to the MFRSR that made the
measurements, and daily total ozone is automatically obtained from a
NASA web site.

![](media/image7.png){width="6.180555555555555in"
height="4.104166666666667in"}

**Figure 7. Time Series of two-month mean Langley Vo calibrations,
corrected to a unit earth-sun distance, for five visible spectral
channels of the MFRSR at the Table Mountain SURFRAD station near
Boulder, Colorado. The solid line is the best-fit periodic function that
is used to generate daily Vo calibration values for AOD calculations.
The linear least squares fit (dotted) is shown for reference.**

Molecular scattering is computed using the station pressure measured at
the SURFRAD station. Because AOD can only be computed for times that the
sun\'s beam is cleanly sensed by the MFRSR, the last task of the AOD
algorithm is to subject the daily AOD time series to cloud screening. A
viable method to screen out cloud-contaminated MFRSR measurements has
been developed and applied. The SURFRAD AOD product is available for all
stations on the GMD web site.

Daily averages of 500-nm AOD for Table Mountain are plotted in time
series for a nine-year period in Figure 8. An annual variation showing
high AOD in summer and low values in winter is readily apparent,
although there appears to be a pronounced inter-annual variability. Some
years, such as 1998, 2001, 2002, and 2003, have higher AODs than the
other years. Persistent drought and resultant forest fires in Colorado
are responsible for the largest values in the summer of 2002.

To get a sense of the typical annual cycle of AOD for Table Mountain,
the nine-year time series of AOD data was analyzed to generate an
average, or typical, annual time series. That summary is shown below for
all channels in Figure 9. The most obvious features are the summer
maximum and winter minimum. Another prominent feature is the relative
maximum in March and April that is caused by a persistent springtime
intrusion of Asian dust. Multi-spectral analyses of AOD in this manner
can yield information concerning the relative particle size of the

![](media/image8.png){width="5.934027777777778in"
height="4.0569444444444445in"}

Figure 8. Nine-year time series of 500-nm daily averaged AOD for the
Table Mountain, Colorado, SURFRAD station.

aerosols through the Angstrom exponent\--the absolute value of the slope
of the natural log of AOD versus the natural log of the measurement
wavelength for two or more channels. The Angstrom exponent is inversely
proportional to the mean particle size of the aerosols. A value of 0
indicates very large particles, and a value of 4 is representative of
molecules. Continental aerosols typically have a value of about 1.3. For
Table Mountain, the mean Angstrom exponent for July is 1.52, and that
for April, when Asian dust is usually present in Colorado, is 0.98.
These values are reasonable and indicate that in the summertime, when
aerosols are dominated by smoke and photochemistry, particles are
typically small, and in the spring, local aerosols combine with imported
Asian dust, and are relatively large.

Contrast the Table Mountain results to those from the Goodwin Creek
SURFRAD station (Figure 10). Goodwin Creek is located in northern
Mississippi on the south edge of the region of the U.S. with the highest
aerosol content. The peak AOD at Goodwin Creek is in July and is three
times higher than the July peak at Table Mountain. A small relative
maximum in the spring is evident for Goodwin Creek, but it is not as
pronounced as that at Table Mountain. It is likely that aerosols in
Mississippi are driven by photochemistry and thus characteristically
small. Consequently, the Angstrom exponents for April and July at
Goodwin Creek are similar at 1.2 and 1.3, respectively.

![](media/image9.png){width="6.009722222222222in"
height="6.134722222222222in"}

Figure 9. Annual time series of monthly averaged AOD for the Table
Mountain, Colorado, SURFRAD station for five measurement channels. Nine
years of AOD data were used to generate this mean annual time series.

Development of a Diffuse Horizontal Shortwave Irradiance Working
Standard

**Introduction**

Cavity radiometers measure direct beam irradiances with an uncertainty
of \~ 0.35%, and models and measurements of direct irradiance for clear
conditions generally agree to within the combined uncertainty limits
(Kato et al. 1997; Halthore et al. 1997; Halthore et al. 2004). Diffuse
horizontal shortwave irradiance measurements do not generally agree with
radiative transfer models even for the simple case of clear skies
(Halthore and Schwartz 2000; Halthore et al.

2004). Models for clear skies produce higher surface diffuse irradiance
than our best diffuse

![](media/image10.png){width="5.9in" height="5.973611111111111in"}

Figure 10. Annual time series of monthly averaged AOD for the Goodwin
Creek, Mississippi, SURFRAD station for five measurement channels. Nine
years of AOD data were used to generate this mean annual time series.

irradiance measurements unless the aerosol absorption is artificially
increased beyond reasonable assumptions. The current published estimate
of carefully characterized diffuse irradiance uncertainty is ± (3% of
reading + 1 W/m^2^) (Reda et al. 2003), in other words, 4 W/m^2^ for a
100 W/m^2^ diffuse irradiance sky. This work seeks to lower this
uncertainty. Fundamentally, this is an important problem to resolve
because it is the simplest radiative transfer problem, and the inability
to come to closure on this issue leaves work on the more difficult
problems of cloudy and partially cloudy skies suspect.

