DQRID : D970317.1
Start DateStart TimeEnd DateEnd Time Data Quality Metric
10/13/1995000008/20/19972359Suspect
more
Subject:
downwelling solar irradiance measurement adjustments
DataStreams:sgpsirosE13.a1, sgpbsrnC1.a1
Description:
A comparison of BSRN and SIROS solar radiometers for measuring downwelling 
irradiances at the SGP central facility was made with field standards and two 
absolute cavity radiometers brought to the site or a two-week period in April 
1996 by Mike Rubes (formerly of the National and Oceanic Atmospheric 
Administration, Air Resources Laboratory, Surface Radiation Research Branch in 
Boulder, CO).  A description of this effort can currently be found on the 
World Wide Web at http://www.srrb.noaa.gov/apr96iop/hagsie.html.  Analyses of 
the data from these comparisons have resulted in several observations on the 
quality of data collected at the BSRN and SIROS platforms since October 13, 
1995, which are probably valid to the present time, until these sensors are 
replaced with more recently calibrated sensors.  On Oct. 13, 1995, the two 
BSRN pyranometers (PSPs) were replaced, so the observations do not apply to 
the BSRN measurements of global and diffuse irradiation before that date.  
Another source of information is inspection of the SIROS and BSRN equipment by 
Joe Michalsky (Atmospheric Science Research Center, State University of New 
York at Albany) at various times.  The results of the findings are summarized 
as recommendations in the following several paragraphs.  Some explanation and 
further comments are provided in the parenthetical remarks.

ANALYSIS WHEN THE DIRECT BEAM WAS NOT OBSCURED BY CLOUDS

Direct-beam solar irradiance measured with the BSRN pyrheliometer (NIP) are 
too large by approximately 0.5% compared to the two absolute cavity 
radiometers.  (This small underestimate is within the expected level of 
uncertainty.)  

Direct-beam solar irradiance measured with the SIROS pyrheliometer are too 
small by approximately 2.1% compared to the two absolute cavity radiometers.  
(This large discrepancy is unexplained and will be explored during future 
calibration activities at the SGP Radiation Calibration Facility.)  

Possibly the best estimate of downwelling total hemispherical solar (global) 
irradiance can be made by summing the SIROS pyrheliometer irradiance reading 
multiplied by 1.021 (and by the cosine of the solar zenith angle) and the 
average of the readings for diffuse irradiance from the shaded BSRN and SIROS 
pyranometers.  The direct-beam part can alternatively be computed as 0.995 
times the BSRN pyrheliometer reading.  For data collected in October before 
the 13th, when the BSRN shaded pyrheliometer was replaced, the diffuse 
component is probably best computed directly from the SIROS shaded sensor 
alone.  

Downwelling total hemispherical solar (global) irradiance measured by the BSRN 
unshaded pyranometer is approximately 2% too small (which is within the 
expected level of uncertainty for unshaded pyranometer measurements) compared 
to the values computed from the measured direct-beam and diffuse components.  

(Downwelling total hemispherical solar irradiances measured by the SIROS 
unshaded pyranometer systematically underestimates the global irradiances 
by excessive amounts, i.e., by greater than 3%.)  

The analyses leading to these recommendations are described in an extended 
abstract presented in early February (J. Michalsky et al., "Optimal 
Measurements of Surface Shortwave Irradiance Using Current Instrumentation--
The ARM Experience," in Preprint Volume, Ninth Conference on Atmospheric 
Radiation, Feb. 2-7, Long Beach, California, pp. J5-J9, American 
Meteorological Society, Boston, MA).  Further relevant analyses were conducted 
by Kato et al., (Seiji Kato, Pennsylvania State University) and are described 
in a manuscript submitted for publication ("Uncertainties in Modelled and 
Measured Clear Sky Surface Shortwave Irradiances")." 

UNSHADED PYRANOMETER PERFORMANCE WHEN THE DIRECT BEAM WAS NOT OBSCURED

The above recommendations are based mostly on analyses conducted for 
cloudless, midday conditions.  Because the data reported from the unshaded 
pyranometer were not corrected for cosine response, slight overestimates of 
global irradiance from unshaded pyranometers tend to occur in cloudless 
conditions at solar zenith angles less than 45 deg and slight underestimates 
tend to occur for zenith angles greater than 55 deg.  The maximum deviations 
occur at extreme solar zenith angles and are about 2%.  

TRACKER-SHADING PERFORMANCE

The data user should note, as has been noted in data release statements, that 
analyses of the direct, diffuse, and/or direct beam irradiances should be 
preceded by a check of sensor performances by summing the direct and diffuse 
components and comparing the result to the directly measured global component.  
When this is done, problems with solar tracking are usually apparent.

Because slight misalignments in the tracking and shading devices can be 
difficult to detect, small deviations of the component sum from expected 
behavior are sometimes difficult to explain.  If such deviations tend to recur 
for specific time intervals for several days, one might suspect a tracking or 
shading problem.  For the time period addressed here, the modern tracking- 
shading assembly used with the SIROS sensors appeared to work well.  For the 
BSRN sensors until January 1996, an older tracking-shading system was used 
that was not as reliable as the modern assembly used with the SIROS sensors; 
problems with this BSRN tracking and shading system, were usually evident when 
they occurred.  A modern tracker-shader was installed for the BSRN sensors in 
January 1996. The tracker was not aligned as well as it could be.  Efforts are 
underway to improve tracker alignment checks and procedures at all SIROS sites 
and the BSRN site.  

