SGP99 DOE ARM SGP BSRN Radiation Data 1.0 General Description The Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Baseline Surface Radiation Network (BSRN) is one of various data sets provided for the Southern Great Plains 1999 (SGP99) project. This data set contains 1-min values of Direct-beam normal solar irradiance, Downwelling hemispheric diffuse solar irradiance, Downwelling hemispheric solar irradiance and Downwelling hemispheric infrared irradiance. The instrument is located at the Central Facility. This data set covers the period from 1 July through 31 July 1999. The SGP99 domain is approximately 97W to 99W longitude and 34.5N to 37N latitude. No additional quality control was performed by the University Corporation for Atmospheric Research/Joint Office for Science Support (JOSS). 2.0 Further Information The ARM program does not as yet have any documentation for this data set. Additional information on the BSRN can be obtained from the WMO on the WWW at: http://bsrn.ethz.ch/ 3.0 Instrumentation Meaning of column names VAR Number of radiation quantity MANUFACT Manufacturer of instrument MODEL Model of instrument SER_NR Serial number of instrument (by manufacturer) VALID_SINCE Instrument assigned to VAR since ... VALID_UNTIL Instrument assigned to VAR until ... REMARKS Remarks about instrument VAR MANUFACT MODEL SER_NR VALID_SINCE VALID_UNTIL REMARKS 2 EPPLEY PSP 30670F3 13-OCT-95 24-JUN-97 located ~200 yards from downpointing instruments 2 EPPLEY PSP 30615F3 24-JUN-97 17-SEP-97 located ~200 yards from downpointing instruments 3 EPPLEY NIP 29554E6 01-JAN-95 24-JUN-97 located ~200 yards from downpointing instruments 3 EPPLEY NIP 29011E6 24-JUN-97 17-SEP-97 located ~200 yards from downpointing instruments 4 EPPLEY PSP 30824F3 13-OCT-95 24-JUN-97 located ~200 yards from downpointing instruments 4 EPPLEY PSP 30617F3 24-JUN-97 17-SEP-97 located ~200 yards from downpointing instruments 5 EPPLEY PIR 28630F3 01-JAN-95 24-JUN-97 located ~200 yards from downpointing instruments 5 EPPLEY PIR 30687F3 24-JUN-97 in use located ~200 yards from downpointing instruments 131 EPPLEY PSP 29280F3 01-JAN-95 12-SEP-97 LOCATED ~200 YARDS FROM UPFACING INSTRUMENTS 132 EPPLEY PIR 29665F3 01-JAN-95 in use LOCATED ~200 YARDS FROM UPFACING INSTRUMENTS VAR 002 global radiation 2 (pyranometer) 003 direct radiation 004 diffuse sky radiation 005 longwave downward radiation 131 upward shortwave (reflected) from tower 132 upward longwave radiation from tower 4.0 Data Quality Reports from ARM 4.1 Downwelling solar irradiance measurement adjustments START DATE: 10/13/1995 START TIME: 0 END DATE: 08/20/1997 END TIME: 0 DATASTREAMS AFFECTED: BSRN A comparison of BSRN and SIROS solar radiometers for measuring downwelling irradiances at the SGP central facility was made with field standards and two absolute cavity radiometers brought to the site or a two-week period in April 1996 by Mike Rubes (formerly of the National and Oceanic Atmospheric Administration, Air Resources Laboratory, Surface Radiation Research Branch in Boulder, CO). A description of this effort can currently be found on the World Wide Web at http://www.srrb.noaa.gov/apr96iop/hagsie.html. Analyses of the data from these comparisons have resulted in several observations on the quality of data collected at the BSRN and SIROS platforms since October 13, 1995, which are probably valid to the present time, until these sensors are replaced with more recently calibrated sensors. On Oct. 13, 1995, the two BSRN pyranometers (PSPs) were replaced, so the observations do not apply to the BSRN measurements of global and diffuse irradiation before that date. Another source of information is inspection of the SIROS and BSRN equipment by Joe Michalsky (Atmospheric Science Research Center, State University of New York at Albany) at various times. The results of the findings are summarized as recommendations in the following several paragraphs. Some explanation and further comments are provided in the parenthetical remarks. ANALYSIS WHEN THE DIRECT BEAM WAS NOT OBSCURED BY CLOUDS Direct-beam solar irradiance measured with the BSRN pyrheliometer (NIP) are too large by approximately 0.5% compared to the two absolute cavity radiometers. (This small underestimate is within the expected level of uncertainty.) Direct-beam solar irradiance measured with the SIROS pyrheliometer are too small by approximately 2.1% compared to the two absolute cavity radiometers. (This large discrepancy is unexplained and will be explored during future calibration activities at the SGP Radiation Calibration Facility.) Possibly the best estimate of downwelling total hemispherical solar (global) irradiance can be made by summing the SIROS pyrheliometer irradiance reading multiplied by 1.021 (and by the cosine of the solar zenith angle) and the average of the readings for diffuse irradiance from the shaded BSRN and SIROS pyranometers. The direct-beam part can alternatively be computed as 0.995 times the BSRN pyrheliometer reading. For data collected in October before the 13th, when the BSRN shaded pyrheliometer was replaced, the diffuse component is probably best computed directly from the SIROS shaded sensor alone. Downwelling total hemispherical solar (global) irradiance measured by the BSRN unshaded pyranometer is approximately 2% too small (which is within the expected level of uncertainty for unshaded pyranometer measurements) compared to the values computed from the measured direct-beam and diffuse components. (Downwelling total hemispherical solar irradiances measured by the SIROS unshaded pyranometer systematically underestimates the global irradiances by excessive amounts, i.e., by greater than 3%.) The analyses leading to these recommendations are described in an extended abstract presented in early February (J. Michalsky et al., "Optimal Measurements of Surface Shortwave Irradiance Using Current Instrumentation--The ARM Experience," in Preprint Volume, Ninth Conference on Atmospheric Radiation, Feb. 2-7, Long Beach, California, pp. J5-J9, American Meteorological Society, Boston, MA). Further relevant analyses were conducted by Kato et al., (Seiji Kato, Pennsylvania State University) and are