**Instrumentation**

The pyranometers in Table 6 were used in the second diffuse Intensive
Observation Period (IOP). Most of these instruments are unique in some
way. The two from NASA Langley have thermistors mounted in the heat sink
and on the inner dome to measure temperatures that are used to correct
offsets. These two are the most similar in this study. The table reports
two responsivities, the one sent by the owner and the one derived during
the IOP using the same calibration technique for all pyranometers (see
below). The last two columns contain data for the offsets that will be
discussed further.

The measurements were made at the ARM Southern Great Plains (SGP)
central facility between Billings and Lamont, Oklahoma. Data were
acquired between 8 and 16 October 2003. There was a mix of clear, partly
cloudy, and totally overcast skies during the period. Two days were
exceptionally clean and clear, with aerosol optical depths at 500 nm
between 0.03-0.04. One of these days was used to perform calibrations on
the instruments and to perform capping experiments to test the offset
correction scheme.

Zero Offset Derivation and Correction

The operation of the two most common types of thermopile pyranometers is
explained in detail in Dutton et al. (2001) and references therein.
During the nighttime, pyranometers should have a zero voltage output,
but the output is often less than zero, with maximum negative values on
cloudless nights. The usual explanation for this is that
single-black-detector thermopile pyranometers have cold junctions
attached to the shielded body of the instrument and hot junctions
attached just under the exposed absorbing receiver. The dome covering
the detector cools by radiating to space (at all times), which in turn
cools the detector relative to the body of the instrument and,
therefore, produces a negative signal. On clear nights the radiation
cooling is great, yielding a large offset. For black and white
pyranometers (model 848) the hot and cold thermopile junctions are
attached to the black and white sectors, respectively, of the
detector\'s receiving surface that are both exposed to incoming solar
(and thermal) radiation. Since both junctions experience the same
thermal environment there should be, and generally is, little or no
offset.

Figure 11 illustrates the nighttime offset for two pyranometers of the
single-black-detector type with different constructions. The instrument
in Figure 11 (a) has been designed to have a small offset, and the
instrument in Figure 11 (b) has an older design and a much larger, and
less predictable, offset. The plots in this figure contain all of the
nighttime one-minute data

**Table 6.** Instruments of the IOP, Two Responsivity Measurements, and
Offsets.

+--------------+------+-----------+------------+----------+----------+
| Instrument   | Se   | Response  | Response   | P        | Capped   |
|              | rial | (old)     | (new)      | redicted | offset   |
|              | No.  |           | (µvol      | offset   | (Wm^-2^) |
|              |      | (µvolt    | ts/Wm^-2^) | (Wm^-2^) |          |
|              |      | s/Wm^-2^) |            |          |          |
+--------------+------+-----------+------------+----------+----------+
| cm22-wrc^a^  | 99   | 11.52     | 11.46      | -1 to 0  | -1 to 0  |
|              | 0010 |           | µv/Wm^-2^  |          |          |
+--------------+------+-----------+------------+----------+----------+
| c            | SoDP | 8.72      | 8.66       | -1 to 0  | -1 to 0  |
| m22mod-kz^b^ |      |           |            |          |          |
+--------------+------+-----------+------------+----------+----------+
| cm22-kz^c^   | 01   | 9.52      | 9.54       | -2 to -1 | -2 to 0  |
|              | 0044 |           |            |          |          |
+--------------+------+-----------+------------+----------+----------+
| cm11-kz^d^   | 02   | 5.04      | 5.098      | -3       | -3       |
|              | 7787 |           |            |          |          |
+--------------+------+-----------+------------+----------+----------+
| cm21-kz^e^   | 02   | 11.32     | 11.33      | -3       | -3       |
|              | 1041 |           |            |          |          |
+--------------+------+-----------+------------+----------+----------+
| cm21-msc^f^  | 98   | 18.06     | 18.03      | -5 to    | -5 to -6 |
|              | 0470 |           |            | --6      |          |
+--------------+------+-----------+------------+----------+----------+
| fpp^g^       | fpp  | 4.43      | 4.42       | -2       | -10 to   |
|              |      |           |            |          | -9       |
+--------------+------+-----------+------------+----------+----------+
| 848-cmdl^h^  | 3    | 8.81      | 8.75       | -1 to 0  | 0        |
|              | 2347 |           |            |          |          |
+--------------+------+-----------+------------+----------+----------+
| 848-nrel^i^  | 3    | 9.084     | 9.052      | 0        | 0        |
|              | 2858 |           |            |          |          |
+--------------+------+-----------+------------+----------+----------+
| psp-nrel^j^  | 3    | 7.561     | 7.712      | -12 to   | -12 to   |
|              | 1256 |           |            | --11     | --11     |
+--------------+------+-----------+------------+----------+----------+
| psp-larc2^k^ | 3    | 7.841     | 7.733      | -9       | -8       |
|              | 0849 |           |            |          |          |
+--------------+------+-----------+------------+----------+----------+
| yes^l^       | 102  | 5000      | 4863       | 1        | 1        |
+--------------+------+-----------+------------+----------+----------+
| eko^m^       | f03  | 7.06      | 6.986      | -1       | No       |
|              | 007f |           |            |          | capping  |
+--------------+------+-----------+------------+----------+----------+
| csd^n^       | 8401 | 967       | 1023       | 0        | -5 to -2 |
+--------------+------+-----------+------------+----------+----------+
| psp-larc1^k^ | 3    | 9.117     | 9.061      | -10      | -10      |
|              | 1562 |           |            |          |          |
+--------------+------+-----------+------------+----------+----------+

