SGP/SIRS/E13 - 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Additionally, the Model PSP radiometer has been replaced by a Model 8-48 which uses a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)

• short_diffuse

• down_short_diffuse_hemisp_std
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Maxima(down_short_diffuse_hemisp_max)
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Minima(down_short_diffuse_hemisp_min)
• Radiation, shortwave, downwelling diffuse hemispheric irradiance, 1-min avg(down_short_diffuse_hemisp)

"bsrn" platform at SGP data problem

As reported in ARM Technical Report ARM-TR-002, the 'bsrn' data at the SGP CF
was not in agreement with the co-located SIRS E13 and C1 data. Subsequent
discussion led to the conclusion that ther was a voltage problem with the
'bsrn' data logger. In development and testing of the Diffuse Correction and
Best Estimate Flux VAPs, it has been shown that all 'bsrn' data, all
instruments, suffered this data logger problem. The result is noisy and
slightly offset values, seemingly at random throughout the period from October
1998 through the end of the 'bsrn' data stream and it's replacement by the BRS
system. All this data needs to be colored 'questionable', and all data users
need to be informed that either the SIRS E13 or SIRS C1 data should be used
instead.

• Raw data stream - documentation not supported

• Calculated downwelling hemispheric diffuse solar irradiance(psp1)
• psp2
• Observed direct-beam normal solar irradiance(nip)
• Downwelling hemispheric infrared irradiance(psig)

• snip
• ssig
• vbat
• Observed direct-beam normal solar irradiance(nip)
• psp2
• Calculated downwelling hemispheric diffuse solar irradiance(psp1)
• spsp2
• spsp1
• stdome
• vref
• stcase
• ptcase
• ptdome
• Downwelling hemispheric infrared irradiance(psig)
• svref

• Shortwave Direct Normal Irradiance, Pyrheliometer, Minima(short_direct_normal_min)
• dsdh_dutton_corrected
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)
• down_long_hemisp_std
• down_long_hemisp_max
• dome_temperature
• Radiation, shortwave, direct normal irradiance, 1-min avg(short_direct_normal)
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Minima(down_short_diffuse_hemisp_min)
• detector_flux
• Radiation, shortwave, downwelling total hemispheric irradiance, 1-min avg(down_short_hemisp)
• calculated_down_long_hemisp
• Radiation, shortwave, downwelling diffuse hemispheric irradiance, 1-min avg(down_short_diffuse_hemisp)
• short_direct_normal_std
• effective_temperature
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Maxima(down_short_diffuse_hemisp_max)
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyranometer, Maxima(down_short_hemisp_max)
• down_long_hemisp_min
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyranometer, Minima(down_short_hemisp_min)
• dsdh_full_corrected
• case_temperature
• down_short_hemisp_std
• down_short_diffuse_hemisp_std
• Irradiance, longwave, downwelling, hemispheric(down_long_hemisp)

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

Direct normal broadband shortwave irradiance measured by NIP s/n 30721E6 was found to have 
a 4.1% 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 30721E6 on July 18, 2000 was -2.8% to +3.5%.  The additional uncertainties of the 
field (rather than calibration) data acquisition system and installation contribute to the 
large negative bias found in the data on August 21, 2000.

Data from NIP s/n 30721E6 can be adjusted according to the in-situ comparisons to reduce 
the apparent measurement bias.

• Raw data stream - documentation not supported

• Observed direct-beam normal solar irradiance(nip)

• Observed direct-beam normal solar irradiance(nip)

SGP/SIRS/E13 - SWFANAL Data description error

The long name attribute for the cloud effect fields incorrectly
defines the cloud effect to be the difference between the 
clearskyfit and measured values:
   
"Difference: ***fluxdn_clearskyfit - ***fluxdn_measured (***fcg)" ;
   
The cloud effects are correctly calculated by subtracting the
irradiance calculated in the clear sky fit from the measured 
irradiance.
   
"Difference: ***fluxdn_measured - ***fluxdn_clearskyfit (***fcg)" ;
   
NOTE: this is just a documentation error.  The data are correctly
calculated.

• sswfluxdn_cloudeffect
• difswfluxdn_cloudeffect
• gswfluxdn_cloudeffect

• sswfluxdn_cloudeffect
• difswfluxdn_cloudeffect
• gswfluxdn_cloudeffect