PARTLY CLOUD CONDITIONS

An analysis by Chuck Long (formerly at the Pennsylvania State University and 
now with the University of Colorado and the National Oceanic and Atmospheric 
Administration) indicated that data users who are investigating partly cloudy 
sky conditions will usually find that the BSRN outputs are more reliable for 
short periods of time, say less than 30 min, than are the SIROS outputs.  This 
tends to occur because the SIROS data are recorded only every 20 s while the 
BSRN data represent one-minute averages computed on the basis of sampling once 
per second.  Under partly cloudy conditions, sampling only once every 20 s 
tends to provide inadequate statistical representation of downwelling 
irradiances.  

ESTIMATES FOR CLOUDY CONDITIONS

The component sum technique is not applicable for overcast conditions.  For 
the time period addressed here, the SIROS shaded sensor appears most reliable 
before October 13, 1995.  Thereafter, an average of data from the SIROS shaded 
pyranometer, the shaded BSRN sensor, and the shaded BSRN sensor multiplied by 
1.02 might be the best estimate of global irradiance for cloudy conditions.  
However, a rigorous analysis on the results of this procedure has not been 
carried out, so the data user should approach this technique with caution.  

SOME ADDITIONAL INFORMATION

The excessively large deviations noted above for the pyranometers result in 
part from a mixture of different sources of calibration procedures.   The 
following table lists the sources of calibration:

Sensor       Coefficient used    Calibration     Installation
              to process data     date            date
BSRN PSP DS     BORCAL           Sept. 1995      Oct. 13, 1995
BSRN PSP DD     Eppley           June 1995       Oct. 13, 1995
BSRN NIP        BORCAL           July 1993       March 17, 1994
SIROS PSP DS    Eppley           June 1995       July 25, 1995
SIROS PSP DD    Eppley           June 1995       July 25, 1995
SIROS NIP       BORCAL           Sept. 1994      July 25, 1995

DS =  downwelling solar or global
DD =  downwelling diffuse
PSP = precision spectral pyranometer
NIP = normal incidence pyrheliometer for direct-beam solar
BORCAL = broadband outdoor radiometer calibration, conducted by the National 
         Renewable Energy Laboratory (NREL)
Eppley = denotes calibrations in an integrating sphere by the manufacturer,
         Eppley Laboratory, Inc.

The BORCAL calibrations result in estimates of solar irradiances that are 
typically 1.5% larger than Eppley calibrations, a situation which is under 
investigation by Tom Stoffel at NREL and John Hickey at Eppley.  They are 
working together to document this difference.  This difference helps to 
explain the larger estimates of global irradiance measurement with the BSRN 
sensor than with the SIROS sensor.  

A greater source of concern than over differences between the NREL versus the 
Eppley calibrations at this time is the insufficiently frequent recalibrations 
of sensors in operation at the SGP site.  Although the NIPs are expected to 
hold their calibrations for rather long periods of time, the pyranometers 
typically should be recalibrated at least once every 12 months.  Change out 
with freshly calibrated pyranometers and pyrheliometers at the SGP site will 
begin in 1997, with the goal of routinely replacing every pyranometer and 
pyrheliometer with freshly calibrated sensors once every year.  

Data users can inspect metrics provided on the World Wide Web by the SGP site 
scientist team on data quality at the following address:
http://manatee.gcn.uoknor.edu/metrics/METRICS.html

Other observations/measurements impacted by this problem:

Any derived estimates of downwelling solar radiation components using data 
from central facility SIROS or BSRN sensors (for downwelling solar radiation) 
for the time period indicated.

Suggested Corrections of the Problem: (e.g. change calibration factor and 
recompute, flag data with this comment, etc.)

Use of these recommendations by data users.  Ideally, the component sum 
technique would be applied in a value-added product (VAP) implemented at the 
Experiment Center, but this has not been done yet.  In the meantime, users of 
recent data can inspect plots of component sum technique on the World Wide Web 
site noted above.
Suggestions: 
Measurements:sgpsirosE13.a1:
  • alt
  • hemisp_narrowband
  • down_long_diffuse_hemisp
  • flag_nighttime1-24
  • mfrsr_temp
  • up_short_hemisp
  • time_offset
  • down_long_case_temp
  • logger_temp
  • down_short_diffuse_hemisp
  • flag_zero_divisor1-24
  • lon
  • channel
  • base_time
  • up_long_hemisp
  • up_long_dome_temp
  • direct_norm_broadband
  • up_long_case_temp
  • diffuse_hemisp_broadband
  • flag_ln_error1-24
  • therm_volt
  • short_direct_normal
  • down_long_dome_temp
  • lat
  • hemisp_broadband
  • direct_norm_narrowband
  • diffuse_hemisp_narrowband
  • logger_volt
  • down_short_hemisp
  • flag_zero_cosine1-24
more
sgpbsrnC1.a1:
  • time_offset
  • base_time
  • psp2
  • lon
  • psp1
  • nip
  • lat
  • alt
  • psig
more

Close this window