BOREAS TF-06 SSA-YA Surface Energy Flux and Meteorological Data Summary The BOREAS TF-06 team collected surface energy flux and meteorology data at the SSA-YA site. The data characterize the energy flux and meteorological conditions at the site from 18-Jul to 20-Sep-1994. The data set does not contain any trace gas exchange measurements. The data are available in tabular ASCII files. Table of Contents * 1 Data Set Overview * 2 Investigator(s) * 3 Theory of Measurements * 4 Equipment * 5 Data Acquisition Methods * 6 Observations * 7 Data Description * 8 Data Organization * 9 Data Manipulations * 10 Errors * 11 Notes * 12 Application of the Data Set * 13 Future Modifications and Plans * 14 Software * 15 Data Access * 16 Output Products and Availability * 17 References * 18 Glossary of Terms * 19 List of Acronyms * 20 Document Information 1. Data Set Overview 1.1 Data Set Identification BOREAS TF-06 SSA-YA Surface Energy Flux and Meteorological Data 1.2 Data Set Introduction Meteorological and flux measurements were collected in 1994 by Centre National de Recherches Météorologiques (CNRM) personnel during Intensive Field Campaigns (IFCs) 2 and 3 of the BOReal Ecosystem-Atmosphere Study (BOREAS) at the Southern Study Area (SSA) Young Aspen (YA) site. Energy fluxes were reported as 30-minute averages, and the meteorology data were reported at 10-minute intervals. Carbon dioxide flux data were not collected. 1.3 Objective/Purpose The common aim of the BOREAS surface flux group was to use a network of tower- based observing systems to measure fluxes of heat, momentum, evaporation, and some trace gases over different vegetation types (aspen, jack pine, black spruce, and fen), vegetation age (old and young), and surface wetness. CNRM personnel on the BOREAS Tower Flux (TF) team TF-06 conducted such a study at the SSA-YA site. The YA site is of importance because it is representative of a regenerating forest that occurs widely in the boreal forest region. 1.4 Summary of Parameters Net radiation, total and solar upward and downward radiation, sensible heat flux, latent heat flux, Bowen ratio, soil heat flux, wind speed and direction, air pressure, rainfall, air temperature and humidity above and in canopy, soil temperature at two depths. 1.5 Discussion Flux data were acquired using fast response sensors. The data were collected and stored by a COMPAQ portable PC powered by batteries and solar panels. Meteorological measurements were acquired using a CEIS ESPACE Automated Weather Station (AWS). The sampling rate for all parameters is on the order of a few seconds. Reported variables are arithmetic means, with the exception of mean wind speed and direction, which are estimated using a vector mean and rainfall, which is the cumulative amount of water collected during 10-minute intervals. Flux data were reported every 30 minutes. Meteorological data were reported as 10-minute averages. The instruments operated continuously from 18-Jul to 20- Sep-1994. 1.6 Related Data Sets BOREAS TF-01 SSA-OA Tower Flux Data BOREAS TF-02 SSA-OA Tower Flux Data BOREAS TF-04 SSA-YJP Tower Flux Data 2. Investigator(s) 2.1 Investigator(s) Name and Title Dr. Pierre Bessemoulin METEO-FRANCE/CNRM Toulouse 2.2 Title of Investigation Study of the Boreal Forest Effects on Surface/Atmosphere Fluxes (TF-06) 2.3 Contact Information Contact 1 --------- Pierre Bessemoulin Meteo-France CNRM/Groupe De Meteorologie Experimentale et Instrumentale (GMEI) TOULOUSE FRANCE (33)61079364 (33)61079627 (fax) bessemoulin@meteo.fr Contact 2 --------- Dominique Puech Meteo-France CNRM/GMEI TOULOUSE FRANCE (33)61079364 (33)61079627 (fax) puech@meteo.fr Contact 3 ------------ Karl F. Huemmrich University of Maryland NASA Goddard Space Flight Center Greenbelt, MD (301) 286-4862 (301) 286-0239 (fax) Karl.Huemmrich@gsfc.nasa.gov 3. Theory of Measurements Eddy correlation is a well-known technique that has the advantage of being a direct method of measuring surface energy fluxes (compared to indirect methods such as the Bowen ratio method, or profile methods). The principle of eddy correlation is to measure fluctuations of the three wind components (u,v,w) with a 3-D sonic anemometer, along with air temperature and moisture. The air temperature may be determined from the sonic temperature (Ts). The sonic temperature is derived from a measure of the sound velocity (C). The air temperature may also be measured directly with a thermocouple. Covariances of the above parameters were computed in real time in the field. Fluxes were derived from covariances