SGP/SIRS/E24 - 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Additionally, the Model PSP radiometer has been replaced by a Model 8-48 which uses a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)

• down_short_diffuse_hemisp_std
• 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)

• short_diffuse

SGP/SIRS - SIRS E24 April 1999

Data quality at this site is very good this month. Nearly all 99.6% of the daytime data is 
present. Passing or low-level flags are assigned to 93.5% of the present daytime data. 
High level flags seen in the first two days of the month indicate that the DD is above 
daytime empirical limits. A diurnal plot for the 1st shows that the suntracker has failed: 
the DD is tracking the DS, and the NIP signal is flat.  Low to mid-level flags observed 
from late morning to mid-afternoon late in the month indicate that the DS is too high by 
the three-component test. Alternatively, the flags may signal that the NIP or the DD is too 
low. The DS and DD were forced below daytime or twilight empirical limits by heavy cloud 
cover on a number of occasions. On the 27th at 1657 GMT the US and UIR signal fields are 
filled with the data logger value representing missing data -  -99999 as if the 
instruments were disconnected. The Operations Management Information Systems Instrument and 
Corrective Maintenance Report for this site indicate that the UIR and US were disconnected and 
the 10m tower taken down for upgrade on the 26th. No problems were noted in the DIR signal.

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

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