BOREAS HYD-02 Estimated Snow Water Equivalent (SWE) from Microwave Measurements Summary The surface meteorological data collected at the BOREAS tower and ancillary sites are being used as inputs to an energy balance model to monitor the amount of snow storage in the boreal forest region. The BOREAS HYD-02 team used snow water equivalent (SWE) derived from an energy balance model and in situ observed SWE to compare the SWE inferred from airborne and spaceborne microwave data, and to assess the accuracy of microwave retrieval algorithms. The major external measurements that are needed are snowpack temperature profiles, and in situ snow areal extent and snow water equivalent data. The data in this data set were collected during February 1994 and cover portions of the SSA, NSA, and the transect areas. The data are available as comma delimited 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 HYD-02 Estimated Snow Water Equivalent (SWE) from Microwave Measurements 1.2 Data Set Introduction The Estimated Snow Water Equivalent data set contains SWE as obtained via airborne measurements. The time period of the experiment was 2 February to 18 February 1994 during the BOReal Ecosystem-Atmosphere Study (BOREAS) winter focused field campaign (FFC-W). The instrumentation used was a series of microwave radiometers (specifically the 18, 37 and 92 GHz channels) that were mounted on a Twin Otter aircraft. The data set also contains other relevant data regarding the conditions under which each measurement was taken. For example, the temperature and dew point at the time of the measurement as well as the pitch and roll of the aircraft are also included. 1.3 Objective/Purpose The objective of this investigation was to quantify the storage of water in snowpacks beneath the forest canopy. Ground water measurements were used as validation for the airborne and spaceborne snow water equivalent algorithm. This data set was created based on airborne microwave measurements to help address the question of the extent to which differences in surface cover affect snow storage. 1.4 Summary of Parameters The specific parameters under observation for this experiment are the snowpack temperature profiles, the snow areal extent and the snow water equivalent measured. 1.5 Discussion During the 1994 Winter Field Campaign, 14 Twin Otter flights were made for the BOREAS project. Detailed flight plans and flight lines were reported by Ian MacPherson of the Flight Research Laboratory, Canadian Establishment of Aeronautic Research, Ottawa, Canada. Three microwave radiometers (18, 37, and 92 GHz) were mounted on-board the aircraft, in addition to video cameras and a PRT-5 thermal sensor. 1.6 Related Data Sets Nimbus-7 SMMR derived global snow depth maps (available through the National Snow and Ice Data Center (NSIDC), http://www-nsidc.colorado.edu/NASA/GUIDE) HYD-03 Snow Measurements HYD-04 Standard Snow Course Data HYD-04 Special Snow Course Data 2. Investigators Principal Investigator: Dr. Alfred T. C. Chang NASA Goddard Space Flight Center Co-Investigators: Dr. Dorothy K. Hall NASA Goddard Space Flight Center Dr. James L. Foster NASA Goddard Space Flight Center 2.2 Title of Investigation Validation of a passive microwave snow water equivalent algorithm using an energy balance model 2.3 Contacts Contact 1: -------------- Mr. Hugh Powell NASA Goddard Space Flight Center Greenbelt, MD (301) 286-2310 Hugh.Powell.1@gsfc.nasa.gov Contact 2: -------------- Dr. Alfred T. C. Chang NASA Goddard Space Flight Center Greenbelt, MD (301) 286-8997 Alfred.T.Chang.1@gsfc.nasa.gov Contact 3 -------------- David Knapp Raytheon STX Corporation NASA GSFC Greenbelt, MD Phone: (301) 286-1424 FAX: (301) 286-0239 email: David.Knapp@gsfc.nasa.gov 3. Theory of Measurements Microwave signatures have been used to infer snow water equivalent values over Canadian Prairie with some success (Goodison and Walker, 1993). Microwave radiation emanates from features on or near the surface of Earth at an intensity that is proportional to the product of the physical temperature and the