UV-RSS - PMOD/WRC
UV-RSS - PMOD/WRC
UV-RSS - PMOD/WRC
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DDR(380nm)<br />
Direct-to-diffuse <strong>UV</strong> solar irradiance ratio for a <strong>UV</strong> rotating shadowband spectroradiometer<br />
(<strong>UV</strong>-<strong>RSS</strong>) and a <strong>UV</strong> multi-filter rotating shadowband radiometer (<strong>UV</strong>-MFRSR)<br />
K. Lantz, P. Disterhoft, and P. Kiedron<br />
Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, USA<br />
J. Slusser<br />
Natural Research Ecology Laboratory, Colorado State University, Fort Collins, Colorado, USA<br />
Abstract. The purpose of the work is to compare<br />
radiative transfer model calculations (T<strong>UV</strong>) with the<br />
results from the <strong>UV</strong>-Rotating Shadowband<br />
Spectroradiometer (<strong>UV</strong>-<strong>RSS</strong>) to estimate direct-to-diffuse<br />
solar irradiance ratios (DDR) that are used to evaluate the<br />
possibility of retrieving aerosol single scattering albedo<br />
(SSA) under a variety of atmospheric conditions: large<br />
and small aerosol loading, large and small surface albedo.<br />
Results from the parameterization of the diffuse and direct<br />
irradiance are used to correct global horizontal irradiance<br />
for the effects of non-Lambertian angular response of<br />
instruments such as Brewer spectrophotometers within the<br />
NOAA-EPA <strong>UV</strong> Network and the U111 spectroradiometer<br />
at Table Mountain.<br />
Introduction<br />
The purpose of the work is to compare radiative<br />
transfer model calculations (T<strong>UV</strong>) with the results from<br />
the <strong>UV</strong>-Rotating Shadowband Spectroradiometer<br />
(<strong>UV</strong>-<strong>RSS</strong>) to estimate direct-to-diffuse solar irradiance<br />
ratios (DDR). The direct to diffuse ratio is a powerful<br />
tool as an indicator of atmospheric conditions and can<br />
provide information on aerosol loading, changing surface<br />
albedo, and cloud conditions [Kiedron et al., 2005]. The<br />
direct to diffuse solar irradiance ratios are used to evaluate<br />
the possibility of retrieving aerosol single scattering albedo<br />
(SSA) under a variety of atmospheric conditions: large<br />
and small aerosol loading, large and small surface albedo<br />
[Krotkov et al., 2005; Petters et al., 2003]. As an<br />
example, results from the parameterization of the diffuse<br />
and direct irradiance are used to correct global horizontal<br />
irradiance for the effects of non-Lambertian angular<br />
response of instruments such as Brewer<br />
spectrophotometers within the NOAA-EPA <strong>UV</strong> Network at<br />
Table Mountain. The impact of the corrections will be<br />
evaluated in the context of accuracy of erythemal dose<br />
from those instruments.<br />
NOAA-EPA Brewer Network<br />
The new NOAA-EPA Brewer Network (NEUBrew)<br />
consists of six sites across the continental United States<br />
and includes Bondville, IL, Fort Peck, MT, Raleigh, NC,<br />
Mountain Research Station, CO, Boulder, CO, and<br />
Houston, TX. The network locations were chosen because<br />
of their proximity to aerosol and cloud instrumentation<br />
sites associated with programs at the National Oceanic and<br />
Atmospheric Administration’s (NOAA) SURFRAD<br />
Program, the USDA <strong>UV</strong> Monitoring and Research<br />
Program [Bigelow et al, 1998], and several U.S.<br />
universities. Table 1 lists the sites, location, and<br />
elevation.<br />
