BOREAS TF-08 NSA-OJP and SSA-OBS Ceilometer Data Summary The BOREAS TF-08 team used ceilometers to collect data on the fraction of the sky covered with clouds and the cloud height. Included with these data is the surface-based lifting condensation level, derived from temperature and humidity values acquired at the flux tower at the NSA-OJP site. Ceilometer data were collected at the NSA-OJP site in 1994 and at the NSA-OJP and SSA-OBS sites in 1996. 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-08 NSA-OJP and SSA-OBS Ceilometer Data 1.2 Data Set Introduction Ceilometers emit pulses of laser light and measure the time it takes for the photons to return after being scattered off of the cloud base. The ceilometer provides data on cloud base height and cloud cover. The cloud fraction is the time fraction of cloud cover based on ceilometer reports every minute (30 per half hour). The BOReal Ecosystem-Atmosphere Study (BOREAS) Tower Flux (TF)-08 team collected these data to obtain a seasonal record of cloud fraction and cloud type. Data were collected at the Northern Study Area (NSA)-Old Jack Pine (OJP) site in 1994 and at the NSA-OJP and Southern Study Area (SSA)-Old Black Spruce (OBS) sites in 1996. 1.3 Objective/Purpose The objective was to collect information on cloud characteristics in conjunction with tower flux measurements. 1.4 Summary of Parameters Measurements include fraction of cloud cover, the height of the cloud base, backscatter range, and the lifting condensation level. In 1996, cloud heights and backscatter ranges were reported for two cloud layers. 1.5 Discussion Ceilometers were located near the flux towers to measure the fraction of cloud cover and the height of the cloud base. In 1994 a single Belfort ceilometer operated at the NSA-OJP site. That ceilometer collected data from 31-May-1994 to 20-Sep-1994. In 1996, two Vaisala ceilometers were used, one at the NSA-OJP, the other at the SSA-OBS site. The NSA-OJP ceilometer operated from 3-Jun-1996 to 10-Nov-1996. The SSA-OBS ceilometer operated from 14-Jul-1996 to 17-Oct-1996. 1.6 Related Data Sets BOREAS TF-08 NSA-OJP Tower Flux, Meteorological, and Soil Temperature Data BOREAS TF-03 NSA-OBS Tower Flux, Meteorological, and Soil Temperature Data BOREAS AFM-05 Level-2 Upper-Air Network Standard Pressure Level Data BOREAS AFM-06 NOAA/ETL 35-GHz Cloud/Turbulence Radar GIF Images 2. Investigator(s) 2.1 Investigator(s) Name and Title David R. Fitzjarrald Research Associate Atmospheric Sciences Research Center Kathleen E. Moore Research Scientist Atmospheric Sciences Research Center 2.2 Title of Investigation Surface Exchange Observations in the Canadian Boreal Forest Region 2.3 Contact Information Contact 1 --------- Kathleen E. Moore Research Scientist Atmospheric Sciences Research Center Albany, NY (518) 437-8732 (518) 437-8758 (fax) moore@asrc.cestm.albany.edu Contact 2 --------- David R. Fitzjarrald Research Associate Atmospheric Sciences Research Center Albany, NY (518) 437-8735 (518) 437-8758 (fax) fitz@asrc.cestm.albany.edu Contact 3 --------- K. Fred Huemmrich University of Maryland NASA GSFC Greenbelt, MD (301) 286-4862 (301) 286-0239 (fax) Karl.Huemmrich@gsfc.nasa.gov 3. Theory of Measurements The operating principle of the laser ceilometer is based on measuring the time needed for a short pulse of light to traverse the distance through the atmosphere from the transmitter of the ceilometer to a backscattering cloud base and back to the ceilometer's receiver. From that time measurement, the height of the cloud base is calculated. The instantaneous magnitude of the return signal provides information on the backscatter properties of the atmosphere at a certain height. The cloud fraction is the time fraction of cloud cover based on ceilometer reports every minute (30 per half hour). 4. Equipment 4.1 Sensor/Instrument Description 4.1.1 Collection Environment Measurements were collected at the NSA-OJP site from late May through mid- September 1994 and early June through mid-November of 1996. At the SSA-OBS site, data were collected from mid-June through mid-October of 1996. Over the entire time period of data collection, temperature conditions from less than –15 °C to over 30 °C were experienced, as well as both rain and snow. 4.1.2 Source/Platform The ceilometers were placed on wooden platforms. The platforms were less than 0.5 m off the ground and within 25 m of the flux towers. 4.1.3 Source/Platform Mission Objectives The objective was to provide stable and level support for the ceilometer. 4.1.4 Key Variables The ceilometers measured fraction of cloud cover, the height of the cloud base, and backscatter range. In 1996, cloud heights and backscatter ranges were reported for two cloud layers. The cloud fraction was the time fraction of cloud cover based on ceilometer reports every minute (30 per half hour). The surface- based lifting condensation level was derived from temperature and humidity values acquired at 22.68 m above ground level on the NSA-OJP flux tower. 