^a^Kipp & Zonen, Inc. CM 22; World Radiation Center ventilation with
heated air flow.

^b^Kipp & Zonen, Inc. CM 22; Kipp & Zonen CV2 ventilator; sapphire outer
dome.

^c^Kipp & Zonen, Inc. CM 22; Kipp & Zonen CV2 ventilator.

^d^Kipp & Zonen, Inc. CM 11; Kipp & Zonen CV2 ventilator.

^e^Kipp & Zonen, Inc. CM 21; Kipp & Zonen CV2 ventilator.

^f^Kipp & Zonen, Inc. CM 21; Meteorological Services of Canada-designed
ventilator.

^g^Eppley un-ventilated prototype.

^h^Eppley ventilated 8-48 from NOAA/Climate Monitoring and Diagnostics
Laboratory.

^i^Eppley un-ventilated 8-48 from DOE/National Renewable Energy
Laboratory.

^j^Eppley ventilated PSP from DOE/National Renewable Energy Laboratory.

^k^Eppley ventilated PSP from NASA/Langley Research Center; measured
case/inner dome temperatures.

^l^Yankee Environmental Systems, Inc. prototype; YES ventilator.

^m^EKO Instruments Co., Ltd. 802 EKO ventilator.

^n^Carter-Scott Design EQ08-A; not ventilated.

collected over the eight days of the comparison. The x-axis is the
pyrgeometer thermopile reading, which responds to net received thermal
radiation, and the y-axis is the pyranometer reading with all data taken
with the sun at least 7º below the horizon (more than 5000 data points).
The linear, least-squares fit to the data is drawn as a dark, solid
line, and the linear, least-squares fit with the intercept forced to
zero is the red, dashed line. The pyranometer reading forced to zero is
shown because this is what one would expect when the net infrared
reading from the co-located pyrgeometer is zero, that is, when the dome
and receiver of the pyrgeometer are at the same temperature. For this
case, the pyranometer dome and receiver should also be at the same
temperature, assuming the time constant of the two processes is
equivalent. The instrument in Figure 11 (a) comes closer to this
expectation than the instrument in Figure 11 (b).

For all of the instruments, the nighttime data were used to predict
offsets at all times using the two-term regression equation, with the
exception of the instrument in Figure 1 (b), where the fit was forced
through zero. This is the prescribed correction for the Eppley PSP based
on Dutton et al. (2001). For the two NASA Langley Research Center PSPs
in Table 1 we used the correlation between the dome and case temperature
measurements' difference each raised to the fourth power versus offset
at night to predict the offset (see Haeffelin et al. (2001) for further
details).

During the clear afternoon of 12 October 2003 we capped each of the
instruments so that no light reached the detector. This was done quickly
and the signal sampled rapidly at 10-sec intervals to measure the
largest negative response. This provides the daytime offset because the
detector response is much more rapid than a temperature change in the
dome. The temperature reaches a valley and eventually rises as the dome
and receiver temperatures equilibrate under the cap. The capping was
repeated three times, with enough time between cappings to allow the
instruments to return to their natural uncapped equilibrium. The
equations derived using nighttime data were then used to predict the
offset at the net infrared reading measured by the pyrgeometer during
the cappings, approximately -150 W/m^2^.

![](media/image11.png){width="6.5in" height="4.333333333333333in"}

In the next to last column of Table 6 the predicted offset using the
equations based on nighttime data and a net infrared of --150 W/m^2^ is
tabulated, and the measured offset using the lowest reading during the
cappings appears in the last column. With three exceptions the offset
predictions are within 1 W/m^2^. Both the Carter-Scott Design EQ08-A and
the Eppley FPP have modest offset predictions using nighttime data, but
have much larger capped offsets. The EKO 802 had a dome that was larger
than the others, and it could not be tested with the available caps.

