Scientific Sponsorship Statement: USDA Ultraviolet Radiation Monitoring and Research Program File Date: 25 January 2002 UV-MFRSRs The UV-MFRSR is a seven channel ultraviolet version of the Multi-Filter Shadowband Radiometer described by Harrison et al. (1994). This new shadowbanded instrument contains separate solid-state detectors which utilize Barr Associates, Inc. ion-assisted-deposition filters, each with a nominal 2 nm full-width at half-maximum (FWHM) bandwidth. The seven filters have nominal center wavelengths at each of 300, 305, 311, 317, 325, 332 and 368 nm. The actual center wavelengths of each filter as measured by the NOAA Central UV Calibration (CUCF) facility in Boulder, CO, is reported to the nearest 0.01 nm with each data record. Each detector shares a common diffuser, thereby allowing total horizontal (no blocking) and diffuse horizontal (direct beam blocked) irradiance to be measured simultaneously at each passband. Direct normal irradiance is derived in near-real time by firmware included within the data logging component of the instrument. All three measurements are returned for each 3-minute interval The Lambertian response of MFRSR instruments has been described by Harrison et al. (1994) and Michalsky et al. (1995). Precise angular corrections are applied to the direct-beam measurement as well as the diffuse (using the isotropic sky assumption) based upon an independent radiometric characterization of individual detectors made through the diffuser along two orthogonal planes (Michalsky et al. 1995). The relative spectral response function (SRF) of the detector is determined by focusing a 300 Watt Xenon arc lamp onto the entrance slit of an Instruments SA U1000 1.0 m double monochromator equipped with a 1200 grooves per mm grating. (Slusser et al., 2000) The monochromatic beam passes through the exit slits and strikes a beam-splitter which directs half of the light to a reference NIST-traceable photodiode and the other half to the detector (the diffuser/filter/photodiode combination). The irradiance determined by the reference photodiode is used to adjust the signal for the spectral characteristics of the lamp in order to yield the detector's relative spectral response at that wavelength. It is assumed that the differences between the spectral response of the two photodiodes is negligible over the narrow passband being measured. The measurement is repeated over the passband by scanning the wavelength of the monochromator in 0.1 nm steps in the UV and 1.0 nm steps in the visible. Calibration of the UV-MFRSR is established through the use of a standard lamp (Early et al., 1998). The SFR of a detector is measured as described in the previous paragraph. Next the voltage of the detector (diffuser/filter/ photodiode combination) is recorded with the detector exactly 50 cm from a 1000 Watt NIST-traceable FEL lamp spectral irradiance standard. The SRF is convolved with the known absolute lamp spectral irradiance to give the effective power incident on the detector. Dividing the measured voltage by the effective power produces the spectral responsivity for each individual channel of the detector in Volts/Watts per meter-squared per nanometer (YES, 1994). Langley analysis of each detector is also pursued as a means of calibrating instruments and is currently being applied routinely to check the radiometric stability of network radiometers (Slusser et al., 2000; Bigelow and Slusser, 2000). Only preliminary estimates of instrument stability, precision and bias are available for the instrument due to the short network measurement history. Bigelow et al. (1998) have reported precision of < 3% within a single passband based upon side-by-side comparisons of two prototype UV-MFRSR instruments. However, biases between the same two instruments from -10% to +3% depending upon the passband being investigated. Comparisons to a Brewer on a single clear day have yielded biases of 1% to 20%. Comparisons of 9 months of Langley calibrations to NIST standard lamp calibrations for two UV-MFRSRs at Mauna Loa Observatory, Hawaii yielded ratios from 1.00 to 1.07, again depending upon the specific passband being measured (Slusser et al., 2000). The following references document the techniques used by the USDA Ultraviolet Radiation Monitoring and Research Program to obtain calibrations and check the radiometric stability of network instrumentation. Bigelow, D.S., J.R. Slusser, A.F. Beaubien and J.H. Gibson, 1998, The USDA Ultraviolet Radiation Monitoring Program, Bull. Amer. Meteor. Soc. 79, 601-615. Bigelow, D.S. and J.R. Slusser, 2000, Establishing the Stability of Multi-filter UV Rotating Shadowband Radiometers, J. Geophys. Res. 105, 4833-4840. Early E., A. Thompson, and P. Disterhoft, 1998, Field Calibration Unit for Ultraviolet Spectroradiometers, Appl. Opt., 37, pp. 6664-6670. Slusser, J.R., J.H. Gibson, D.S. Bigelow, D. Kolinski, P. Disterhoft, K. Lantz and A. Beaubien, 2000, Langley Method of Calibrating UV Filter Radiometers, J.Geophys. Res. 105, 4841-4849. Yankee Environmental Systems, Inc., 1994. Yankee Environmental Systems Optical Calibration Facilities. YES Turners Falls, MA. p16 UV-MFRSR instrumentation is calibrated at least annually. The instruments are calibrated by the NOAA Central UV Calibration Facility (CUCF) at Boulder, Colorado. Some instruments have participated in the NOAA/NIST sponsored North American Interagency Spectroradiometer Intercomparisons. Routine annual calibrations include a cosine response characterization, spectral response function characterization, and calibration against a standard lamp. As an aid to data users who may wish to identify superbly clear and/or totally overcast days at various stations, USDA Ultraviolet Radiation Monitoring and Research Program field technicians visually inspect the data from a subset of days at each site and code them if one of the following criteria are satisfied: a. a "clear sky sunny" day is one which has a flawless bell-curve shape for all three components (total horizontal, diffuse horizontal, direct normal) with sunrise and sunset tails defined relative to the electronic noise level of specific channels and/or the instrument. Thus, it is a totally blue-sky day. b. a "totally overcast" day is one for which the direct normal component is zero for the entire day with zero defined relative to the electronic noise level of specific channels and/or the instrument. Thus, it is a total, heavy overcast day. When a given day has been coded as meeting one of the criteria, a comment is placed at the end of the corresponding data file submitted to the WOUDC. Additionally, these days are listed in files residing in the same directory as this document. Users should note that due to the random nature of the data inspections only a subset of days that would meet the criteria are identified. REFERENCES Harrison, L., J. Michalsky and J. Berndt, 1994. Automated multi-filter rotating shadowband radiometer: an instrument for optical depth and radiation measurements. Applied Optics, 32, 343-349. Long,C.S., A.J. Miller, H. Lee, J.D. Wild, R.C. Przywarty and D. Hufford, 1996. Ultraviolet Index Forecasts Issued by the National Weather Service. Bull, Amer. Meteor. Soc., 77,729-748. Michalsky, J.J., L.C. Harrison and W.E. Berkheiser, 1995. Cosine response characteristics of some radiometric and photometric sensors, Solar Energy, 54, 397-402.