by applying different corrections: • Coordinate rotations to remove possible misalignment of the sonic anemometer structure with respect to the local mean wind streamlines. • w'T's corrected from the influence of latent and momentum fluxes to derive the sensible heat flux. • Webb and frequency response corrections (including the effect of limited response of the sensors, time constants, path length averaging, and sensor separation); • Contamination of humidity measurements when using a Krypton hygrometer by sensible heat flux. Fluxes were estimatedevery 30-minutes, using a sampling rate of 21 Hz. Fluctuations were computed by referencing the actual signals to running means (recursive filter where the time constant was 200 seconds). 4. Equipment 4.1 Sensor/Instrument Description 4.1.1 Collection Environment Measurements were taken continuously from 18-Jul to 20-Sep-1994. The tower extended above the canopy and was exposed to direct sunlight and weather. Because the site operated only through late summer, the temperature conditions were mild. 4.1.2 Source/Platform Two portable pole-type towers, stabilized with guy wires, were used. One tower supported the meteorological instruments; the other supported the sonic anemometer. A third structure at the site, in the form of an "A"-shaped folding ladder, supported the solar cells that powered the instruments. 4.1.3 Source/Platform Mission Objectives The purpose of the tower was to provide a stable platform extending above the canopy from which surface flux observations could be collected. 4.1.4 Key Variables Net radiation, total and solar upward and downward radiation, sensible heat flux, latent heat flux, Bowen ratio, soil heat flux, wind speed and direction, air pressure, rainfall, air temperature and humidity above and in canopy, soil temperature at two depths. 4.1.5 Principles of Operation Eddy correlation using a 3-D sonic anemometer. 4.1.6 Sensor/Instrument Measurement Geometry Instruments were attached to the tower at the following heights (negative values are underground): Sonic anemometer at 6.0 m (agl) Moisture sensor, Krypton hygrometer, at 6.0 m (agl) Response time (path length) were the following: Sonic anemometer: 0.05 s (14.9 cm) Hygrometer: 0.01 s (1.3 cm) The separation between the two sensors was 18 cm. Wind speed and direction at 10 m (agl) Temperature at 9.2 m (agl) Humidity at 9.2 m (agl) Upward/downward total radiation at 9.2 m (agl) Upward/downward solar radiation at 9.2 m (agl) Temperature at 2.0 m (agl) Humidity at 2.0 m (agl) Rainfall measured at 6 m (above canopy) Soil temperature at -1 cm Soil temperature at -5 cm Soil heat flux plate at -3 cm 4.1.7 Manufacturer of Sensor/Instrument Sonic anemometer - Gill Solent3D sonic anemometer Moisture sensor - Campbell Scientific KH20 Krypton hygrometer Campbell Scientific, Inc. 815 W. 1800 N. Logan, UT 84321-1784 (801) 753-2342 Wind speed and direction - R.M. Young wind monitor AQ type Manufacturer: R.M. Young Company Distributor: Campbell Scientific, Inc. 815 W. 1800 N. Logan, UT 84321-1784 (801) 753-2342 Humidity sensors - Vaisala HMP35A Manufacturer: Vaisala, Inc., Woburn, MA Distributor: Campbell Scientific, Inc. 815 W. 1800 N. Logan, UT 84321-1784 (801) 753-2342 Upward/downward total radiation - Schenk 8111 pyranometer Upward/downward solar radiation - Schenk 8101 pyranometer Rainfall - Weathermeasure tipping bucket Surface pressure - AIR DB2A Soil heat flux plate - Thornthwaite plate 4.2 Calibration 4.2.1 Specifications The Campbell Scientific KH20 hygrometer was connected to one analog input of the sonic anemometer. Data available on the serial port of the sonic anemometer, which was set to MODE 1 (calibrated UVW) were used. In this mode, the three vector speeds were corrected to allow for the effects of the framework and transducers. Accordingly, calibrations of the manufacturer were used for the sonic anemometer. Regular checks wre made in the field (comparison with wind speed measured by the nearby AWS did not reveal any discrepancy). For the KH20 hygrometer, calibration information provided by Campbell Scientific was used. Only mean moisture values could be checked, compared to humidity measurements conducted at the AWS. 4.2.1.1 Tolerance Not given. 4.2.2 Frequency of Calibration Not given. 4.2.3 Other Calibration Information Not given. 