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
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 .
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Additionally, the Model PSP radiometer has been replaced by a Model 8-48 which uses a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 . The radiometer replacement at this SIRS location was
completed on the ending date of this DQR.
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.
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)