TITLE: CEOP CPPA ARM SGP Flux Data Set CONTACT: Raymond McCord Building 1507 PO Box 2008, MS 6407 Oak Ridge, TN 37831-6407 Telephone: (865) 574-7827 Fax: (865) 574-4665 E-Mail: email@example.com 1.0 DATA SET OVERVIEW This data set contains 30-minute resolution flux data from the Coordinated Energy and Water cycle Observation Project (CEOP) Global Energy and Water Cycle Experiment (GEWEX) Climate Prediction Program for the Americas (CPPA) Reference Site operated by the Atmospheric Radiation Measurement (ARM) Program at its Southern Great Plains (SGP) facility in Kansas and Oklahoma. This data set includes flux observations from 23 locations. This data set covers the time period 1 October 2002 through 31 December 2009. Further information about the ARM SGP site is available at the following URL: http://www.arm.gov 1.1 Station Locations Site State Elev (m) Latitude Longitude Surface Type -------------------------------------------------------------------------------- E1_Larned KS 632 38.202 N 99.316 W Wheat E2_Hillsboro KS 450 38.306 N 97.301 W Grass E3_Le_Roy KS 338 38.201 N 95.597 W Wheat and Soybeans (rotated) E4_Plevna KS 513 37.953 N 98.329 W Rangeland (ungrazed) E5_Halstead KS 440 38.114 N 97.513 W Wheat E6_Towanda KS 409 37.842 N 97.020 W Alfalfa E7_Elk_Falls KS 283 37.383 N 96.180 W Pasture E8_Coldwater KS 664 37.333 N 99.309 W Rangeland (grazed) E9_Ashton KS 386 37.133 N 97.266 W Pasture E10_Tyro KS 248 37.068 N 95.788 W Alfalfa E12_Pawhuska OK 331 36.841 N 96.427 W Native Prairie E13_Lamont OK 318 36.605 N 97.485 W Pasture and Wheat E14_Lamont OK 315 36.607 N 97.488 W Pasture and Wheat E15_Ringwood OK 418 36.431 N 98.284 W Pasture E16_Vici OK 602 36.061 N 99.134 W Wheat E18_Morris OK 217 35.687 N 95.856 W Pasture (ungrazed) E19_El_Reno OK 421 35.557 N 98.017 W Pasture (ungrazed) E20_Meeker OK 309 35.564 N 96.988 W Pasture E21_Okmulgee OK 240 35.615 N 96.065 W Forest E22_Cordell OK 465 35.354 N 98.977 W Rangeland (grazed) E24_Cyril OK 409 34.883 N 98.205 W Wheat (gypsum hill) E26_Cement OK 400 34.957 N 98.076 W Pasture E27_Earlsboro OK 300 35.269 N 96.740 W Pasture Note: E19 has a slightly different location from the start of collection through 6 May 2003 (35.549 N and 98.020 W). From 7 May 2003 to the end of collection the location is as above. 1.2 Time Period Covered by Data Site Start Date End Date ------------------------------------------------ E1_Larned 8 Mar 2004 14 Oct 2009 (data collection ended) E2_Hillsboro 1 Oct 2002 20 Oct 2009 (data collection ended) E3_Le_Roy 10 Mar 2004 24 Oct 2009 (data collection ended) E4_Plevna 1 Oct 2002 31 Dec 2009 E5_Halstead 1 Oct 2003 2 Nov 2009 (data collection ended) E6_Towanda 1 Oct 2003 31 Dec 2009 E7_Elk_Falls 1 Oct 2002 31 Dec 2009 E8_Coldwater 1 Oct 2002 10 Nov 2009 (data collection ended) E9_Ashton 1 Oct 2002 31 Dec 2009 E10_Tyro 3 Oct 2003 31 Dec 2009 E12_Pawhuska 1 Oct 2002 31 Dec 2009 E13_Lamont 1 Oct 2002 31 Dec 2009 E14_Lamont 1 Oct 2003 31 Dec 2009 E15_Ringwood 1 Oct 2002 31 Dec 2009 E16_Vici 1 Oct 2003 31 Dec 2009 E18_Morris 1 Oct 2002 17 Nov 2009 (data collection ended) E19_El_Reno 1 Oct 2002 31 Dec 2009 E20_Meeker 1 Oct 2002 31 Dec 2009 E21_Okmulgee 11 Feb 2004 31 Dec 2009 E22_Cordell 1 Oct 2002 1 Dec 2009 (data collection ended) E24_Cyril 18 Mar 2004 14 Nov 2009 (data collection ended) E26_Cement 1 Oct 2002 17 Dec 2009 (data collection ended) E27_Earlsboro 7 May 2003 4 Dec 2009 (data collection ended) 1.3 Temporal Resolution All data are 30-minute resolution. See the instrumentation section for further information. 2.0 INSTRUMENTATION DESCRIPTION The ARM flux measurements are obtained from two different instrumentation packages, the Eddy Correlation (ECOR) and Energy Balance Bowen Ratio (EBBR) systems. The following sites used the ECOR instrumentation: E1, E3, E5, E6, E10, E14, E16, E21 and E24 The ECOR instrumentation provides measurements of CO2 flux, latent heat flux and sensible heat flux. The soil heat flux is not measured at these stations. The ECOR sensors are located at 3.00 m height at all stations except E21 where the sensors are located at 21.00 m height. These stations started collecting data at varying times during EOP-4. The following sites used the EBBR instrumentation: E2, E4, E7, E8, E9, E12, E13, E15, E18, E19, E20, E22, E26 and E27 The EBBR instrumentation provides measurements of latent heat flux, sensible heat flux and soil heat flux. The CO2 flux is not measured at these stations. The soil heat flux is measured at -0.05 m height. The sensible and latent heat flux values are shown as at a missing (-999.99 m) height in the data file since the measurements used to derive these parameters are at multiple heights. 