emissivity of the surface. The measured value, referred to as the brightness temperature (TB) can simply be expressed as: TB = ( R * Tsky + ( 1 - R ) * Tsurf) e-t + Tatm (1) where e-t is the atmospheric transmissivity, R is the surface reflectivity, Tsky is the sky radiation, Tsurf is the surface emission, and Tatm is the emission from the intervening atmosphere. In the microwave region both Tsky and Tatm are small and can be neglected. Thus, the observed TB is directly related to surface features. Based on radiative transfer calculations (Chang et al., 1987), a relationship between brightness temperature and the number of snow crystals was developed for SWE retrieval. The differences between the 18 and 37 GHz horizontal polarization brightness temperature is linearly related to the SWE values when SWE is less than 200 mm. The scattering information comes largely from the 37 GHz signal. The 18 GHz signal serves as the background reference. The SWE - brightness temperature relationship of a homogeneous snow layer with crystals having a mean radius of 0.3 mm and density of 300 kg/m3 for SMMR data can be expressed as follows (Chang et al., 1987); SWE = 4.8 x ( T18H - T37H ) (2) where SWE is the snow water equivalent in mm of equivalent water, T18H and T37H are the brightness temperatures for the 18 and 37 GHz horizontal polarizations, respectively. Both vertical and horizontal polarization will give generally similar results in Eq (2). Due to differences in the surface snow characteristics, researchers have used either vertical or horizontal polarization (Hallikainen and Jolma, 1992; Goodison and Walker, 1994) in retrieving the SWE. Rott and Aschbacher (1989) proposed a more generalized relationship of snow water equivalent and brightness temperature: SWE = A + B * DTB (3) where A and B are the offset and slope for brightness temperature difference and DTB is the brightness temperature difference between a high scattering channel (37 or 85 GHz) and a low scattering channel (18 or 19 GHz) vertical or horizontal polarization channels. Based on ground measurements of SWE in forests, A and B were determined for the airborne sensor in the boreal region. For this experiment, A and B are 0.0 and 1.7 respectively, when using the 18 and 37 GHz vertical polarization data. The brightness temperature difference for forest covered areas will cancel out if the emissivities of forest for both the high scattering and the low scattering channels are approximately the same. This is based on the findings that the emissivities for forest in Finland at 37 and 18 GHz are very similar and have the values of 0.9 to 0.92 (Hallikainen et al., 1988). Thus, only the snow covered fraction contributes to the brightness temperature difference. For a footprint with a fraction of forest cover (f) and fraction snow cover (1 - f), Eq (3) will become SWE = 1.7 * DTB /(1 - f) (4) Over the forested pixels, Eq (3) would underestimate the SWE if not corrected for the forest cover. The amount of underestimation depends on the fraction of forest cover in Eq (4). Due to the low sun angles for early February in the BOREAS test sites, accurate forest cover determination is difficult to obtain from the video. Therefore, fractional forest cover corrections were not included in this data set. Users could have a better estimate of the fractional forest cover in the sites in which they are interested and apply Eq(4) to correct for the forest cover. 4. Equipment 4.1 Sensor/Instrument Description Three dual polarization microwave radiometers at 18, 37, and 92 GHz were mounted onto a Canadian Twin Otter aircraft. Thermal radiation in the microwave region was measured using Dicke-type radiometers with two reference sources in 18, 37, and 92 GHz. The microwave radiation was received by square wave detectors. A PRT-5 infrared (IR) radiometer was also mounted on the aircraft. 4.1.1 Collection Environment The data were collected during the BOREAS experiment focused field campaign- winter (FFC-W), which occurred from 2 February to 18 February 1994. The area over which the data were collected was both the BOREAS Northern Study Area (NSA) and the BOREAS Southern Study Area (SSA). There were 14 flyovers for this particular project. 