4.1.5 Principles of Operation The Belfort ceilometer used a 20-watt near-infrared Gallium-Arsenide laser operating at a wavelength of 0.91 microns. It employed 1,024 range gates, which yielded a vertical resolution of 7.62 m (25 feet) up to a maximum altitude of 7,802 m (25,600 feet). The fields of view of the transmitter and receiver were approximately 1°. The time interval between consecutive observations was set at 1 minute. The Vaisala CT12K ceilometer digitally sampled the return signal every 100 ns and had a vertical resolution of 15.24 m (50 feet) from ground level to an altitude of 3,901 m (12,500 feet). 4.1.6 Sensor/Instrument Measurement Geometry The ceilometers sat on a wooden platform within openings in the forest canopy allowing an unobstructed view of the sky. 4.1.7 Manufacturer of Sensor/Instrument Belfort Instrument Company 727 South Wolfe Street Baltimore, MD 21231 USA (410) 342-2626 (410) 342-7028 (fax) http://www.belfort-inst.com/ Vaisala Inc. U.S. Office 100 Commerce Way Woburn, MA 01801-1068 USA (781) 933-4500 4.2 Calibration 4.2.1 Specifications None given. 4.2.1.1 Tolerance The Belfort ceilometer had a vertical resolution of 7.62 m (25 feet), and the Vaisala CT12K ceilometer had a vertical resolution of 15.24 m (50 feet). 4.2.2 Frequency of Calibration Unknown. 4.2.3 Other Calibration Information None given. 5. Data Acquisition Methods For each ceiling observation, the Belfort ceilometer goes through a cycle of measurements. The laser is fired 5,120 times, and the reflected signal is sampled for each range gate. On each of the 5,120 scans, for each range gate, the signal is compared with the appropriate noise level for that gate. Depending on whether the received signal is above, below, or within the noise level band, an integer value of 1, -1, or zero is assigned to that gate. These assigned integers are then summed for each gate and thus represent a modified histogram of counts versus height. A peak-location algorithm is then applied to this product to produce a first-order estimate of the cloud ceiling. For the Vaisala CT12K ceilometer, the return signal strength is derived from the Lidar Equation. A threshold for the backscatter coefficient (a measure of reflection) is based upon assumed visibilities in cloud being in the range of 15 to 150 m. 6. Observations 6.1 Data Notes None. 6.2 Field Notes None. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage The ceilometers measured cloud conditions at a single point. In 1994, data were collected only at NSA-OJP; in 1996, data were collected at both NSA-OJP and SSA- OBS sites. The ceilometers were located within 25 m of the flux towers. The North American Datum of 1983 (NAD83) coordinates for the sites were: Latitude Longitude Elevation NSA-OJP tower 55.92842° N 98.62396° W 255.1 m SSA-OBS tower 53.98717° N 105.11779° W 628.9 m. 7.1.2 Spatial Coverage Map Not applicable. 7.1.3 Spatial Resolution The ceilometer laser was aimed vertically and the fields-of-view of the transmitter and receiver were approximately 1°. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics 7.2.1 Temporal Coverage In 1994, a single Belfort ceilometer operated at the NSA-OJP. That ceilometer collected data from 31-May-1994 to 20-Sep-1994. In 1996, two Vaisala ceilometers were used, one at the NSA-OJP, the other at the SSA-OBS. The NSA-OJP ceilometer operated from 03-Jun-1996 to 10-Nov-1996. The SSA-OBS ceilometer operated from 14-Jul-1996 to 17-Oct-1996. 7.2.2 Temporal Coverage Map Not applicable. 7.2.3 Temporal Resolution The time interval between consecutive observations was set at 1 minute for the Belfort ceilometer. The Vaisala CT12K ceilometer digitally sampled the return signal every 100 ns. The cloud fraction was the time fraction of cloud cover based on ceilometer reports every minute (30 per half hour). One-minute ceilometer data and 20-minute lifting condensation level data were interpolated to the half-hour averages provided in the data set. 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (tf08ceil.def). 7.4 Sample Data Record Sample data format shown in the companion data definition file (tf08ceil.def). 8. Data Organization 8.1 Data Granularity All of the NSA-OJP and SSA-OBS Ceilometer Data are contained in one data set. 8.2 Data Format The data file contains numerical and character fields of varying length separated by commas. The character fields are enclosed with single apostrophe marks. There are no spaces between the fields. Sample data records are shown in the companion data definition file (tf08ceil.def). 9. Data Manipulations 9.1 Formulae None. 9.1.1 Derivation Techniques and Algorithms None given. 9.2 Data Processing Sequence 9.2.1 Processing Steps 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 One-minute ceilometer data and 20-minute lifting condensation level data were interpolated to half-hour averages. 