SGP/SIRS/E3 - 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Additionally, the Model PSP radiometer has been replaced by a Model 8-48 which uses a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)

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

• short_diffuse

SGP/SIRS/E4 - 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)

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

• short_diffuse

SGP/SIRS/E3

Data this site-month is good except for Global (DS), which is pretty bad. DS goes 
below empirical limits every single night of the month. Its nighttime offset average is 
about 
-11 W/m^2. DS also dipped below daytime empirical limits intermittently throughout the 
day on the 3rd, 8th, and 9th and on the morning of the 17th. There is also a noticeable 
but 
unascertainable daytime offset in the DS which becomes evident when the NIP is near zero. 
Diffuse (DD) 
also goes below empirical limits on the morning of the 9th. The DD nighttime offset is 
about -5 W/m^2. 


too high and the DS is too low. This could be caused by the use of a 
constant pyranometer responsivity for all solar zenith angles, or the switchover from 
daylight to twilight SERI QC testing regimes. The daytime negative offset in the DS PSP is 
also a 
probable factor in the error. It should also be noted that the calibration factors for the 
PSP's have a 
tolerance of plus or minus 5% and the NIP has a tolerance of plus or minus 3%. Upwelling 
Longwave goes below the empirical limits on the local nights of the 23rd and 30th 
(24th and 31st GMT) for extended periods of time. The winter Longwave empirical limit
of 250 W/m^2 is currently under further review. (A reading of 250 W/m^2 suggests a 
temperature 
between 5 and 20 degrees Fahrenheit.)

• Radiation, shortwave, downwelling diffuse hemispheric irradiance, 1-min avg(down_short_diffuse_hemisp)
• short_direct_normal_std
• down_long_hemisp_max
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyranometer, Maxima(down_short_hemisp_max)
• time_offset
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)
• Upwelling Longwave Hemispheric Irradiance, Unventilated Pyrgeometer, Maxima(up_long_hemisp_max)
• lon
• Radiation, shortwave, downwelling total hemispheric irradiance, 1-min avg(down_short_hemisp)
• up_short_hemisp_std
• vBatt
• Upwelling Shortwave Hemispheric Irradiance, Unventilated Pyranometer, Maxima(up_short_hemisp_max)
• lat
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Maxima(down_short_diffuse_hemisp_max)
• Radiation, shortwave, direct normal irradiance, 1-min avg(short_direct_normal)
• up_long_hemisp_std
• alt
• Shortwave Direct Normal Irradiance, Pyrheliometer, Minima(short_direct_normal_min)
• Upwelling Longwave Hemispheric Irradiance, Unventilated Pyrgeometer, Minima(up_long_hemisp_min)
• down_short_hemisp_std
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyranometer, Minima(down_short_hemisp_min)
• Radiation, longwave, at 10-m height, upwelling hemispheric irrad., 1-min avg(up_long_hemisp)
• Radiation, shortwave, at 10-m height, upwelling hemispheric irrad., 1-min avg(up_short_hemisp)
• base_time
• Upwelling Shortwave Hemispheric Irradiance, Unventilated Pyranometer, Minima(up_short_hemisp_min)
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Minima(down_short_diffuse_hemisp_min)
• Irradiance, longwave, downwelling, hemispheric(down_long_hemisp)
• down_long_hemisp_std
• down_long_hemisp_min
• down_short_diffuse_hemisp_std

• base_time
• alt
• Irradiance, longwave, downwelling, hemispheric(down_long_hemisp)
• lon
• down_long_case_resist
• Radiation, shortwave, at 10-m height, upwelling hemispheric irrad., 1-min avg(up_short_hemisp)
• Radiation, longwave, at 10-m height, upwelling hemispheric irrad., 1-min avg(up_long_hemisp)
• Observed direct-beam normal solar irradiance(nip)
• time_offset
• up_long_dome_resist
• up_long_case_resist
• lat
• Radiation, shortwave, downwelling total hemispheric irradiance, 1-min avg(down_short_hemisp)
• short_diffuse
• down_long_dome_resist

SGP/SIRS/E4

Direct too low by visual inspection and Diffuse too high. Downwelling and upwelling IR, 
global and upwelling shortwave all indicate clear or mostly clear sky conditions during 
daylight hours, yet direct is near zero. Probable cause: sun tracker failure. There was 
a couple of ultimately futile attempts to fix the problem on the 5th and 19th.

• Radiation, shortwave, downwelling diffuse hemispheric irradiance, 1-min avg(down_short_diffuse_hemisp)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Minima(short_direct_normal_min)
• short_direct_normal_std
• Radiation, shortwave, direct normal irradiance, 1-min avg(short_direct_normal)
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Minima(down_short_diffuse_hemisp_min)
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Maxima(down_short_diffuse_hemisp_max)
• down_short_diffuse_hemisp_std

• Observed direct-beam normal solar irradiance(nip)
• short_diffuse

SGP/SIRS/E4  -

Data quality this site-month is terrible. This is due to the repeated 
failure of the sun tracker. The longwave looks good and there are no notable 
problems for those 2 parameters. Global (DS) has a nighttime offset of about 
-4 W/m^2. DS data look good. Disregard the three component test flags for DS. Something 
happens with the Upwelling Shortwave (US) on the 3rd where
it exceeds its daytime empirical limit. Snowy conditions is the likely cause. US also dips 
below its nighttime 
empirical limit on the local night of the 15th after sunset.