4.1.2 Source/Platform Radiometers were mounted on the right side of the Twin Otter aircraft with 45 degree look-angle. 4.1.3 Source/Platform Mission Objective The mission of the Twin Otter was to serve as a platform for the brightness temperature measurements. 4.1.4 Key Variables Brightness temperatures, IR temperature, and aircraft locations. 4.1.5 Principles of Operation Dicke-type radiometers with two reference sources were used to measure brightness temperature. Microwave radiation was received by square wave detectors. 4.1.6 Instrument Measurement Geometry The radiometers were set-up such that a 45 degree angle looking out of the aircraft to the right was achieved. The pitch and roll of the aircraft were also recorded. 4.1.7 Manufacturer of Instrument The radiometers were assembled at Goddard Space Flight Center (GSFC) using commercial parts. An Intel 486 IBM-compatible personal computer (PC) was used as the data logger. 4.2 Calibration During normal data taking cycle, warm and cold calibration readings were taken each minute. Pre- and post-mission calibrations were taken at GSFC to better characterize the brightness temperature calibrations. 4.2.1 Specifications Radiometers were calibrated with clear sky, liquid nitrogen, and warm ecosorb targets. 4.2.1.1 Tolerance Accuracy of the radiometers is about 2 Kelvin (K) in nominal temperature range. 4.2.2 Frequency of Calibration During a flight, calibration was done for six seconds out of every minute of data recording. External calibration was done twice during the mission. 4.2.3 Other Calibration Information Losses for each component were measured in the laboratory in 1992. 5. Data Acquisition Methods Microwave brightness temperatures were taken by aircraft from takeoff to landing, nominally lasting about two hours. Data were collected in one minute blocks, which included six seconds of calibration and 54 seconds of data from target. Data were recorded on the hard disk of a PC. These data are copied to other computers for further processing. 6. Observations 6.1 Data Notes At the beginning of February 1994, the temperatures were very cold (about -40 degrees Celsius), the snowpack should have been dry. The temperature warmed up slowly during these two weeks of experimentation. In the Flight 14, the air temperatures were close to 0 degrees Celsius, thus surface melting is possible. Data were taken over the NSA and SSA sites during the winter FFC. 6.2 Field Notes None. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage Data were taken over the BOREAS Northern Study Area (NSA), Southern Study Area (SSA), and transect areas. NSA Corner Longitude Latitude Northwest 98.82W 56.247N Northeast 97.24W 56.081N Southeast 97.49W 55.377N Southwest 99.05W 55.540N SSA Corner Longitude Latitude Northwest 106.23W 54.319N Northeast 104.24W 54.223N Southeast 104.37W 53.419N Southwest 106.30W 53.513N 7.1.2 Spatial Coverage Map Not available. 7.1.3 Spatial Resolution These data were taken from an aircraft altitude of 2500 feet which resulted in a spatial resolution of approximately 350 feet at the 45 degree viewing angle. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics Most of the fourteen flight lines were covered once during the mission. 7.2.1 Temporal Coverage The data were collected from 06-Feb-1994 to 13-Feb-1994. 7.2.3 Temporal Resolution In each minute, data values were collected once per second for 53 seconds; the remaining time was used for instrument calibration. 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (h02swed.def). 7.4 Sample Record Sample data format shown in the companion data definition file (h02swed.def). 8. Data Organization 8.1 Data Granularity The Estimated Snow Water Equivalent (SWE) from Microwave Measurements data are contained in 14 datasets. There is one data file for each filght. 