ARM/GCIP NESOB-96 Net Radiation and PAR Composite 1.0 General Description This 30 minute Net Radiation and Photosynthetically Active Radiation (PAR) Composite is one of several surface-layer data sets provided in the Atmospheric Radiation Measurement(ARM)/Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project (GCIP) Near Surface Observation Data Set - 1996 (NESOB-96). This Radiation Composite was formed from two data sources: net radiation derived from the Solar and Infrared Radiation Observing System (SIROS) 20 second data of Upwelling and Downwelling Longwave and Shortwave Irradiance; and 30 minute data from the GCIP National Oceanic and Atmospheric Administration (NOAA)/Atmospheric Turbulence and Diffusion Division (ATDD) Little Washita, Oklahoma long term flux monitoring site. This composite was developed by the merging of the computed 30-minute averaged values of Net Radiation as derived by University Corporation for Atmospheric Research/Joint Office for Science Support (UCAR/JOSS) from the 20-second values provided by ARM for its SIROS stations, and the 30-minute averaged values of Incoming/Outgoing PAR and Net Radiation as provided by NOAA/ATDD for its Little Washita station. UCAR/JOSS computed standard deviations for the averaged data when at least 15 observations were available within the 30-minute averaging interval. JOSS did not do any other quality control on the data set. This radiation composite contains data within the NESOB-96 domain (100.5W to 94.5W longitude and 34N to 39N latitude) and time period (01 April 1996 through 30 September 1996). 2.0 Detailed Data Description The NESOB-96 Radiation and PAR Composite is composed of data from two sources which report data at different frequencies. The ARM/CART SIROS provides measurements of upwelling and downwelling hemispherical solar irradiances, direct-beam solar irradiance, diffuse hemispherical solar irradiance from the sky and upwelling and downwelling hemispherical infrared irradiances via its collection of instruments including the Multifilter Rotating Shadowband Radiometer (MFRSR), pyranometer, pyrheliometer, and pyrgeometer at 19 of the ARM extended facilities (including the central facility). The NOAA/ATDD long-term flux monitoring site was established within the Little Washita Watershed, near Chickasha, Oklahoma. The tower is located 1/4 mile north of State Road 19 within a grazed pasture. Pasture surrounds the 3 meter tower in all sectors providing a minimum fetch of 200 meters over gently rolling terrain. Its collection of instruments include Radiation and Energy Balance Systems (REBS) Q*7 for net radiation, and the LI-COR LI-190 SB for PAR. 2.0.1 ARM Data Quality Reports A few problems occurred with instrumentation during NESOB-96 which affected the quality of the data from SIROS sites. Summarized below are the problems reported within the NESOB-96 time period, as well as the cause and possible resolution. This information was taken from Data Quality Reports provided by ARM (ARM, 1999). 2.0.1.1 SIROS 2.0.1.1.1 Site E4 MFRSR data missing from 05/08/1996 to 06/12/1996 The MFRSR head was damaged at site E4. MFRSR data was not collected from 960508 to about 960612. The head was replaced on 960612 about 1700 GMT and data collection again started. During the time the data was NOT collected, zeros will appear for all MFRSR data fields. 2.0.1.1.2 Site E16 calibration of pyranometers from 04/01/1996 to 09/30/1996 The pyranometers at this SIROS were installed before ARM/CART calibrations could be performed. Thus, calibration coefficients provided by vendor were used to process the data. Analyses by the National Renewable Energy Laboratory (NREL) and the National Oceanic and Atmospheric Administration, (NOAA) however, indicate that use of vendor calibration data tend to produce estimates of irradiances that are systematically 1-3% smaller, averaging about 1.5%, than if calibration data produced by NREL for ARM/CART were used. This discrepancy is currently being investigated. Hence, users of the pyranometer data from this site should consider increasing the measured irradiances by about 1.5%. This change is within the targeted +/-3% uncertainty estimated for pyranometer calibrations. 