ARM/GCIP NESOB-97 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 - 1997 (NESOB-97). This Radiation Composite was formed from three 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; net radiation derived from the Solar Infrared Station (SIRS) 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 and SIRS 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-97 domain (100.5W to 94.5W longitude and 34N to 39N latitude) and time period (01 April 1997 through 31 March 1998). 2.0 Detailed Data Description The NESOB-97 Radiation and PAR Composite is composed of data from three sources which report data at different frequencies. The ARM/CART SIROS and SIRS 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-97 which affected the quality of the data from SIROS and SIRS sites. Summarized below are the problems reported within the NESOB-97 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 & SIRS 2.0.1.1.1 Site E4 MFRSR data missing from 05/08/1997 to 06/12/1997 The MFRSR head was damaged at site E4. MFRSR data was not collected from 970508 to about 970612. The head was replaced on 970612 about 1700 GMT and data collection again started. During the time the data was NOT collected, zeros will appear for all MFRSR data fields. Diffuse Pyranometer Thermal Offsets from 11/10/1997 to 02/14/2001 Broadband downwelling shortwave diffuse (sky) irradiance measurements available from SIRS during the period of this Data Quality Report (DQR), require adjustment for thermal offsets. These thermal, or 'zero' offsets refer to the generally reduced output signals from a shaded pyranometer due to the exchange of longwave (infrared) irradiance between the single black thermopile detector, the protective glass domes surrounding the detector, and the atmosphere. Originally considered an acceptable nighttime response of thermopile-type pyranometers, the generally negative bias is now recognized to significantly effect the accuracy of SIRS diffuse irradiance data during daylight periods. Studies of the Eppley Laboratory, Inc. Model PSP (Precision Spectral Pyranometer), used for the SIRS measurements of diffuse irradiance, suggest the thermal offset correction can range from near 0 to as much as 30 Watts per square meter, depending on the coincident net longwave, or infrared irradiance [1, 2]. Under very clear-sky conditions, the diffuse irradiance from a shaded PSP can be less than the minimum physical limit defined by radiative transfer model estimates based only on Rayleigh scattering effects. A correction method has been developed for adjusting SIRS diffuse irradiance data [3]. The resulting Value Added Product (VAP) will be applied to SIRS data for the period of this DQR. The VAP will not be applied to SIROS data collected before the instrument platform was converted to SIRS. Additionally, the Model PSP radiometer has been replaced by a Model 8-48 which uses a black and white thermopile detector known to reduce the thermal offset errors to less than 2 Watts per square meter [3]. The radiometer replacement at this SIRS location was completed on the ending date of this DQR. References: 1. Gulbrandsen, A., 1978: On the use of pyranometers in the study of spectral solar radiation and atmospheric aerosols. J. Appl. Meteorol., 17, 899-904. 2. Cess, R. D., X. Jing, T. Qian, and M. Sun, 1999: Validation strategies applied to the measurement of total, direct and diffuse shortwave radiation at the surface. J. Geophys. Res. 3. Dutton, E.G., J. Michalsky, T. Stoffel, B. Forgan, J. Hickey, D. Nelson, T. Alberta, and I. Reda, 2001: Measurement of Broadband Diffuse Solar Irradiance Using Current Commercial Instrumentation With a Correction for Thermal Offset Errors. J. Atmos. Oceanic Tech. Vol 18, No. 3, 297-314. (March 2001) 2.0.1.1.2 Site E16 data missing due to hw/communications problems from 04/03/1997 to 04/09/1997 Between 4/3 and 4/9 only 1-5% expected data volume where collected from the SIROS at E16 (Vici). Site maintenance found the system inoperable 4/9 and power-cycled. calibration of pyranometers from 04/01/1997 to 08/21/1997 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 04/01/1997 to 08/20/97 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 1997 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/apr97iop/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 1997. 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 SIROS site E2 solar tracker inoperative 04/12/1997 to 04/17/1997 On about 97/04/12 the solar tracker at site E2 stopped working. The solar tracker exhibits its broken state in a particular striking fashion on 97/04/14 - an almost cloud-free day. On this day, the NIP reads zero and the diffuse and direct SW radiation are about the same. The fact that these two data steams are not exactly the same indicates a calibration problem - probably with the diffuse instrument. This poor calibration is not typical of other ARM sites! On about 97/04/17 (1400 GMT) the solar tracker was fixed. (Note: we cannot pinpoint the exact time or date when the solar tracker went out and the date quoted above {97/04/12} should be regarded as approximate) Reprocess: wrong calibration 11/06/1997 to 11/26/1997 Mike Splitt found the following in the Site Ops OMIS data base from E2 on 11/26/97 : Found that the wrong calibration sticker was placed