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