2.0.1.1.3 Site E13 downwelling solar irradiance measurement adjustments from 04/01/1996 to 09/30/96 A comparison of Broadband Solar Radiation Network (BSRN) and SIROS solar radiometers for measuring downwelling irradiances at the Southern Great Plains (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 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 (Michalsky, 1997), and further relevant analyses were conducted by Kato et al. (Kato, 1997). 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. For the time period addressed here, the modern tracking- shading assembly used with the SIROS sensors appeared to work well. 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. 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 DD Eppley June 1995 Oct. 13, 1995 SIROS PSP DD Eppley June 1995 July 25, 1995 SIROS NIP BORCAL Sept. 1994 July 25, 1995 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://www.res.sgp.arm.gov/sst/dq_monitor/DISPLAYS.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. 2.0.1.1.4 Site E10 Reprocess: Wrong calibration factor applied to NIP from 04/01/1996 to 09/30/96 From about 950721 to about 970204 (19:12 GMT) the wrong calibration factor was applied to the normal incidence pyrheliometer (NIP) for direct-beam solar at site E10. The calibration factor that was used was 1.00E-06 Volts/(W m^2). The correct calibration factor is 8.338E-06 Volts/(W m^2). Thus the calibrated NIP values are too large by a factor of 8.338. This affects the calculated net radiation value, which uses the short direct normal parameter. 2.0.2 ARM Algorithms The algorithms used by ARM to produce the radiation data are not currently available. 2.0.3 ATDD Algorithms The algorithms used by ATDD to produce the radiation data are not currently available. 2.0.4 UCAR/JOSS Algorithms The following equation was used to derive the net radiation for the ARM SIROS sites: Q = K + k - r - Lu + Ld where the parameters are: net radiation = Q short_direct_normal = K down_short_diffuse_hemisp = k up_short_hemisp = r up_long_hemisp = Lu down_long_diffuse_hemisp = Ld Standard deviation is reported for the net radiation parameter when a minimum of 15 observations are reported in the 30 minute time period. 2.1 Detailed Format Description The NESOB-96 Net Radiation and PAR Composite Dataset contains eight metadata parameters and nine data parameters and flags. The metadata parameters describe the date, time, network, station and location at which the data were collected. Data values are valid for the 30 minutes preceeding the time of observation. All times are UTC. Table 1 below details the data parameters. The data parameters have an associated QC flag but UCAR/JOSS does not Quality Control the data at the present time. The Quality Control flag is set to "U" for "Unchecked", unless the datum is missing, in which case the flag is set to "M". The table below details each parameter in the composite data set. Parameter Unit --------- ---- Date of Observation UTC Time of Observation UTC Network Identifier Abbreviation of platform name Station Identifier Network Dependent Latitude Decimal degrees, South is negative Longitude Decimal degrees, West is negative Station Occurrence Unitless Station Elevation Meters Net Radiation W/m2 QC flag U or M Standard Deviation W/m2 Incoming PAR (0.4-0.7 um) in uE/m2/s QC flag U or M Standard Deviation uE/m2/s Outgoing PAR (0.4-0.7 um) in uE/m2/s QC flag U or M Standard Deviation uE/m2/s 2.2 Data Remarks None. 3.0 Quality Control Processing No QC was performed on this dataset by the University Corporation for Atmospheric Research/Joint Office for Science Support (UCAR/JOSS). 4.0 References ARM, 1999: Data quality reports for SIROS. Kato, S., Ackerman, T. P., Clothiaux, E. E., Mather, J. H., Mace, G. G., Wesely, M. L., Murcray, F., and Michalsky, J., 1997: Uncertainties in modeled and measured clear-sky surface shortwave irradiances, J. Geophys. Res., Vol. 102, pp. 25881. Michalsky, J., Rubes, M., Stoffel, T., Wesely, M., Splitt, M., and DeLuisi, J., 1997: Optimal measurements of surface shortwave irradiance using current instrumentation--the ARM experience. Preprints, Ninth Conference on Atmospheric Radiation, Boston, MA, Amer. Meteor. Soc., J5-J9.