on the nip and the wrong Cal factor had been entered in the program. Also while editing the program I found that I had the wrong Cal factor for the DS. I inserted the correct Cal factors for the DS and Nip, complied and down loaded the new program into SIRS_02 at 1720GMT on 11/26/97 This entry was made by Craig Webb. The problem was reported in P971121.1 2.0.1.1.5 SIROS sites E1, E4, E6, E9, E11 NIP affected by precipitation 06/23/1997 to 06/24/1997 Comparison of the dowelling hemispheric solar broadband flux estimated from the unshaded PSP (down_short_hemisp) and that derived from the combination of the NIP (short_direct_normal) and shaded PSP (down_short_diffuse_hemisp) showed significant differences (greater than 70 W/m^2) at times on June 23, 1997. These differences appear to be due to questionable data from the NIP (short_direct_normal) that may have been caused by rainfall on that day. The NIP values recorded values as low as -14 W/m^2 in proximity to when the SMOS recorded precipitation. After the first precipitation event, the NIP appeared to be recording lower values during sunnier periods that may be due to collected precipitation in the NIP affecting the amount of solar radiation reaching the sensor. 2.0.1.1.6 SIRS sites E8, E11, E13, E15 SIRS NIPs affected by condensation 09/26/1997 to 09/26/1997 Comparison of data from the unshaded PSP to that derived from the "direct + diffuse" (NIP + shaded PSP) indicated low values of the "direct+diffuse" combination starting at sunrise and lasting for about an hour at E8, E11, E13 and E15. Further examination revealed that the NIPs were reading low and were likely affected by condensation during this brief period. Weather conditions were foggy over the area early that morning, and fog quickly dissipated after sunrise. 2.0.1.1.7 SIRS site E20 MFRSR/SIROS E20 - ingest problems 12/24/1997 to 01/23/1998 From about 971224 to about 980123 the MFRSR ingest and the SIROS ingest were both running at site E20. Needless to say, this caused some data collection mix ups. Sometimes only the MFRSR data were collected while at other times both the MFRSR and broadband data were collected. These data are probably OK, but one should beware of the potential for problems and gaps in the MFRSR and SIROS data streams. The start and end dates indicated above are approximate. 2.0.1.1.8 Site E10 Reprocess: Wrong calibration factor applied to NIP from 04/01/1997 to 09/30/97 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. 2.0.1.1.9 SIRS site E1 SGP/SIRS/E1 - Diffuse Pyranometer Thermal Offsets 11/20/1997 to 02/13/2001 Broadband downwelling shortwave diffuse (sky) irradiance measurements available from SIRS during the period of this Data Quality Report (DQR), require adjustment for thermal offsets. These thermal, or zero offsets refer to the generally reduced output signals from a shaded pyranometer due to the exchange of longwave (infrared) irradiance between the single black thermopile detector, the protective glass domes surrounding the detector, and the atmosphere. Originally considered an acceptable nighttime response of thermopile-type pyranometers, the generally negative bias is now recognized to significantly effect the accuracy of SIRS diffuse irradiance data during daylight periods. Studies of the Eppley Laboratory, Inc. Model PSP (Precision Spectral Pyranometer), used for the SIRS measurements of diffuse irradiance, suggest the thermal offset correction can range from near 0 to as much as 30 Watts per square meter, depending on the coincident net longwave, or infrared irradiance [1, 2]. Under very clear-sky conditions, the diffuse irradiance from a shaded PSP can be less than the minimum physical limit defined by radiative transfer model estimates based only on Rayleigh scattering effects. A correction method has been developed for adjusting SIRS diffuse irradiance data [3]. The resulting Value Added Product (VAP) will be applied to SIRS data for the period of this DQR. The VAP will not be applied to SIROS data collected before the instrument platform was converted to SIRS. Additionally, the Model PSP radiometer has been replaced by a Model 8-48 which uses a black and white thermopile detector know to reduce the thermal offset errors to less than 2 Watts per square meter [3]. The radiometer replacement at this SIRS location was completed on the ending date of this DQR. (References: 1. Gulbrandsen, A., 1978: On the use of pyranometers in the study of spectral solar radiation and atmospheric aerosols. J. Appl. Meteorol., 17, 899-904. 2. Cess, R. D., X. Jing, T. Qian, and M. Sun, 1999: Validation strategies applied to the measurement of total, direct and diffuse shortwave radiation at the surface. J. Geophys. Res. 3. Dutton, E.G., J. Michalsky, T. Stoffel, B. Forgan, J. Hickey, D. Nelson, T. Alberta, and I. Reda, 2001: Measurement of Broadband Diffuse Solar Irradiance Using Current Commercial Instrumentation With a Correction for Thermal Offset Errors. J. Atmos. Oceanic Tech. Vol 18, No. 3, 297-314. (March 2001)) 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 and SIRS 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-97 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 The SIROS stations were changed over to SIRS in August 1997. 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.