SGP/SIRS/E1 - 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Additionally, the Model PSP radiometer has been replaced by a Model 8-48 which uses a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, Minima(down_short_diffuse_hemisp_min)
• Radiation, shortwave, downwelling diffuse hemispheric irradiance, 1-min avg(down_short_diffuse_hemisp)
• Downwelling Shortwave Diffuse Hemisp. Irrad., Ventilated Pyranometer, Maxima(down_short_diffuse_hemisp_max)

SGP/SIRS/E1

Automated testing says this is yellow because there is not enough information from the 
shortwave instruments to ascertain longwave quality, given that the Direct (NIP) is bad. 
Visual inspection says the data are good.

• down_long_case_resist
• up_long_dome_resist
• up_long_case_resist
• Radiation, longwave, at 10-m height, upwelling hemispheric irrad., 1-min avg(up_long_hemisp)
• down_long_dome_resist
• Irradiance, longwave, downwelling, hemispheric(down_long_hemisp)

• Upwelling Longwave Hemispheric Irradiance, Unventilated Pyrgeometer, Maxima(up_long_hemisp_max)
• up_long_hemisp_std
• Upwelling Longwave Hemispheric Irradiance, Unventilated Pyrgeometer, Minima(up_long_hemisp_min)
• down_long_hemisp_max
• down_long_hemisp_min
• down_long_hemisp_std
• Irradiance, longwave, downwelling, hemispheric(down_long_hemisp)
• Radiation, longwave, at 10-m height, upwelling hemispheric irrad., 1-min avg(up_long_hemisp)

SGP/SIRS/E1  -

Data quality this month is pretty good, with some exceptions. Nighttime offsets in the 
Downwelling pyranometer (DS) average about -3 W/m^2 for the month. The Downwelling 
Diffuse pyranometer (DD) averages about -4 W/m^2 for its nighttime offset. There are 
moderate inconsistencies between the Global (DS), Direct (NIP) and Diffuse (DD) 
components for the 6th and 7th in the afternoon and the 4th, 5th, 7th, 11th, 13th, 22nd, 

This could be caused by the use of a 
constant pyranometer responsivity for all solar zenith angles, or the switchover from 

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 empirical limits on the 4th 
from 06:20 to 16:20 GMT and on the 7th from 09:25 to 13:30 GMT. 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.) 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 
belows its nighttime empirical limit
for short durations on the local nights of the 15th, 17th, and 22nd.

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

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