5. Data Acquisition Methods Flux data were acquired using fast response sensors. Data were collected and stored by a COMPAQ portable PC powered by batteries and solar panels. Meteorological measurements were acquired using a CEIS ESPACE AWS. The sampling rate for all parameters is on the order of a few seconds. Parameters appearing in the message delivered by the station are arithmetic means, except for mean wind speed and direction, which are estimated using a vector and rainfall, which is the cumulative amount of water fallen during 10-minute intervals. Flux data were reported every 30-minutes. Met data were 10-minute averages (see above). 6. Observations 6.1 Data Notes Fetch conditions were excellent for the flux measurements. 6.2 Field Notes Four days of data were lost (23-Aug - 26-Aug, out of IFC-3) because some big game destroyed the cable powering the AWS. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage All data were collected at the SSA-YA site. The site was located approximately 53° 39' N; 105° 20'W. The altitude was 550 m. The tower was located in a clearing 2 km long, 1 km wide, covered by dense young aspen (mean height: 2.5 to 3 m), surrounded by large trees (mixed aspen and jack pines, up to 20 m). Near the tower, the tree density was 10 trunks per square meter, with a mean trunk diameter of 3 cm. The biomass was estimated at 20 kg per square meter. 7.1.2 Spatial Coverage Map Not applicable. 7.1.3 Spatial Resolution The data represent point source measurements taken at the given location. The towers were placed in a clearing 2 km long, 1 km wide, covered by dense young aspen (mean height: 2.5 to 3 m). The area was surrounded by large trees (mixed aspen and jack pines, up to 20 m). Fetch conditions were excellent for the flux measurements. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics 7.2.1 Temporal Coverage Measurements are available from 18-Jul to 20-Sep-1994. Four days of data were lost (23-Aug - 26-Aug, out of IFC-3) because some big game destroyed the cable powering the AWS. 7.2.2 Temporal Coverage Map All data were collected at the SSA-YA site. 7.2.3 Temporal Resolution The Gill Solent3D sonic anemometer has a sampling rate of 21 Hz; these data are averaged to 30-minute values. Flux data were reported every 30 minutes. Meteorological data are 10-minute averages. 7.3 Data Characteristics The data from TF-06 are stored in two files, one for the tower flux and one for the meteorology data. Both files are described in the following sections. Data characteristics are defined in the companion data definition file (tf6fxmet.def). 7.4 Sample Data Record Sample data format shown in the companion data definition file (tf6fxmet.def). 8. Data Organization 8.1 Data Granularity All of the SSA-YA Surface Energy Flux and Meteorological Data are contained in one dataset. 8.2 Data Format The data files contain numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe marks. There are no spaces between the fields. Sample data records are shown in the companion data definition files (tf6fxmet.def). 9. Data Manipulations 9.1 Formulae 9.1.1 Derivation Techniques and Algorithms Formulas included statistical procedures for computing means, variances and covariances of signals sampled. Coordinate rotations were simple geometric transformations. 9.2 Data Processing Sequence 9.2.1 Processing Steps Means, variances, and covariances were computed in the field using fluctuations of the quantities computed as the difference between the actual measurement and a recursive mean value updated at each time step. The time constant used for the recursive filter was 200 seconds. Every one or two days, the memory of the acquisition computer was dumped in order to produce covariances corresponding to coordinates where the x axis is aligned with the mean wind speed (u=U; v=0), and the z axis is such that the mean vertical wind speed, w, is zero. BORIS Staff processed these data by: 1) Reviewing the initial data files and loading them online for BOREAS team access. 2) Designing relational data base tables to inventory and store the data. 3) Loading the data into the relational data base tables. 4) Working with the team to document the data set. 5) Extracting the data into logical files. 9.2.2 Processing Changes None. 9.3 Calculations 9.3.1 Special Corrections/Adjustments Further processing included corrections to covariances necessary to obtain fluxes: • Compensation for the spatial separation of sensors and limited frequency response of sensors. • Webb correction for the effect of temperature-induced density fluctuations for the latent heat flux. • Schotanus correction associated with the use of the sonic temperature: the sensible heat flux can be estimated from w'T's provided corrections involving the latent heat and momentum fluxes are applied. • Contamination of the latent heat flux by the sensible heat flux when using the Krypton hygrometer. 