8.2 Data Format The snow water equivalent data set contains 28 columns of data with a line describing the content of the columns beneath it. The data columns are comma delimited. Sample data records are shown in the companion data definition file (h02swed.def). 9. Data Manipulation 9.1 Formula Measured radiometric units have been converted to brightness temperature using the following equations: TB = TH-(dc-hc)/(cc-hc)*(TH-TC) where: dc = data counts cc = cold counts hc = hot counts TC = cold load temperature TH = hot load temperature 9.1.1 Derivation Technique/Algorithm Radiometer calibration was done by pointing the antenna to (1) cold sky and (2) liquid nitrogen bucket as the cold references, ecosorb at ambient temperature is used as the warm reference target. 9.2 Data Processing Sequence The data collected during the flights are processed using the following steps: 1. The raw data counts from each radiometer are read from the collected data file. 2. These data are then converted to antenna temperature using the calibration equations derived. 3. Apply loss corrections to the antenna temperature to create brightness temperature. 4. Derive SWE from equations. 5. Merged with aircraft navigation data and PRT-5 IR data. 6. Output saved on 8 mm tape cartridge. 7. BORIS Staff read the files from tape, added commas to delimit the different columns, and wrote the data back to tape. 9.3 Calculations 9.3.1 Special Corrections/Adjustments None given. 9.3.2 Calculated Variables Please refer to equation (3) in Section 3.0 of this document for the equation used to infer the snow water equivalent values. 9.4 Graphs and Plots Brightness temperatures for each flight are plotted as a function of time, as a quick-look product. Please contact personnel at Hydrological Sciences Branch at Goddard Space Flight Center. 10. Errors 10.1 Source of Error Errors in the calibrated brightness temperature data may arise from several sources: 1. Instrumentation operation temperature-Due to the cold ambient temperature, the instrument temperature cannot be controlled accurately. 2. Stability of the noise diode. 10.2 Quality Assessment 10.2.1 Data Validation by Source Comparisons were made with Special Sensor Microwave Imager (SSM/I) data over the same area and available water targets. 10.2.2 Confidence Level/Accuracy Judgment The 18 and 37 GHz radiometers are believed to be accurate to about +- 3 Kelvin. The 92 GHz radiometer may be accurate to approximately +- 10 Kelvin. For snow water equivalent value, the accuracy is approximately 5 mm. 10.2.3 Measurement Error for Parameters and Variables None given. 10.2.4 Additional Quality Assessment Applied None given. 10.2.5 Data Verification by Data Center None given. 11. Notes 11.1 Limitations of the Data During the aircraft flights it was found that the 92 GHz brightness temperature was not very stable because of the instability in the cold reference load temperature. Therefore the quality of these data is somewhat uncertain. 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 This data set may be used to study the energy balance for the BOREAS sites. 13. Future Modification and Plans There are no reprocessing plans at this time. 14. Software Since this data set is in ASCII format, it can be read with simple read statements. 14.1 Software Description None given. 14.2 Software Access None given. 15. Data Access 15.1 Contact Information Primary contact: Ms. Beth Nelson BOREAS Information System NASA Goddard Space Flight Center Greenbelt, Maryland (301) 286-4005 (301) 286-0239 Elizabeth.Nelson@gsfc.nasa.gov 15.2 Data Center Information See 15.1 15.3 Procedures for Obtaining Data Users may place requests by telephone, electronic mail, or FAX. 15.4 Data Center Status/Plans the HYD-02 SWE data are available from the EOSDIS ORNL DAAC (Earth Observing System Data and Information System) (Oak Ridge National Laboratory) (Distributed Active Archive Center). 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 ASCII files on 8 mm tape. 16.2 Film Products None. 16.3 Other Products Video tapes from the Twin Otter flights are also available. 17. REFERENCES 17.1 Platform/Sensor/Instrument/Data Processing Documentation None given. 