described in a manuscript submitted for publication ("Uncertainties in Modelled and Measured Clear Sky Surface Shortwave Irradiances")." UNSHADED PYRANOMETER PERFORMANCE WHEN THE DIRECT BEAM WAS NOT OBSCURED The above recommendations are based mostly on analyses conducted for cloudless, midday conditions. Because the data reported from the unshaded pyranometer were not corrected for cosine response, slight overestimates of global irradiance from unshaded pyranometers tend to occur in cloudless conditions at solar zenith angles less than 45 deg and slight underestimates tend to occur for zenith angles greater than 55 deg. The maximum deviations occur at extreme solar zenith angles and are about 2%. TRACKER-SHADING PERFORMANCE The data user should note, as has been noted in data release statements, that analyses of the direct, diffuse, and/or direct beam irradiances should be preceded by a check of sensor performances by summing the direct and diffuse components and comparing the result to the directly measured global component. When this is done, problems with solar tracking are usually apparent. Because slight misalignments in the tracking and shading devices can be difficult to detect, small deviations of the component sum from expected behavior are sometimes difficult to explain. If such deviations tend to recur for specific time intervals for several days, one might suspect a tracking or shading problem. For the time period addressed here, the modern tracking-shading assembly used with the SIROS sensors appeared to work well. For the BSRN sensors until January 1996, an older tracking-shading system was used that was not as reliable as the modern assembly used with the SIROS sensors; problems with this BSRN tracking and shading system, were usually evident when they occurred. A modern tracker-shader was installed for the BSRN sensors in January 1996. The tracker was not aligned as well as it could be. Efforts are underway to improve tracker alignment checks and procedures at all SIROS sites and the BSRN site. PARTLY CLOUD CONDITIONS An analysis by Chuck Long (formerly at the Pennsylvania State University and now with the University of Colorado and the National Oceanic and Atmospheric Administration) indicated that data users who are investigating partly cloudy sky conditions will usually find that the BSRN outputs are more reliable for short periods of time, say less than 30 min, than are the SIROS outputs. This tends to occur because the SIROS data are recorded only every 20 s while the BSRN data represent one-minute averages computed on the basis of sampling once per second. Under partly cloudy conditions, sampling only once every 20 s tends to provide inadequate statistical representation of downwelling irradiances. ESTIMATES FOR CLOUDY CONDITIONS The component sum technique is not applicable for overcast conditions. For the time period addressed here, the SIROS shaded sensor appears most reliable before October 13, 1995. Thereafter, an average of data from the SIROS shaded pyranometer, the shaded BSRN sensor, and the shaded BSRN sensor multiplied by 1.02 might be the best estimate of global irradiance for cloudy conditions. However, a rigorous analysis on the results of this procedure has not been carried out, so the data user should approach this technique with caution. SOME ADDITIONAL INFORMATION The excessively large deviations noted above for the pyranometers result in part from a mixture of different sources of calibration procedures. The following table lists the sources of calibration: Sensor Coefficient used Calibration Installation to process data date date BSRN PSP DS BORCAL Sept 1995 Oct. 13, 1995 BSRN PSP DD Eppley June 1995 Oct. 13, 1995 BSRN NIP BORCAL July 1993 March 17, 1994 SIROS PSP DS Eppley June 1995 July 25, 1995 SIROS PSP DD Eppley June 1995 July 25, 1995 SIROS NIP BORCAL Sept 1994 July 25, 1995 DS = downwelling solar or global DD = downwelling diffuse PSP = precision spectral pyranometer NIP = normal incidence pyrheliometer for direct-beam solar BORCAL = broadband outdoor radiometer calibration, conducted by the National Renewable Energy Laboratory (NREL) Eppley = denotes calibrations in an integrating sphere by the manufacturer, Eppley Laboratory, Inc. The BORCAL calibrations result in estimates of solar irradiances that are typically 1.5% larger than Eppley calibrations, a situation which is under investigation by Tom Stoffel at NREL and John Hickey at Eppley. They are working together to document this difference. This difference helps to explain the larger estimates of global irradiance measurement with the BSRN sensor than with the SIROS sensor. A greater source of concern than over differences between the NREL versus the Eppley calibrations at this time is the insufficiently frequent recalibrations of sensors in operation at the SGP site. Although the NIPs are expected to hold their calibrations for rather long periods of time, the pyranometers typically should be recalibrated at least once every 12 months. Change out with freshly calibrated pyranometers and pyrheliometers at the SGP site will begin in 1997, with the goal of routinely replacing every pyranometer and pyrheliometer with freshly calibrated sensors once every year. Data users can inspect metrics provided on the World Wide Web by the SGP site scientist team on data quality at the following address: http://manatee.gcn.uoknor.edu/metrics/METRICS.html Other observations/measurements impacted by this problem: Any derived estimates of downwelling solar radiation components using data from central facility SIROS or BSRN sensors (for downwelling solar radiation) for the time period indicated. Suggested Corrections of the Problem: (e.g. change calibration factor and recompute, flag data with this comment, etc.) Use of these recommendations by data users. Ideally, the component sum technique would be applied in a value-added product (VAP) implemented at the Experiment Center, but this has not been done yet. In the meantime, users of recent data can inspect plots of component sum technique on the World Wide Web site noted above. 5.0 Contacts 5.1 ARM Instrument Mentor Tom Stoffel Email: TomStoffel@nrel.gov