SGP/SIRS/C1 - Diffuse Pyranometer Thermal Offsets

Broadband downwelling shortwave diffuse (sky) irradiance measurements available from SIRS
during the period of this Data Quality Report (DQR), require adjustment for thermal
offsets.  These thermal, or ?zero? offsets refer to the generally reduced output signals
from a shaded pyranometer due to the exchange of longwave (infrared) irradiance between
the single black thermopile detector, the protective glass domes surrounding the
detector, and the atmosphere. Originally considered an acceptable nighttime response of
thermopile-type pyranometers, the generally negative bias is now recognized to
significantly effect the accuracy of SIRS diffuse irradiance data during daylight
periods.

Studies of the Eppley Laboratory, Inc. Model PSP (Precision Spectral Pyranometer), used
for the SIRS   measurements of diffuse irradiance, suggest the thermal offset correction
can range from near 0 to as much as 30 Watts per square meter, depending on the
coincident net longwave, or infrared irradiance [1, 2].  Under very clear-sky conditions,
the diffuse irradiance from a shaded PSP can be less than the minimum physical limit
defined by radiative transfer model estimates based only on Rayleigh scattering effects.

A correction method has been developed for adjusting SIRS diffuse irradiance data [3]. 
The resulting Value Added Product (VAP) will be applied to SIRS data for the period of
this DQR.  The VAP will not be applied to SIROS data collected before the instrument
platform was converted to SIRS.

Additionally, the Model PSP radiometer has been replaced by a Model 8-48 which uses a
black and white thermopile detector known to reduce the thermal offset errors to less
than 2 Watts per square meter [3].  The radiometer replacement at this SIRS location was
completed on the ending date of this DQR.

References:
1. Gulbrandsen, A., 1978:  On the use of pyranometers in the study of spectral solar
radiation and atmospheric aerosols.  J. Appl. Meteorol., 17, 899-904.
2. Cess, R. D., X. Jing, T. Qian, and M. Sun, 1999:  Validation strategies applied to the
measurement of total, direct and diffuse shortwave radiation at the surface.  J. Geophys.
Res.
3. Dutton, E.G., J. Michalsky, T. Stoffel, B. Forgan, J. Hickey, D. Nelson, T. Alberta,
and I. Reda, 2001:  Measurement of Broadband Diffuse Solar Irradiance Using Current
Commercial Instrumentation With a Correction for Thermal Offset Errors. J. Atmos. Oceanic
Tech.   Vol 18, No. 3, 297-314.   (March 2001)

• Shaded pyranometer voltage(short_diffuse)

• Downwelling Shortwave Diffuse Hemispheric Irradiance, Ventilated Pyranometer(down_short_diffuse_hemisp)
• Downwelling Shortwave Hemispheric Irradiance, Pyranometer, Standard Deviation(down_short_diffuse_hemisp_std)
• Downwelling Shortwave Diffuse Hemispheric Irradiance, Ventilated Pyranometer,
  Maxima(down_short_diffuse_hemisp_max)
• Downwelling Shortwave Diffuse Hemispheric Irradiance, Ventilated Pyranometer,
  Minima(down_short_diffuse_hemisp_min)

SGP/SIRS/C1 - Discrepancy between measured and derived downwelling shortwave hemispheric

Measured values of downwelling hemispheric shortwave radiation were up to 40 W/m^2 higher 
than derived values during the afternoon hours.

Analysis reveals that this is likely an artifact of the cosine response of the particular 
installed PSP coupled with typical uncertainties of this set of installed radiometers.   
That is, this particular instrument cluster seems to be producing cumulative errors on 
the high side of accepted values.

The PSP contains the most uncertainty in measurement at this station. Users should use a 
derived value for the downwelling shortwave hemispheric irradiance (DS). Where DS = 
diffuse component + direct normal * cos(SZA) component described in the SIRS handbook Sec 7.2 
available at: http://www.arm.gov/publications/tech_reports/handbooks/sirs_handbook.pdf

• Shortwave Direct Normal Irradiance, Pyrgeometer(short_direct_normal)
• Shortwave Direct Normal Irradiance, Pyrgeometer, Standard Deviation(short_direct_normal_std)
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyranometer, Standard
  Deviation(down_short_hemisp_std)
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyranometer, Minima(down_short_hemisp_min)
• Downwelling Shortwave Diffuse Hemispheric Irradiance, Ventilated Pyranometer,
  Maxima(down_short_diffuse_hemisp_max)
• Downwelling Shortwave Diffuse Hemispheric Irradiance, Ventilated Pyranometer,
  Minima(down_short_diffuse_hemisp_min)
• Downwelling Shortwave Hemispheric Irradiance, Pyranometer, Standard Deviation(down_short_diffuse_hemisp_std)
• Down-welling unshaded pyranometer voltage(down_short_hemisp)
• Shortwave Direct Normal Irradiance, Pyrgeometer, Maxima(short_direct_normal_max)
• Shortwave Direct Normal Irradiance, Pyrgeometer, Minima(short_direct_normal_min)
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyranometer, Maxima(down_short_hemisp_max)
• Downwelling Shortwave Diffuse Hemispheric Irradiance, Ventilated Pyranometer(down_short_diffuse_hemisp)

• Instantaneous Downwelling Hemispheric Shortwave, Unshaded Pyranometer Thermopile
  Voltage(inst_global)
• Instantaneous Uncorrected Downwelling Shortwave Diffuse, Shaded Pyranometer
  Thermopile Voltage(inst_diffuse)
• Instantaneous Direct Normal Shortwave Irradiance, Pyheliometer Thermopile
  Voltage(inst_direct_normal)

SGP/SIRS/C1 - Reprocess: Adjust Direct Normal Irradiance

Direct normal broadband shortwave irradiance measured by NIP s/n 30722E6 was found to have 
a 0.62%% negative bias when compared in-situ with absolute cavity radiometer 
measurements taken on August 21, 2000.  The estimated measurement uncertainty of the calibration of 
NIP s/n 30722E6 on July 18, 2000 was -2.6% to +4.8%.  

Data from NIP s/n 30722E6 can be adjusted according to the in-situ comparisons to reduce 
the apparent measurement bias.

• Raw data stream - documentation not supported(Raw data stream - documentation not supported)

• Measured Downwelling Direct Shortwave Irradiance (dir)(dirfluxdn_measured)

• Raw data stream - documentation not supported(Raw data stream - documentation not supported)

• Measured Downwelling Direct Shortwave Irradiance (dir)(dirfluxdn_measured)

SGP/SIRS/C1 - Reprocessed: Longwave Calibration error

Modified pyrgeometer calibration procedures were implemented beginning in December 2003. 
These modified procedures introduced a calibration bias in the longwave data. The previous 
procedures were re-implemented at all sites between December 2005 and February 2006 to 
restore proper calibrations. 

The data collected while the incorrect procedures were in place have been reprocessed to 
remove the calibration bias.  The reprocessed 60 second averaged data are based on 3 
instantaneous 20 second data records rather than on 60 1 second instantaneous data records. 
Still, these data are considered far superior to the originally processed data. The 
reprocessed data were archived in October 2006.

• Downwelling Longwave Hemispheric Irradiance, Shaded Pyrgeometer(down_long_hemisp_shaded)
• Downwelling Longwave Hemispheric Net Infrared(down_long_netir)
• Upwelling (10 meter) Longwave Hemispheric Irradiance, Pyrgeometer(up_long_hemisp)
• Upwelling Longwave Hemispheric Net Infrared(up_long_netir)