Table 1. NEU-Brew site locations<br />
http://esrl.noaa.gov/gmd/grad/neubrew/<br />
Site Latitude, Longitude Elevation<br />
Bondville, IL 40.040, 88.360 213 m<br />
Ft Peck, MT 40.310, 105.100 634 m<br />
MRS, CO 40.031, 105.533 2896 m<br />
Raleigh, NC 35.720, 78.680 124 m<br />
Boulder, CO 40.125, 105.236 1689 m<br />
Houston, TX 29.720, 95.343 18+ stories<br />
Instrumentation and RT Model<br />
A <strong>UV</strong> rotating shadowband spectroradiometer<br />
(<strong>UV</strong>-<strong>RSS</strong>) measures once a minute simultaneous direct and<br />
diffuse irradiances in the nominal wavelength range of 297<br />
nm – 385 nm at 734 pixels with resolution varying from<br />
0.32 nm to 0.61 nm (FWHM) [Kiedron et al., 2001].<br />
The instrument is deployed at Table Mountain, Boulder<br />
Colorado since June 2003. The <strong>UV</strong>-<strong>RSS</strong> is collocated<br />
with a <strong>UV</strong> multifilter rotating shadowband radiometer<br />
(<strong>UV</strong>-MFRSR), a high resolution (0.1 nm FWHM) USDA<br />
U111 scanning spectroradiometer, and 3 MKIV Brewer<br />
spectrophotometers from a new NOAA-EPA <strong>UV</strong> network.<br />
The <strong>UV</strong>-MFRSR is part of the USDA <strong>UV</strong> monitoring<br />
network and the instrument deployed at Table Mountain<br />
has been set-up to measure once a minute total and diffuse<br />
solar irradiance in nominal 2-nm channel bands in 7<br />
channels at 300, 305, 311, 317, 325, 332, 368 nm<br />
[Harrison et al., 1994]. The radiative transfer model used<br />
in the analysis is the T<strong>UV</strong> radiative transfer code<br />
developed by Madronich [1993].<br />
Discussion of Future Work<br />
Our work will analyze the diffuse to direct ratio<br />
(DDR) under a variety of atmospheric conditions at the<br />
Table Mountain Test Site, 15.0 km north of NOAA,<br />
4<br />
3<br />
2<br />
1<br />
0<br />
1.0<br />
1.5<br />
2.0<br />
UpperEnvelope=0.34225+7.7623*exp(-0.87249*m)<br />
2.5<br />
Airmass - m<br />
Figure 1. Direct to Diffuse (DDR) at 380 nm from the<br />
<strong>UV</strong>-<strong>RSS</strong> at Table Mountain for 5412 spectra.<br />
3.0<br />
3.5<br />
4.0
DDR/DDR(368nm)<br />
Resid [%]<br />
Mt, August 31, 2005<br />
Table<br />
GMT= 19.0166<br />
Hour<br />
58.2835¡<br />
SEA=<br />
1.17479<br />
Airmass=<br />
DDR(<strong>UV</strong>MFRSR)<br />
3rd degree polynomial<br />
Fit<br />
Irradiance [W/m2/nm]<br />
Boulder, CO.<br />
The <strong>UV</strong>-<strong>RSS</strong> has been located at Table Mountain since<br />
June, 2003. The <strong>UV</strong>-MFRSR 00286 has been located at<br />
Table Mountain since July, 2005. Radiative transfer<br />
calculations of DDR will be compared to the<br />
measurements under a variety of atmospheric conditions.<br />
Figure 1 shows the DDR at 380 nm from the <strong>UV</strong>-<strong>RSS</strong> for<br />
5412 spectra from June, 2003 – July, 2006 and gives an<br />
indication of the typical values seen under a variety of<br />
conditions.<br />
One application of this work will be to correct global<br />
solar irradiance for the non-Lambertian response of the<br />
Brewer spectrophotometers in the NEUBrew Network.<br />
The correction of the angular response requires knowledge<br />
of the partitioning of solar irradiance into direct and<br />
diffuse components. All of the NEUBrew sites have are<br />
collocated with a <strong>UV</strong>-MFRSR; however the <strong>UV</strong>-MFRSR<br />
measures the diffuse and total solar irradiance in discrete<br />
