Scientific Theme: Advanced Modeling and Observing Systems

Theme report: Advanced Modeling and Observing Systems
triangular slit function. This method was compared to previous years that used a simple method of convolving with a
common slit function. In addition, this year the reference spectrum was determined from a routine from Gardiner et
al. [1997]. Although the routine is arduous, it has the advantage of providing an unbiased standard procedure for
determining the reference spectrum. A draft of the spectroradiometer paper has been prepared. A comparison of the
solar irradiance from the filter radiometers is presented in a second forthcoming paper.
MILESTONE GMD01.3:
Update UV-MFRSR calibration data files from 1997 to 2005 for CUCF horizontal scale
corrections, NIST vertical scale corrections, and out-of-band corrections.
ACCOMPLISHMENTS FOR GMD01.3:
Previously, the CUCF determined that the UV-MFRSR channels 3-6 were in poor agreement with the U111
spectroradiometer. These channels changed from Gallium Phosphide (GaP) photodetectors to Silicon (Si) to
accommodate the failure of the GaP photodetectors in the field. Work indicated that the problem was primarily that
the Si photodetectors were allowing out-of-band light from the NIST traceable halogen-tungsten lamps to contribute
to the calibration signal, which resulted in an inaccurate calibration.
Studies showed that the contribution of out-of-band light to the signal depended on several factors. First, it depended
on the channel, with the 317-nm channel typically being the most affected, i.e., average of 22.1%. Secondly, it
depended on the instrument, where each instrument was different but grouped in about four different patterns, i.e.,
for the 317-nm channel the out-of-band light ranged from 37.7% to 6.6%. Lastly, the signal depended on the lamp
that was used and likely due to the different color temperatures of the lamps, i.e., approximately 0-5% difference
between lamps at 317-nm.
All 47 UV-MFRSR radiometers were characterized for out-of-band light. The last two radiometers came through the
CUCF in December 2006. There were 193 UV-MFRSR calibration files corrected for out-of-band light since 2001.
The UV-MFRSR calibration files were also corrected for the 1-2% NIST adjustment to the lamp irradiance scale and
the 4-5% adjustment to the CUCF irradiance scale last year. In November 2006, 300+ corrected calibration files
were sent to the USDA to replace the old files in their system. The USDA UV monitoring programs have updated
the UV solar irradiance from their instruments, using the corrected calibration files. The corrected data are now
available on their website including a web page to describe the corrections to the calibration files.
GMD02: Surface Radiation Network
GOAL:
Collect long-term research-quality up-welling and down-welling broadband solar and infrared radiation data at
seven U.S. sites. Collect long-term, broadband ultraviolet radiation data to evaluate variations in the erythemal
doses. Collect long-term, spectral filter data to measure column aerosol optical depth and cloud optical depth.
Collect cloud cover data to assess the effect of clouds on the surface radiation budget.
MILESTONE GMD02.1:
Analyze 10-year plus aerosol optical depth record of SURFRAD sites and publish the results.
ACCOMPLISHMENTS FOR GMD02.1:
Results from a recent decadal study of aerosol optical depth (AOD) over the U.S. reveal a general nationwide
decrease, but also geographic differences. These new data from NOAA‘s SURFRAD surface radiation budget
network show high AOD in summer and low AOD in winter. The western stations have the lowest AOD and the
eastern stations have the highest. Western stations also show a secondary springtime maximum caused by Asian
dust. The abundance of aerosols in the east and their relatively small size is attributed to organic emissions, high
humidity, and the predisposition for stagnant air in summer. These characteristics agree with previously published
results, however, national and station decadal AOD tendencies show interesting features that may have climate
change implications.
Nationally, 500nm AOD decreased from 1997 through 2006 by about 0.02, which is similar to recent results
reported for the oceans. However, not all U.S. stations share this tendency. Results show that aerosols are decreasing
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