9.3.2 Calculated Variables The Bowen ratio is the ratio of the sensible heat flux to the latent heat flux. 9.4 Graphs and Plots None. 10. Errors 10.1 Sources of Error The main source of trouble comes from the Campbell hygrometer, which does not work correctly when the windows are wet. 10.2 Quality Assessment 10.2.1 Data Validation by Source Intercomparison of the flux computation algorithms were conducted. The physical constants used in the derivation of the fluxes are widely accepted ones. The algorithms used to calculate fluxes at this site produced exactly the same results as the algorithm agreed upon for the intercomparison analysis. A roving net radiometer was used at all BOREAS TF sites in order to evaluate the intercomparability of net radiation measurements conducted at the tower sites. 10.2.2 Confidence Level/Accuracy Judgment None given. 10.2.3 Measurement Error for Parameters None given. 10.2.4 Additional Quality Assessments None given. 10.2.5 Data Verification by Data Center Data were examined to check for spikes, values that are four standard deviations from the mean, long periods of constant values, and missing data. 11. Notes 11.1 Limitations of the Data The data set does not contain any trace gas exchange measurements. The measurement period covers only mid to late summer. 11.2 Known Problems with the Data None given. 11.3 Usage Guidance None given. 11.4 Other Relevant Information None given. 12. Application of the Data Set These data are useful for the study of water and energy exchange in a regenerating aspen stand. 13. Future Modifications and Plans None. 14. Software 14.1 Software Description None given. 14.2 Software Access None given. 15. Data Access 15.1 Contact Information Ms. Beth Nelson BOREAS Data Manager NASA GSFC Greenbelt, MD (301) 286-4005 (301) 286-0239 (fax) beth@ltpmail.gsfc.nasa.gov 15.2 Data Center Identification See Section 15.1. 15.3 Procedures for Obtaining Data Users may place requests by telephone, electronic mail, or fax. 15.4 Data Center Status/Plans These data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The BOREAS contact at ORNL is: ORNL DAAC User Services Oak Ridge National Laboratory (865) 241-3952 ornldaac@ornl.gov ornl@eos.nasa.gov 16. Output Products and Availability 16.1 Tape Products None. 16.2 Film Products None. 16.3 Other Products The data are available as tabular ASCII text files. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation None. 17.2 Journal Articles and Study Reports Sellers, P. and F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1994-3.0, NASA BOREAS Report (EXPLAN 94). Sellers, P., F. Hall, and K.F. Huemmrich. 1996. Boreal Ecosystem-Atmosphere Study: 1994 Operations. NASA BOREAS Report (OPS DOC 94). Sellers, P., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den Hartog, J. Cihlar, M.G. Ryan, B. Goodison, P. Crill, K.J. Ranson, D. Lettenmaier, and D.E. Wickland. 1995. The boreal ecosystem-atmosphere study (BOREAS): an overview and early results from the 1994 field year. Bulletin of the American Meteorological Society. 76(9):1549-1577. 17.3 Archive/DBMS Usage Documentation None. 18. Glossary of Terms None. 19. List of Acronyms ASCII - American Standard Code for Information Interchange AWS - Automated Weather Station BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOREAS Information System CNRM - Centre National de Recherches Météorologiques DAAC - Distributed Active Archive Center EOS - Earth Observing System EOSDIS - EOS Data and Information System GMT - Greenwich Mean Time GSFC - Goddard Space Flight Center GMEI - Groupe De Meteorologie Experimentale et Instrumentale IFC - Intensive Field Campaign NASA - National Aeronautics and Space Administration ORNL - Oak Ridge National Laboratory SSA - Southern Study Area URL - Uniform Resource Locator YA - Young Aspen 20. Document Information 20.1 Document Revision Dates Written: 04-MAR-1998 Revised: 08-MAY-1998 20.2 Document Review Dates BORIS Review: 09-MAR-1998 Science Review: 20.3 Document ID 20.4 Citation The observations were collected by P. Bessemoulin, D. Puech, G. Bouhours, G. Lachaud, E. Gizard, and J. Marcel. 20.5 Document Curator 20.6 Document URL Keywords ASPEN TOWER FLUX SENSIBLE HEAT FLUX LATENT HEAT FLUX NET RADIATION SOIL HEAT FLUX METEOROLOGY AIR PRESSURE AIR TEMPERATURE SOIL TEMPERATURE RELATIVE HUMIDITY WIND SPEED RAINFALL TOTAL RADIATION SOLAR RADIATION TF06_Flux_Met.doc Page 1 of 1 05/26/98