2-nm bands and not across the same wavelength region as<br />
the Brewer spectrophotometer. The <strong>UV</strong>-<strong>RSS</strong> in<br />
proximity to the <strong>UV</strong>-MFRSR at Table Mountain and in<br />
concert with RT models provides a means of<br />
parameterizing the DDR across the Brewer’s spectral<br />
range. The parameterized DDR with the collocated<br />
<strong>UV</strong>-MFRSR measurements at 368 nm can be used for the<br />
angular response corrections to the Brewer’s total solar<br />
irradiance.<br />
Figure 2 shows the spectral diffuse to direct ratio<br />
(DDR) relative to the DDR at 368 nm of the <strong>UV</strong>-<strong>RSS</strong> from<br />
300 – 380 nm on a clear-day, August 31, 2005 at 19 UTC.<br />
Also shown is the relative DDR for the <strong>UV</strong>-MFRSR in the<br />
seven channels. The DDR is fitted with a 3 rd degree<br />
polynomial with the residuals given above the main plot.<br />
The right hand side of the plot gives the total and diffuse<br />
solar irradiances from the spectrograph and filter<br />
instrument.<br />
Acknowledgments. The authors wish to thank Charles Wilson<br />
of the Central <strong>UV</strong> Calibration Facility for <strong>UV</strong>-MFRSR<br />
calibrations. This work was funded through a U.S.<br />
Environmental Protection Agency Star Grant #RD833224.<br />
References<br />
Bigelow, D.S., J. Slusser, A. Beaubein, J. Gibson, The USDA<br />
ultraviolet radiation monitoring program, Bull. Am. Meteorol.<br />
Soc., 79, 1-15, 1998.<br />
Kiedron, P., L. Harrison, J. Berndt, J. Michalsky, A. Beaubien,<br />
Specification and Performance of <strong>UV</strong> Rotating Shadowband<br />
Spectroradiometer, Proceedings of SPIE, Ultraviolet<br />
Space-based Measurements, Models, and Effects, vol. 4482,<br />
249-258, 2001.<br />
Krotkov, N. A., P. K. Bhartia, J. R. Herman, J. Slusser, G. Scott, G.<br />
Labow, A. P. Vasilkov, T. F. Eck, O. Dubovik, B. N. Holben,<br />
Aerosol ultraviolet absorption experiment (2002 to 2004), part<br />
2: absorption otpical thickness, refractive index, and single<br />
scattering albedo, Optical Engineering, 44(4), 2005.<br />
Harrison, L. J. Berndt, J. Michalsky, A. Beaubien, USDA<br />
Reference spectroradiometric network: Current performance<br />
and operational experience, Proceedings of SPIE, Ultraviolet<br />
Space-based Measurements, Models, and Effects, vol. 4482,<br />
23-37, 2001.<br />
Madronich, S., <strong>UV</strong> Radiation in the natural and perturbed<br />
atmosphere, Environmental Effects of <strong>UV</strong> Radiation, M. Tevini,<br />
Ed., Lewis Publisher, Boca Rotan, FL, 17-69, 1993<br />
Petters, J., V. Saxena, J. Slusser, B. Wenny, S. Madronich,<br />
Aerosol single scattering albedo retrieved from measurements<br />
ofsurface irradiance and a radiative transfer model, J. Geophys.<br />
Res., 108(D9), 4288, 2003.<br />
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Diffuse <strong>RSS</strong><br />
<strong>UV</strong>MFRSR<br />
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Total <strong>RSS</strong><br />
<strong>UV</strong>MFRSR<br />
DDR(<strong>RSS</strong>)<br />
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Figure 2. Relative Diffuse to Direct Ratio (DDR) of the <strong>UV</strong>-<strong>RSS</strong> and <strong>UV</strong>-MFRSR as a function of wavelength<br />
on August 31, 2005 (green curve and purple squares). Right hand side of plot shows total and diffuse solar<br />
irradiance of the <strong>UV</strong>-<strong>RSS</strong> (lines) and <strong>UV</strong>-MFRSR (squares).