2.1 ARM EBBR Instrumentation Further information on the EBBR instrumentation is included in the ARM EBBR Handbook: http://www.joss.ucar.edu/data/ceop/docs/GAPP/GAPP_SGP_EBBR.pdf Highlights from that document are included here. 2.1.1 Expected uncertainties The expected uncertainty for the sensible, latent and soil heat fluxes is 10%. They define uncertainty as the range of probable maximum deviation of a measured value from the true value within a 95% confidence interval. 2.1.2 EBBR Components The accuracies cited below are generally those stated by the manufacturer. They are sensor absolute accuracies and do not include the effects of system (i.e., datalogger) accuracies. Although it is not known how some of the manufacturers have determined sensor accuracy, it is properly the root square sum of any nonlinearity, hysteresis, and nonrepeatablity, usually referenced as percentage of full scale. The detection limit is normally restricted to the range (sometimes called Calibrated Operating Range) over which the accuracy applies. In the case of the EBBR, some of the detection limits are those determined by the vendor (REBS) who performed the calibration, not by the manufacturer of the sensor. Some manufacturers also specify an Operating Temperature Range in which the sensor will physically and electronically function, even though the calibration may not be appropriate for use throughout that range. When no detection limits have been listed by the manufacturer or the calibrating vendor, none are stated below. Air temperature: Chromel-constantan thermocouple, Omega Engineering Inc., REBS Model # ATP-1, Detection Limits -30 to 40 deg C, Accuracy +/- 0.5 deg C. Temperature/Relative Humidity Probe: Operating Temperature Range -20 to 60 deg C. Temperature: Platinum Resistance Temperature Detector (PRTD); Detection Limits -30 to 40 deg C, Accuracy +/- 0.2 deg C Relative Humidity: Capacitive element, Vaisala Inc., Model #s HMP 35A and HMP 35D; Detection Limits 0% to 100% RH, Accuracy +/- 2% (0-90% RH), +/- 3% (90-100%), uncertainty of RH calibration +/- 1.2%. Soil Temperature: Platinum Resistance Temperature Detector, MINCO Products, Inc., REBS Model # STP-1, MINCO Model # XS11PA40T260X36(D), Detection Limits -30 to 40 deg C, Accuracy +/- 0.5 deg C. Soil Moisture: Soil Moisture Probe (fiberglass and stainless steel screen mesh sandwich), Soiltest, Inc., REBS Model # SMP-2, Soiltest Model # MC-300, Accuracy not specified by manufacturer (varies significantly depending on soil moisture and soil type). Detection limits for this sensor are limited by the ability to fit a polynomial to the calibration data; for the SGP CART Site the detection limits are approximately 1% to 50% by volume. Soil Heat Flow: Soil Heat Flow Probes, Radiation & Energy Balance Systems, Inc., Model #s HFT-3, HFT3.1, Accuracy not specified by manufacturer. Barometric Pressure: Barometric Pressure Sensor, Met One Instruments, Model #s 090C-24/30-1, Detection Limits 24 to 30 kPa; 090C-26/32-1, Detection Limits 26 to 32 kPa; 090D-26/32-1, Detection Limits 26 to 32 kPa; Accuracy for all +/- 0.14 kPa. Net Radiation: Net Radiometer, Radiation & Energy Balance Systems, Inc., Model Q*6.1 or Q*7.1, Accuracy +/- 5% of full-scale reading. Wind Direction: Wind Direction Sensor, Met One Instruments, Model #s 5470, 020C, Detection Limits 0 to 360 deg physical (for greater than 0.3 ms-1 wind speed), 0 to 356 deg electrical, Accuracy +/- 3 deg. Wind Speed: Wind Speed Sensor, Met One Instruments, model #s 010B and 010C, Operating Temperature Range -50 to 85 deg C, Detection Limits 0.27 to 50 ms-1, Accuracy +/- 1% of reading. Operational Limit on speed 60 ms-1. Datalogger: Campbell Scientific, Inc., Model CR10, Detection Limits vary by voltage range selected, Accuracy +/- 0.1% of full scale reading. 2.1.3 EBBR System Configuration The meteorological observations made with the EBBR system are: Air temperature at two heights (1 m separation) Relative humidity at two heights (1 m separation) Net radiation (at 2 m typical) Soil moisture at 2.5 cm depth Soil heat flow at 5 cm depth Soil temperature, integrated 0 to 5 cm Barometric pressure Wind direction at 2.5 m Wind speed at 2.5 m Reference temperature of control box The EBBR sensors (except for soil probes) are mounted on a triangular pipe framework that sits on the soil surface. The net radiometer mount extends from the south end of the EBBR frame. A unique aspect of the system is the automatic exchange mechanism (AEM), which helps to reduce errors from instrument offset drift. The AEM extends from the north end of the frame. Aspirated radiation shields (which house the air temperature and relative humidity probes) are attached to the AEM. The openings of the aspirated radiation shields face north to reduce radiation error from direct sunlight. The