Common Calibration

A favored method to calibrate pyranometers is to alternately shade and
unshade the horizontal pyranometer and compare the difference to an
absolute cavity radiometer that simultaneously measures the direct beam
irradiance. The difference in voltages with the

Figure 11. Nighttime one-minute data (\> 5000 points) during the IOP
with pyranometer offset plotted versus net infrared irradiance from a
co-located pyrgeometer for two pyranometers. The dark line is a linear
least-squares fit to the data (offset and slope are black numbers) and
the red, dashed line is a linear fit with a forced zero intercept (slope
in red). The large spread of points in (b) is typical for this
instrument model.

pyranometer shaded and unshaded is produced by the component of the
direct irradiance incident on the pyranometer, which is the direct
irradiance measured by a cavity radiometer multiplied by the cosine of
the solar-zenith angle. Preferably, this measurement is made with the
solar-zenith angle near 45°, and repeated as often as practical.
Philipona (2002) has recently provided compelling experimental
justification for this method.

Fortunately, on 12 October 2003 at the SGP site the minimum solar-zenith
angle was 44.04º at solar noon. The sun moved between this minimum and
45.96º or within 45 ± 0.96º over a two-hour period providing many
opportunities to repeat a two-minute shade, two-minute unshade sequence.
An Eppley pyrheliometer (model NIP) measured the direct beam on this day
and then its calibration was confirmed against absolute cavity
measurements on the next clear day, which occurred two days later. The
change in the pyranometers' calibrations from the original calibrations
assigned for the instruments by the owner can be seen in Table 6.

Results

Figure 12 is an overplot of eleven diffuse irradiance measurements for a
very clear day (14 October 2003) after correcting for zero offsets and
applying the shade/unshade calibrations. Unfortunately, one tracker
carrying three instruments started late each morning and stopped before
sunset. The high, stray points before 287.3 and after 287.7 in Figure 12
are due to this failure. Since the horizontal grid lines in this figure
are 5 W/m^2^ apart one can see that the spread is the lower eight is
only ± 1 W/m^2^ among these pyranometer readings at solar noon. The four
instruments that do not appear in Figure 2 are the Eppley FPP (fpp), the
NREL Eppley PSP (psp-nrel), the EKO Trading Co. 802 (eko) and the
Carter-Scott Design EQ08-A (csd). The fpp and psp-nrel instruments are
low throughout most of the day and are noisier than the group of eight
pyranometers. The csd instrument reads lower than the group of eight
during most of the day, and then reads higher in the late afternoon. The
signal is noisier than most instruments. The eko is only slightly lower,
but noisier, than the eight stable pyranometers. This was also the
instrument that could not be capped to confirm or deny the veracity of
the derived nighttime offset correction equation. The three remaining
instruments have higher readings throughout the day, but the signals are
stable in comparison to the noisy signals of the four instruments just
discussed. The Yankee Environmental Systems prototype (yes) and the NREL
Eppley 8-48 (848-nrel) are the highest while the CMDL Eppley 8-48
(848-cmdl) has an intermediate value when compared to the eight
instruments and the yes and 848-nrel. Possible explanations for these
differences are discussed below.

In Figure 13 instruments are compared for a cloudy day. Fourteen
pyranometer outputs are plotted as differences from the mean value of
the group for this 3.5-hour cloudy period in which the irradiance varied
between 40 and 220 W/m^2^. The instrument responsivities used were those
obtained using the shade/unshade method during the IOP. Most of the data
fall within ± 2% of the mean. Not shown in this figure is the fifteenth
pyranometer, the Dutton et al. (2001) corrected PSP (psp-nrel), which
significantly underestimated the offset correction for both cloudy days.
The fairly close agreement on cloudy days for a large range of
irradiances as shown in Figures 13 and the large fractional disagreement
on clear days that can be inferred from

Figure 12 requires explanation. The possibilities are investigated next.

Discussion

Cloudy sky diffuse irradiance differs from clear sky diffuse irradiance
spectrally with the clearest skies having a deep blue spectrum and
cloudy skies having a spectral distribution much like the full solar
spectrum. Cloudy skies also have a different spatial distribution of
skylight. For heavy overcast, the zenith is the brightest point in the
sky and the horizon is the darkest. For clear, clean skies, the
brightest and darkest parts of the sky depend on the solar position. For
clear, hazy skies, the horizon is bright. For partly cloudy skies, the
spatial distribution, of course, changes unpredictably. Since the
instruments have different angular responses, this could lead to
relative differences in measured irradiance for these changing radiance
distributions.

![](media/image12.png){width="6.5in" height="3.986111111111111in"}Figure
12. Overplot of eight instruments named on plot after correction for
offset and with a common shade/unshade calibration applied. One tracker
that carried three instruments started tracking late and ended tracking
early exposing pyranometers to direct sunlight, and these points are
removed from consideration in our analysis. Three instruments appear
stable, but high relative to these eight. Four instruments that read
lower than the eight and had noisy signals are not plotted.

Another effect that should be considered is the difference in the
shading of detectors having different sizes. This was assumed to be a
minor effect, but if there are several small differences that add in the
same direction, all must be considered as possibilities.