17.2 Journal Articles, Study Reports, etc. Chang, A.T.C., J.L. Foster, D.K. Hall, A.E. Walker, B.E. Goodison, J.R. Metcalfe (1996). "Snow Parameters Derived From Microwave Measurements During the BOREAS Winter Field Campaign", 22nd Conference on Agricultural and Forest Meteorology (AMS Conference), Atlanta, GA, Jan. 1996 Chang, A.T.C., J.L. Foster and D.K. Hall, 1987, Nimbus-7 derived global snow cover parameters, Annuals of Glaciology, 9, 39-44. Chang, A.T.C., J.L. Foster and D.K. Hall, Effect of vegetation on microwave snow water equivalent estimates, "Proceedings of the International Symposium on Remote Sensing and Water Resources", Enschde, The Netherlands, 137-145, 1990 Goodison, B.E., and A.E. Walker, Canadian development and use of snow cover information from passive microwave satellite data, Passive Microwave Remote Sensing of Land-Atmosphere Interactions,(Eds. Choudhury, Kerr, Njoku and Pampaloni), VSP, 245-262, 1994. Goodison, B.E., and A.E. Walker, Use of snow cover derived from satellite passive microwave data as an indicator of climate change. Annals of Glaciology, 17, 137-142, 1993. Goodison, G., A.E. Walker and F.W. Thirkettle, Determination of snowcover on the Canadian prairies using passive microwave data, "proceedings for the International Symposium on Remote Sensing and Water Resources", Enschede, The Netherlands, 127-136, 1990 Hall, D.K. , J.L. Foster and A.T.C. Chang, "Mapping snow cover during the BOREAS Winter Experiment," AGU Annual Fall Meeting, 1994. Hallikainen, M.T., and P.A. Jolma, Comparison of algorithms for retrieval of snow water equivalent from Nimbus-7 SMMR data in Finland. IEEE Trans. on Geoscience and Remote Sensing, 30, 124-131, 1992. Hallikainen, M.T., P.A. Jolma and J.M. Hyyppa, Satellite microwave radiometry of forest and surface types in Finland. IEEE Trans. on Geoscience and Remote Sensing, 26, 622-628, 1988. Rott, H. and J. Aschbacher, On the use of satellite microwave radiometers for large-scale hydrology, Proc. IASH 3rd Int. Assembly on Remote Sensing and large Scale Global Processes, Baltimore, 21-30, 1989. Walker, A.E. and B.E. Goodison, Discrimination of a wet snow cover using passive microwave data, "Annals of Glaciology", 17, 307-311, 1993. Wang, J.R., R. Meneghini, H. Kumagai, T.T. Wilheit, W.C. Boncyk, P. Racette, J.R. Tesmer and B. Maves, Airborne active and passive microwave observations of super typhoon Flo, "IEEE Trans. Geoscience and Remote Sensing", 32, 231-242, 1994. 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 C - degrees Celsius DAAC - Distributed Active Archive Center EOS - Earth Observing System EOSDIS - EOS Data and Information System FFC-W - Focused Field Campaign - Winter GPS - Global Positioning System GSFC - Goddard Space Flight Center INS - Inertial Navigation System IR - InfraRed K - Kelvin MTPE - Mission to Planet Earth MW - MicroWave NASA - National Aeronautics and Space Administration NSA - Northern Study Area NSIDC - National Snow and Ice Data Center ORNL - Oak Ridge National Laboratory PC - Personal Computer PRT5 - Borneo Model PRT-5 radiation thermometer SMMR - Scanning Multichannel Microwave Radiometer SSA - Southern Study Area SSMI - Special Sensor Microwave Imager SWE - Snow Water Equivalent URL - Uniform Resource Locator 20. DOCUMENT INFORMATION 20.1 Document Revision Date Written: 23-Jan-1997 Revised: 02-Jul-1998 20.2 Document Review Date BORIS Review: 02-Jul-1998 Science Review: 20.3 Document 20.4 Citation The microwave brightness temperature data set was to provide accurate measurements of thermal microwave radiation from snow fields. This data set was developed with support from NASA's Mission to Planet Earth (MTPE) BOREAS Project. Thanks are due to BORIS at Goddard Space Flight Center for distributing the data; Drs. Chang, Hall, and Foster of the Hydrological Sciences Branch, NASA/GSFC for producing these data products. 20.5 Document Curator [DAAC WILL FILL IN.] 20.6 Document URL [DAAC WILL FILL IN.] KEYWORDS --------- SNOW WATER EQUIVALENT BRIGHTNESS TEMPERATURE MICROWAVE HYD02_Aircraft_SWE.doc 07/07/98