soil probes are buried just outside the view of and in an arc to the south of the net radiometer. Heights of individual sensors are listed in Primary Quantities Measured with System. Heights of air temperature and relative humidity sensors vary from site to site and are dependent on vegetation height; these heights can vary seasonally as vegetation height changes. The reference temperature sensor, barometric pressure sensor, datalogger, storage module, and communication equipment are located in the control box, which is attached at the northeast corner of the EBBR frame. The local area of influence upon Bowen ratio measurements is contained within a horizontal distance of approximately 20 times the height of the top aspirated radiation shield on the AEM. This distance varies among the different extended facilities and for different times of the year because of differences in vegetation height, and therefore the height at which the AEM is installed. The manufacturer's (REBS) name for these systems is SEBS (Surface Energy Balance System) ; this is the name that appears in their systems documentation. 2.2 ECOR Instrumentation Detailed information on the ECOR instrumentation is included in the ECOR Handbook developed by ARM. Selected highlights from that document are included here. 2.2.1 Uncertainties The expected uncertainties are as follows: Sensible heat flux - 6% Latent heat flux - 5% CO2 flux - 4% 2.2.2 ECOR Components Ultrasonic anemometer: WindMaster Pro by Gill Instruments Ltd Open-path CO2/H2O IRGA: LI-7500 by LI-COR, Inc. 2.2.3 ECOR System Configuration In a typical arrangement, the ECOR system is placed on the north side of a wheat field; sonic and IRGA sensor heads are mounted on a small tower at 3 m above ground level, at the end of a horizontal boom pointing south. The computer and communication devices are installed in an enclosure with basic temperature control (ventilation or heating). One exception to the usual arrangement is the Okmulgee site (E21), where the ECOR system is installed on a tall tower (15 m above ground, about 3 m above the canopy) in a hardwood forest. The IRGA provides fast-response measurements of water vapor density and CO2 concentration in digital and analog form; the sonic anemometer provides three wind components and the SOS data in digital form (retrieved via serial link) at a rate of 10 Hz and performs synchronous digitization of the IRGA analog outputs. The digital data stream from the IRGA is also recorded by the data acquisition computer; it is used to extract IRGA diagnostics values and as a second copy of the water vapor density and CO2 concentration data. The raw data stream is recorded into raw data files by 30-min portions and is processed every half hour by the ECOR computer. The raw and processed data files are transferred to the Central Facility for ingest (conversion into the netCDF format and incorporation of QC flags) and shipment to the ARM Archive. 3.0 DATA COLLECTION AND PROCESSING 3.1 ARM Data Collection and Processing Full information on the ARM data collection and processing is available in the EBBR Handbook and the ECOR Handbook. 3.2 UCAR/JOSS Data Processing The University Corporation for Atmospheric Research/Joint Offfice for Science Support (UCAR/JOSS) converted the data from the raw format provided by ARM into the CEOP EOP-3 data format agreed to by the CEOP Scientific Steering Committee. This format is described in detail as part of the CEOP Reference Site Data Set Procedures Report which is available at the following URL: http://www.joss.ucar.edu/ghp/ceopdm/refdata_report/ceop_flux_format.html As part of this conversion, JOSS multiplied the flux values from both the EBBR and ECOR systems by -1.0 so the sign conventions match the remainder of the CEOP flux data sets. Otherwise JOSS takes the 30 minute observations from the EBBR and ECOR systems without change. 4.0 QUALITY CONTROL PROCEDURES 4.1 ARM Quality Control Procedures For detailed information on EBBR quality control procedures please see the EBBR Handbook and the ECOR Handbook. 