In the first diffuse irradiance IOP paper (Michalsky et al. 2003)
differences that result from different sized detectors' ability to see
more or less of the circumsolar radiation, when the shading geometry is
held constant, were modeled using the framework described in Major
(1994). The geometry was held constant in this experiment, as well. In
this IOP the largest difference in receiver diameter is 32 mm versus 11
mm, and for a clear-sky diffuse irradiance of 65 W/m^2^, as in Figure
12, the calculated irradiance differs by about 0.7 W/m^2^. The Eppley
8-48 has the largest diameter detector; therefore, a larger diffuse
irradiance signal is expected for this instrument. Although the
magnitudes of the differences in Figure 12 are much larger than this,
the tendency for the 8-48 to read higher explains a portion of the
difference.

Angular responses of pyranometers differ among manufacturers and even
somewhat for the same instrument models. Angular responses are usually
plotted normalized with respect to the perfect Lambertian response, that
is, the response of a perfect irradiance receiver should decrease as the
cosine of the angle with respect to the normal to the surface of the
receiver. Michalsky et al. (1995) illustrates the cosine responses of
several types of detectors including some of the instruments in this
study. Using two instruments with very different angular

![](media/image13.png){width="6.497916666666667in"
height="3.5729166666666665in"}

**Figure 13. Fourteen instruments are compared to their mean for this
3.5-hour cloudy period while the diffuse irradiance varies between 40
and 220 W/m^2^. The pyranometers use the shade/unshade calibration for
these data. The variation is almost entirely within ± 2% of the mean.**

responses, a sensitivity analysis was conducted. Assuming that the
instruments are calibrated with the sun near 45º solar-zenith angle, the
nominal cosine responses can cause a maximum difference in response
between cloudy and clear skies that is on the order of 0.8% at 45º
solar-zenith angle. The 8-48 reads higher in clear skies if, in cloudy
conditions, it agrees with an instrument that has a lower response at
high solar-zenith angles, but the difference is only \~ 0.5 W/m^2^ for
the case in Figure 12. As true for the shade geometry bias the 8-48
reads higher in clear skies, but the magnitude of the difference, when
combined with the geometry results, only explains about 1.2 W/m^2^ of
the differences in Figure 12.

Can the instruments' spectral sensitivities cause the difference? Since
the instruments are calibrated with the shade/unshade technique they are
responding to the full solar spectrum. When they measure a cloudy-sky
diffuse irradiance signal they are seeing a similar spectral
distribution attenuated quasi uniformly across the spectrum. When the
diffuse irradiance is from a clear sky, the spectrum is shifted so that
almost all of the energy is in the blue and visible regions. For
all-black thermocouple instruments the response depends on the
absorptivity of the receiver and the transmission of the dome or domes.
For the Eppley 8-48 the response depends on the dome transmission and
the difference in reflectivity of the black and white surfaces that are
in contact with the hot and cold junctions of the receiver. By knowing
the details of the transmissions and absorptions as a function of
wavelength, the difference in responsivities to full sun and to blue sky
can be calculated. Using available manufacturers' and literature data,
we evaluated the relative responsivities to scattered clear-sky
irradiance and full-sun irradiance. Calculations yielded transmission
and reflectance differences of less than 0.5%. Clearly, more effort with
specific instrument data, especially the reflectance of the white
portion of the 8-48, is required.

Summary

Fifteen pyranometers representing most current manufacturers plus a few
prototypes made diffuse horizontal shortwave irradiance measurements for
nine days for a variety of cloud conditions from completely overcast to
completely clear skies. The instruments were calibrated simultaneously
over a two-hour period using a shade/unshade technique while the sun
position remained within 1º of 45° solar-zenith angle. Zero offset
corrections were determined using nighttime data and checked using
daytime cappings with 12 of 14 pyranometers yielding offsets within 1
W/m^2^ of that predicted. One was not checked because a cap large enough
to fit over the dome was unavailable.

Eight pyranometers agreed to within ±2% for clear-sky conditions. Three
others have a high bias on clear days relative to these eight, but all
eleven agree within ±2% on overcast days. Four pyranometers were
eliminated from consideration as part of a standard at this juncture
because we could not predict offsets during the day adequately, the
instruments had noisy signals, or their biases with respect to the
majority of the instruments were variable.

The three high instruments did not agree with the majority, but could
not be eliminated because of noisy or erratic readings. Attempts to
explain the remaining differences included shading geometry differences,
angular response differences, and spectral response differences.
Geometry and angular responses account for a fraction of the difference.
Spectral response differences appear to be small if the manufacturers'
data are correct. In the calculations of angular and spectral response
we used generic responses. In order to estimate actual differences we
would need to know the details of the angular and spectral response of
each pyranometer; information that we do not currently possess. At this
point the unexplained differences in responses when clear skies prevail
are not completely resolved.

Figure 14 summarizes the bias and root-mean-square differences from the
mean of the eleven instruments that were not eliminated for all-sky
conditions during the IOP. Clearly, most of the bias and
root-mean-square difference can be attributed to the three instruments
that read high on clear days.