9.3.2 Calculated Variables The surface-based lifting condensation level was derived from temperature and humidity values acquired at 22.68 m above ground level on the NSA-OJP flux tower. The cloud fraction was the time fraction of cloud cover based on ceilometer reports every minute (30 per half hour). 9.4 Graphs and Plots None. 10. Errors 10.1 Sources of Error None given. 10.2 Quality Assessment None given. 10.2.1 Data Validation by Source None given. 10.2.2 Confidence Level/Accuracy Judgment None given. 10.2.3 Measurement Error for Parameters The Belfort ceilometer had a vertical resolution of 7.62 m (25 feet) and the Vaisala CT12K ceilometer had a vertical resolution of 15.24 m (50 feet). 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 were four standard deviations from the mean, long periods of constant values, and missing data. 11. Notes 11.1 Limitations of the Data None given. 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 The ceilometer data provide a continuous record of cloud conditions that can be linked with surface observations of heat and moisture fluxes measured from the flux towers. 13. Future Modifications and Plans None. 14. Software 14.1 Software Description None given. 14.2 Software Access None. 15. Data Access 15.1 Contact for Data Center/Data Access Information These BOREAS data are available from the Earth Observing System Data and Information System (EOS-DIS) 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 15.2 Procedures for Obtaining Data BOREAS data may be obtained through the ORNL DAAC World Wide Web site at http://www-eosdis.ornl.gov/ or users may place requests for data by telephone, electronic mail, or fax. 15.3 Output Products and Availability Requested data can be provided electronically on the ORNL DAAC's anonymous FTP site or on various media including, CD-ROMs, 8-MM tapes, or diskettes. The complete set of BOREAS data CD-ROMs, entitled "Collected Data of the Boreal Ecosystem-Atmosphere Study", edited by Newcomer, J., et al., NASA, 1999, are also available. 16. Output Products and Availability 16.1 Tape Products None. 16.2 Film Products None. 16.3 Other Products These data are available on the BOREAS CD-ROM series. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation None given. 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. and F. Hall. 1996. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1996-2.0, NASA BOREAS Report (EXPLAN 96). 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, and K.F. Huemmrich. 1997. Boreal Ecosystem-Atmosphere Study: 1996 Operations. NASA BOREAS Report (OPS DOC 96). 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. Sellers, P.J., F.G. Hall, R.D. Kelly, A. Black, D. Baldocchi, J. Berry, M. Ryan, K.J. Ranson, P.M. Crill, D.P. Lettenmaier, H. Margolis, J. Cihlar, J. Newcomer, D. Fitzjarrald, P.G. Jarvis, S.T. Gower, D. Halliwell, D. Williams, B. Goodison, D.E. Wickland, and F.E. Guertin. 1997. BOREAS in 1997: Experiment overview, scientific results, and future directions. Journal of Geophysical Research, 102(D24):28,731-28,770. 17.3 Archive/DBMS Usage Documentation None. 18. Glossary of Terms None. 19. List of Acronyms ASCII - American Standard Code for Information Interchange BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOREAS Information System CD-ROM - Compact Disk-Read-Only Memory DAAC - Distributed Active Archive Center EOS - Earth Observing System EOSDIS - EOS Data and Information System GMT - Greenwich Mean Time GSFC - Goddard Space Flight Center HTML - HyperText Markup Language IFC - Intensive Field Campaign NAD83 - North American Datum of 1983 NASA - National Aeronautics and Space Administration NOAA - National Oceanic and Atmospheric Administration NSA - Northern Study Area OBS - Old Black Spruce OJP - Old Jack Pine ORNL - Oak Ridge National Laboratory PANP - Prince Albert National Park SSA - Southern Study Area TF - Tower Flux TGB - Trace Gas Biogeochemistry URL - Uniform Resource Locator 20. Document Information 20.1 Document Revision Date Written: 06-May-1999 Revised: 25-May-1999 20.2 Document Review Date(s) BORIS Review: 07-May-1999 Science Review: 20.3 Document ID 20.4 Citation When using these data, please include the following acknowledgment: These data were provided by Drs. David R. Fitzjarrald and Kathleen E. Moore. If using data from the BOREAS CD-ROMs please also reference the data as: Dr. David R. Fitzjarrald and Dr. Kathleen E. Moore, "Surface Exchange Observations in the Canadian Boreal Forest Region." in Collected Data of The Boreal Ecosystem-Atmosphere Study. Eds. J. Newcomer, D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J. Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers. CD-ROM. NASA, 1999. To cite the BOREAS CD-ROM set as a published volume, use: J. Newcomer, D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J. Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers, eds. Collected Data of The Boreal Ecosystem-Atmosphere Study. CD-ROM. NASA, 1999. 20.5 Document Curator 20.6 Document URL Keywords: CEILOMETER CLOUD COVER CLOUD HEIGHT CLOUDS CONDENSATION LEVEL TF08_Ceilometer.doc 06/09/99