4.2 UCAR/JOSS Quality Control Procedures UCAR/JOSS converted the ARM QC flags into the CEOP QC flags in the following manner. If a parameter failed one of the ARM QC checks it was flagged as Questionable/Dubious ("D") and if it failed two or more ARM QC checks it was flagged as Bad ("B"). Additionally, ARM issues Data Quality Reports (DQRs) anytime problems are noticed within the data stream (e.g. failing instruments, calibration periods, etc). UCAR/JOSS has examined the DQRs issued by ARM over this time period (over 140 DQRs were issued) and determined when the parameters included within this data set may have been impacted and flagged the data either "D" or "B" based on the description of the problem included in the DQR. The ARM DQR's are provided as part of the data set: DQR_SGP_EBBR_200206_200303.html, DQR_SGP_EBBR_200304_200309.html, DQR_SGP_EBBR_200310_200412.html, DQR_SGP_ECOR_200310_200412.html, DQR_SGP_ECOR_200501_200504.html. Additionally, UCAR/JOSS conducted two primary quality assurrance/control procedures on the reference site data. First the data has been evaluated by a detailed QA algorithm that verifies the format is correct, examines any QC flags, and conducts basic checks on data values. Second, JOSS conducts a manual inspection of time series plots of each parameter. Additional data quality flags are applied during this stage. 5.0 GAP FILLING PROCEDURES No gap filling procedures were applied to these data by either ARM or UCAR/JOSS. 6.0 DATA REMARKS 6.1 EBBR Typical Problems to Watch For The EBBR system is capable of producing estimates of 30-minute average sensible and latent heat fluxes accurate to approximately +/- 10% of the estimated value or +/- 10 watts per meter squared, whichever is larger, at the 95% confidence level. Offset and calibration drift errors in the measurements of the temperature and relative humidity gradients are significantly reduced by the use of the AEM, which switches the positions of the upper and lower sensors every 15 minutes. There are a number of conditions under which the primary measurements may be incorrect. The data user should examine the data quality flags (described in Automated Quality Control (flagging contained within netCDF files)) and data quality reports (DQR) to determine whether significant malfunctions have occurred in the EBBR system. The more frequent sources of error are described briefly here. Generally, when the AEM is not functioning properly, the sensible and latent heat flux estimates are unreliable and should not be used. Examine the data quality flags for AEM home signals to ensure that only flux data produced when the AEM is functioning properly is used. Another source of error in the sensible and latent heat flux estimates lies in net radiation measurement errors. An error in the net radiation results in the same percentage error in the heat flux estimates. Condensation or frost sometimes forms on the upper net radiometer dome (called a "windshield" by the manufacturer) at night, especially when the relative humidity is high, wind speeds are very low, and the sky is clear (allowing rapid radiational cooling). This can persist until early afternoon, especially on the side of the net radiometer that is away from the sun. If the net radiometer dessicant is not in good condition, condensation can also occur inside the dome during the nighttime and can persist until early afternoon. Dessicant in poor condition can desorb water vapor into the dome area during the nighttime portion of the natural diurnal "breathing" process through the dessicant; some air passes through the dessicant and into the domes as the ambient temperature decreases with the approach of nightfall. Conversely, in the morning, the air in the domes warms and some moves through the dessicant in the opposite direction, thereby reducing the water vapor content of the air. This "breathing" process is intended to keep the air inside the domes dry. Condensation or frost inside or outside the domes can cause a decrease in net radiation to be measured during the day and an increase at night. During the day, the condensation or frost obstructs the passage of solar and infrared radiation through the top dome, thereby causing too low a value to be measured. At night, the condensation or frost prevents the cooling of the upper sensing surface of the net radiometer, thereby resulting in too large a net radiation value. The nighttime error in net radiation has, in extreme conditions, been measured to be as much as 30 watts per meter squared. Within a year of installation of the first EBBR systems it was obvious that the dessicant was degrading and had to be replaced more frequently than should have occurred. The cause was found to be a poor seal of the domes and the sensor head, a result of a poor sealing design. During the nighttime, undessicated ambient air would leak into the dome area around the seals. The manufacturer subsequently modified the design, resulting in fewer occurrences. Furthermore, a procedure to test the seal on the domes every 3 months and at each dome change was instituted. The polyethylene domes (particularly the upper one) become cloudy from exposure to sunlight, most rapidly in the first several months of use. After 6 months, the vendor states that degradation may result in as much as a -5% error in net radiation during the day, but with a negligible error at night. The domes are