**Shortwave Radiative Closure Studies for Clear Skies During the ARM
2003 Aerosol Intensive Observation Period**

## Introduction

Achieving agreement between clear-sky shortwave broadband irradiance
models and measurements is one requirement for validating models, and it
is the first step to further use of these models in more complicated
cloudy-sky analyses. However, recent efforts to achieve broadband
shortwave radiative closure have met with some difficulties. Kato et al.
(1997) looked at clear-sky data in the mid 1990s and found that direct
beam irradiance (hereafter, direct irradiance) models and measurements
agreed well. Halthore et al. (1997) reached a similar conclusion. Kato
et al.'s (1997) diffuse horizontal broadband shortwave irradiance
(diffuse irradiance) measurements, however, were well below the modeled
irradiance even though the uncertainties in the inputs and errors due to
model assumptions were taken into account. Halthore et al. (1998)
concluded, likewise, that models of diffuse irradiance were higher than
measurements. This led to a reassessment of offsets in thermal
pyranometers caused by the temperature gradient between the dome and
thermopile in pyranometers that had been noted decades earlier (e.g.,
Gulbrandsen, 1978), but largely ignored. A series of papers resolved the
![](media/image14.png){width="6.488888888888889in"
height="4.2659722222222225in"}

Figure 14. Summary plot of root-mean-square errors and biases from the
mean of the eleven stable pyranometers using offset-corrected and
shade/unshade calibrated data.

**Shortwave Radiative Closure Studies for Clear Skies During the ARM
2003 Aerosol Intensive Observation Period**

offset issue by proposing methods to correct the error (Bush et al.
2000; Haeffelin et al. 2001; Dutton et al. 2001; and Michalsky et al.
2003). Halthore and Schwartz (2000) found that even with corrected
offsets, diffuse irradiance measurements were persistently lower than
models.

Barnard and Powell (2002) confirmed the Halthore and Schwartz (2000)
result for the Southern Great Plains (SGP) Climate Research Facility
(CRF), however, using data from the North Slope of Alaska, Atmospheric
Radiation Measurement (ARM) site near Barrow, they were able to achieve
closure to within the measurement and model uncertainty. In a recent
paper using improved aerosol property inputs to the model, Halthore et
al. (2004) were still unable to achieve closure at the SGP CRF in
northern Oklahoma. In another recent paper where model input aerosol
optical properties were carefully scrutinized, Henzing et al. (2004)
were able to achieve closure between direct beam measurements and
models, but found that models produced much higher diffuse irradiance
for a mid-latitude site in the Netherlands.

This study uses data from a major aerosol field experiment conducted
between May 5, 2003, and May 30, 2003, at and above the ARM SGP CRF,
which is located midway between Lamont and Billings, Oklahoma, at 36.61°
N and 97.49° W. A major goal of this aerosol intensive observation
period (AIOP) was to obtain redundant measurements of each aerosol
optical property needed as model input to radiative transfer codes. This
study features computations of direct and diffuse irradiance using six
radiative transfer codes and carefully scrutinized inputs. Notable
improvements include better asymmetry parameters and better spectral
surface albedo specifications than in prior studies. The comparisons
cover a wide range of aerosol optical depths, solar zenith angles, and
water vapor amounts. These model results are compared with careful
measurements of direct and thermal-offset-corrected diffuse irradiances.

## Model Data Input and Radiation Measurements

Skies were screened for totally cloud-free periods using three sources
of data. First, time series plots of broadband direct and diffuse data
were used to screen for clear skies. This was followed with plots of the
multi-filter rotating shadow-band radiometer (MFRSR) 870-nm diffuse data
(Harrison et al. 1994). This wavelength has little Rayleigh scattering,
and even light cirrus noticeably affects the shape of the time series
plots. A final step was to use sky images from the YES, Inc. Total Sky
Imager (TSI) to confirm the selection of clear-sky period
identifications. Thirty cases were identified that included low to high
sun angles and aerosol optical depth that ranged over those typically
observed for this site.

The most important input variables in clear-sky solar radiation modeling
of surface irradiance include aerosol information, specifically, column
aerosol optical depth, single scattering albedo, and asymmetry
parameter, all as a function of wavelength. The other key variables are,
in approximate order of importance, column water vapor, the spectral
albedo of the surface that surrounds the sites where irradiance
measurements are made, and column ozone.

Aerosol optical depth measurements were made with a normal incidence
multi-filter radiometer (NIMFR), which is a modified MFRSR that looks
directly at the sun with a 5.7º field of view. Optical depths are
measured at five wavelengths including 415, 500, 615, 673, and 870 nm.
Measurements are made every 20 seconds during all daylight hours. CIMEL
sun-photometer measurements are also made at the ARM CRF as part of the
AERONET network (Holben et al. 1998). For 13 of the 30 cases we compared
CIMEL and NIMFR measurements at or near three wavelengths\--500, 670,
and 870 nm. Of the 39 comparisons two were not within the 95%
uncertainty limits, about as expected, and there was negligible bias
between the measurement sets. The NIMFR aerosol optical depth was used
in all calculations.