• down_short_diffuse_hemisp_std
• Radiation, longwave, at 10-m height, upwelling hemispheric irrad., 1-min avg(up_long_hemisp)
• Radiation, shortwave, downwelling diffuse hemispheric irradiance, 1-min avg(down_short_diffuse_hemisp)
• up_short_hemisp_std
• short_direct_normal_std
• Radiation, shortwave, downwelling total hemispheric irradiance, 1-min avg(down_short_hemisp)
• time_offset
• lon
• Radiation, shortwave, at 10-m height, upwelling hemispheric irrad., 1-min avg(up_short_hemisp)
• Upwelling Longwave Hemispheric Irradiance, Unventilated Pyrgeometer, Maxima(up_long_hemisp_max)
• alt
• Irradiance, longwave, downwelling, hemispheric(down_long_hemisp)
• up_long_hemisp_std
• down_long_hemisp_min
• Radiation, shortwave, direct normal irradiance, 1-min avg(short_direct_normal)
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Minima(down_short_diffuse_hemisp_min)
• lat
• vBatt
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyranometer, Minima(down_short_hemisp_min)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Minima(short_direct_normal_min)
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyranometer, Maxima(down_short_hemisp_max)
• Upwelling Shortwave Hemispheric Irradiance, Unventilated Pyranometer, Maxima(up_short_hemisp_max)
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Maxima(down_short_diffuse_hemisp_max)
• Upwelling Shortwave Hemispheric Irradiance, Unventilated Pyranometer, Minima(up_short_hemisp_min)
• Upwelling Longwave Hemispheric Irradiance, Unventilated Pyrgeometer, Minima(up_long_hemisp_min)
• base_time
• down_long_hemisp_max
• down_short_hemisp_std
• down_long_hemisp_std

• down_long_case_resist
• Irradiance, longwave, downwelling, hemispheric(down_long_hemisp)
• Radiation, longwave, at 10-m height, upwelling hemispheric irrad., 1-min avg(up_long_hemisp)
• up_long_dome_resist
• lat
• base_time
• up_long_case_resist
• short_diffuse
• down_long_dome_resist
• alt
• Radiation, shortwave, downwelling total hemispheric irradiance, 1-min avg(down_short_hemisp)
• lon
• time_offset
• Observed direct-beam normal solar irradiance(nip)
• Radiation, shortwave, at 10-m height, upwelling hemispheric irrad., 1-min avg(up_short_hemisp)

SGP/SIRS/E24 - NIP shaded near sunrise

The NIP is shaded by the EF laptop computer enclosure near sunrise during parts of the 
Spring and Summer.  This has occurred from 04/08 to 05/20 and from 07/26 to 09/06 every year 
since 2001 and will occur during these date ranges in the future as well.  The shading 
occurs for a few minutes each day between 1200 and 1300 GMT depending on the time of the 
year.  For the early dates, shading occurs a little after 1200 GMT and by 05/20 shading 
happens nearer to 1300 GMT. In late July the shading occurs near 1300 GMT and closer to 
1200 GMT by 09/06.

• Shortwave Direct Normal Irradiance, Pyrheliometer, Minima(short_direct_normal_min)
• short_direct_normal_std
• Radiation, shortwave, direct normal irradiance, 1-min avg(short_direct_normal)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)

• Radiation, shortwave, direct normal irradiance, 1-min avg(short_direct_normal)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Minima(short_direct_normal_min)
• short_direct_normal_std

• inst_direct_normal

SGP/SIRS/E24 - NIP shaded near sunrise

The NIP is shaded by the EF laptop computer enclosure near sunrise during parts of the 
Spring and Summer.  This has occurred from 04/08 to 05/20 and from 07/26 to 09/06 every year 
since 2001 and will occur during these date ranges in the future as well.  The shading 
occurs for a few minutes each day between 1200 and 1300 GMT depending on the time of the 
year.  For the early dates, shading occurs a little after 1200 GMT and by 05/20 shading 
happens nearer to 1300 GMT. In late July the shading occurs near 1300 GMT and closer to 
1200 GMT by 09/06.

• Shortwave Direct Normal Irradiance, Pyrheliometer, Minima(short_direct_normal_min)
• Radiation, shortwave, direct normal irradiance, 1-min avg(short_direct_normal)
• short_direct_normal_std
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)

• Radiation, shortwave, direct normal irradiance, 1-min avg(short_direct_normal)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Minima(short_direct_normal_min)
• short_direct_normal_std

• Shortwave Direct Normal Irradiance, Pyrheliometer, Minima(short_direct_normal_min)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)
• Radiation, shortwave, direct normal irradiance, 1-min avg(short_direct_normal)
• short_direct_normal_std

• inst_direct_normal