scheduled to be replaced every 3 months to minimize this error. At first, the spare domes obtained from the manufacturer appeared to vary significantly in thickness, a possible small source of error (within the accuracy stated by the manufacturer). The nature of the manufacturing process (forming polyethylene hemispheres from a sheet of material) lends itself to some variation in thickness of the final product. The manufacturer was subsequently asked by ARM to inspect future domes for abnormalities before they are sent to the SGP CART Site. The net radiation measurement is also affected by the amount of cooling produced by wind speeds of various magnitudes. The cooling effect is greatest on the top portion of the net radiometer (whose surface is warmest during the day because of solar radiational heating), primarily during daylight hours. After April 1996, a correction to net radiation, based on wind speed, was included in the CR10 software used in each EBBR calibration changeout kit. The changeouts were completed in November 1996. In 1997, specially designed ventilators are to be added to the net radiometers. The ventilators provide a non-wind-dependent airflow, the effect of which is easily included in the CR10 programming. This constant airflow will reduce the number of occurrences of condensation and frost on the net radiometer domes, thereby reducing the amount of incorrect net radiation data at night and in the morning. Furthermore, an error occurs when the Bowen ratio has a value near -1. Another source of error can be possible inaccuracies of the soil measurements (soil temperature change with time, soil moisture, soil heat flow at 5 cm) used to calculate estimated soil heat flux; aging of the sensors with time can result in changes in calibration or offset. When such changes become significant, the standard procedure is to replace the sensor and submit a DQR indicating the amount of error involved. Rarely is the error sufficiently large to declare the sensible and latent heat fluxes incorrect. The soil heat flux is normally a small fraction of the total energy exchange at the surface. Inaccuracies in the soil heat flux estimates normally cause less than a +/- 5% error in the sensible and latent heat fluxes. However, a total failure of a soil sensor (this is always reported in a DQR) often requires that the sensible and latent heat fluxes be recalculated using soil heat flux determined from the functioning sets of soil sensors only. The information needed to perform the recalculation is provided in the DQR. The alternative is to not use the sensible and latent heat flux estimates when a DQR has reported a soil sensor failure. One condition that may be hard for the data user to detect involves the failure of one of the air temperature thermocouples from which temperature gradient is determined. A failed thermocouple will typically result in offscale top and bottom air temperatures, thereby producing a zero temperature gradient and no sensible heat flux. The latent heat flux that is reported is then incorrect. It is possible for the sensible and latent heat fluxes to be recalculated using the relative humidity probe temperatures in place of the thermocouple temperatures; this method may add a small amount of error to the resulting fluxes, but should produce fluxes that are within 20% of those that would have been calculated using the thermocouples. Another condition that needs to be "looked out for" is rapid changes in net radiation. Rapid changes during the basic measurement averaging time of 12 minutes per quarter hour can result in non-linear changes that the 30 second sampling interval does not handle well; in other words, the average of one or all of net radiation, temperature gradient, and vapor pressure gradient do not reflect a correct average of the rapid changes; this is a result of undersampling. Normally, this condition is difficult to distinguish from normal variations, but on occasion it has been pronounced, even causing an unexpected sign of one heat flux (usually sensible) and a very large value for the other (usually latent). This condition most commonly occurs with saturated soil and partly cloudy conditions. 7.0 REFERENCE REQUIREMENTS To support the continuation of this program, please include the following 'credit line' in the acknowledgments of your publication: "Data were obtained from the Atmospheric Radiation Measurement (ARM) Program sponsored by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Environmental Sciences Division." 8.0 REFERENCES Further information on ARM SGP EBBR and ECOR instrumentation (including QC, calibration, maintenance, theory of operation, and references) can be found in the EBBR Handbook and the ECOR Handbook.