Values for single scattering albedo (*ϖ~o~*) and asymmetry parameter
(*g*) used in the radiative transfer calculations were derived from
ground-based *in situ* measurements made with the CRF's Aerosol
Observing System (AOS). A comprehensive explanation of the AOS can be
found in Sheridan et al. (2001). On occasion, when AOS data were
unavailable, data from a backup system in the guest instrument facility
(GIF) within 200 m of the CRF were used. The two systems produced data
that agreed to within their uncertainties when both were operating.

The water vapor column was obtained from the data archive
[www.archive.arm.gov]{.underline}. These measurements are made every 20
seconds using a microwave radiometer, however, the five-minute-averaged
data are used for this study. The derivation of and uncertainty in
column water vapor (± 0.05 cm) is explained in Liljegren (1994). The
column water vapor was adjusted downward from the current ARM archived
values by 3% following a suggestion by Liljegren (2005) based on
improved microwave retrievals. Ozone data are obtained from the Total
Ozone Mapping Spectrometer (TOMS) web site
[toms.gsfc.nasa.gov](../toms.gsfc.nasa,gov). For the two cases when TOMS
data were not available Dobson data from Boulder obtained from the
Climate Monitoring and Diagnostics Laboratory
[www.cmdl.noaa.gov/ozwv/dobson](http://www.cmdl.noaa.gov/ozwv/dobson)
were used.

Spectral surface albedo input data were parameterized from the two
downward looking multi-filter radiometer (MFR) heads that are on the
10-m tower over un-grazed pasture and at the 25-m level of the 60-m
tower over a wheat field. MFR data were divided by up-looking MFRSR data
to determine surface albedos at six wavelengths. Equal weighting of the
surface albedos used in the calculations was based on the subjective
observation that the site is surrounded mostly by wheat and pasture in
equal proportions. The parameterization used for the surface albedo
between 300 and 3000 nm is described in Michalsky et al. (2003). Since
clear-sky surface albedos show solar-zenith angle and wavelength
dependencies, the instantaneous surface albedo is determined for each
observation.

The ground-based direct beam and diffuse horizontal shortwave irradiance
data used in model comparisons was obtained from the CRF's radiation
calibration facility (RCF) and GIF. An absolute cavity radiometer makes
the preferred measurement of direct beam irradiance with an uncertainty
of 0.4% or about 3-4 W/m^2^. Cavity measurements were made from the RCF
for seven of the 30 cases studied. For the other direct beam
measurements a pyrheliometer, Eppley model NIP, labeled A on the GIF
observing stand was used with an uncertainty of about 1-1.5% or 8-12
W/m^2^. The diffuse horizontal irradiance data were obtained from two
instruments on the GIF observing stand. The diffuse value used was an
average of an Eppley 8-48 shaded pyranometer, which has no offset, and
an offset-corrected Kipp & Zonen CM22. The small offset correction
applied in the latter case is a two-parameter fit as explained in
Michalsky et al. (2005). The estimated uncertainty in diffuse irradiance
is about 4% or about 4 W/m^2^.

Models Used in the Downwelling Shortwave Calculations

The models used for the comparisons have low to moderate spectral
resolution. The modeled direct beam and diffuse horizontal spectral
irradiances are spectrally integrated to compare with broadband
shortwave measurements. The extraterrestrial spectral irradiance
assembled by Gueymard (2004) was used as the input spectrum, except for
one model, and this input irradiance was scaled to the actual solar
distance. A surface albedo file that spanned the

**Table 7.** Models and their descriptions used in this clear-sky
comparison with broadband solar direct and diffuse measurements.

![](media/image15.png){width="6.488888888888889in"
height="3.602777777777778in"}

**Table 8.** The table contains key model inputs, except for surface
albedos, where CST is central standard time, *ϖ~o~* is the single
scattering albedo, and *g* is the asymmetry parameter. Water vapor and
ozone are column values.

![](media/image16.png){width="6.492361111111111in"
height="7.115972222222222in"}

300-3000 nm wavelength region was provided for each case. Table 7
contains information regarding the models and how inputs were applied
for the 30 cases in this study.

## Comparison Results

Thirty clear-sky cases were selected from 13 days during the AIOP. Table
8 contains most of the input parameters used to model the direct and
diffuse horizontal irradiances detected at the surface. This table
indicates the range of conditions that were modeled. Times ranged from
early morning to late afternoon, including times near solar noon,
consequently, solar elevations were as low as 12º and as high as 75º.
Aerosol optical depth ranged from 0.055 to 0.493 at 500 nm with some
significant variations in the wavelength dependence of the aerosol
extinction. A common way to express the aerosol optical depth wavelength
behavior is as the slope of a linear least squares fit to a plot of the
natural log of the aerosol optical depth versus the natural log of the
wavelength in μm. That slope is commonly designated *α*, which is listed
in Table 8 for all 30 cases. For comparison a typical value of *α* for
the ARM site is 1.3 (Michalsky et al. 2001). Integrated water vapor
column amounts varied between 1.0 and 3.5 cm. Ozone had a modest
variation during the selected days with extreme values of 291 and 350
Dobson units. Of particular note is the fact that single scattering
albedos (column labeled *ϖ~o~*) were unexceptional with all mid-visible
values between 0.88 and 0.97, and most were in the middle of this range.
The aerosol asymmetry parameters (column labeled *g*) ranged between
0.51 and 0.69.The model and measurement results are displayed
graphically in Figures 15 (top) and 15 (bottom) for direct irradiance
and Figures 16 (top) and 16 (bottom) for diffuse irradiance. Figures 15
(top) and 16 (top) are scatter plots of six modeled direct and diffuse
irradiances, respectively, for the 30 cases versus the measured direct
and diffuse irradiances. The one-to-one line is drawn in each plot to
demonstrate that the correlation is high for both direct and diffuse,
that the range of direct models intersect the one-to-one line in most of
the thirty cases, and that the range of diffuse models intersect the
one-to-one line in slightly fewer, but still the majority of cases.
Figures 16 (bottom) and 16 (bottom) are plots of the differences in
terms of (model -- measurement) for the 30 cases in the order that they
appear in Tables 8. This display allows one to see amplified differences
that are not obvious on the scatter plots.

The results are generally encouraging in that the measurements, for the
most part, fall within the range of all six models' results for both the
direct and diffuse irradiance. The exceptions are clearly seen in
Figures 15 (bottom) and 16 (bottom). The relatively low-aerosol optical
depth days (\<0.10) of May 5^th^, 11^th^, and 12^th^ show mixed results
relative to the problem of model-overestimated diffuse for these
clearest of days discussed in Halthore and Schwartz (2000) and Halthore
et al. (2004). While May 5^th^ had the lowest aerosol optical depth,
only three points of the six (cases 2-4) during the day had somewhat
high diffuse values. Moreover, the direct models are low for those same
points suggesting the possibility that the aerosol optical depth input
values may have been high, which, if true, would raise the direct and
lower the diffuse. Note that the deviations on May 27^th^, cases 23 and
24, although not low aerosol optical depth cases, could be improved with
a similar explanation. Nothing in Figures 15(bottom) and 16(bottom)
suggest that May 12^th^ results (cases 19-20), for this low aerosol day,
are exceptional. May 11^th^ (cases 16-18), therefore appears to be the
exception with three high diffuse values and no corresponding low direct
values. From Table 8 it appears that we could improve the diffuse
comparison by using the AERONET values for aerosol optical depth,
however, the modeled direct would be much too high if we were to use
AERONET aerosol data.

The overall modeled and measured differences for the direct beam and
diffuse horizontal irradiances in terms of root-mean-square (RMS)
differences and biases are summarized in Figure 17. The hatched boxes
are RMS differences for direct normal and diffuse irradiances. The solid
boxes represent the biases in terms of (modeled -- measured)
irradiances. RMS differences for direct normal irradiance range between
0.9 and 1.2%; biases range between --0.7 and 0.7%. RMS differences for
diffuse irradiances range between 3.2 and 4.1%; biases range between
--0.5 and

![](media/image17.png){width="6.100694444444445in"
height="4.067361111111111in"}

![](media/image18.png){width="6.127777777777778in"
height="4.079166666666667in"}

**Figure 15. Scatter plot of six modeled irradiances versus measured
direct irradiance (top plot). Difference plot (model -- measurement) of
direct irradiance as a function of case number in Table 2 (bottom
plot).**

![](media/image19.png){width="6.271527777777778in"
height="4.054861111111111in"}![](media/image20.png){width="6.289583333333334in"
height="4.097222222222222in"} Figure 16. Scatter plot of six modeled
irradiances versus measured diffuse irradiance (top plot). Difference
plot (model -- measurement) of diffuse irradiance as a function of case
number in Table 2 (bottom plot).

1.8%. Results for both direct and diffuse fall within the combined
uncertainties of the models and measurements.

In summary, solar radiation models and measurements seem to have merged
as a result of better specification of input parameters and better
measurements of irradiances than in prior studies. However, improving
the uncertainties of the inputs and irradiance measurements is an
unfinished task. Further effort is warranted in testing more cases for
very low aerosol optical depth days that tend to occur in mid to late
Autumn at the SGP CRF. Finally, these models should be tested spectrally
against a carefully calibrated shortwave spectral irradiance data set in
order to ensure that cancellation of errors in different portions of the
spectrum is not responsible for the excellent agreement achieved in this
study.

![](media/image21.png){width="6.488888888888889in"
height="4.223611111111111in"}

**Figure 17. Thirty-cases of (modeled -- measured) results in W/m^2^ for
direct beam and diffuse horizontal irradiances.**

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