A new way of using data from satellite-based radar is providing
scientists with a unique insight into the effects of climate change on
ice caps, plant life and land surface.
As ESA’s Mark Drinkwater explained “for the first time, in this
scatterometer climate-record pathfinder study, we have generated a
consistent, long-term global microwave radar data set. With this data,
scientists will be able to monitor long-term trends over the entire land
surface of the Earth. Importantly, we have assembled data from a variety
of satellites, going back to the late 1970s, with which one can begin to
piece together a clear picture of global change over decades.”
The new technique, pioneered by Drinkwater and colleague David Long of
Brigham Young University, United States uses data gathered by a wind
scatterometer — a radar instrument carried aboard ERS-1 and 2. The
scatterometer sends out a radar pulse and measures the characteristics
of the echo that bounces back from the planet’s surface below.
Originally designed to monitor ocean winds by the distinctive echoes
from the waves they whipped up on the sea surface, the scatterometer
is proving much more versatile.
“We’ve been doing a lot of work looking at polar ice,” explains
Drinkwater, “but now we’re starting to understand how the radar echo
is affected by different types of vegetation and soil moisture levels
over land, which opens up valuable new opportunities to look at how
ecosystems change with time.”
Although the scatterometer does not provide the kind of high-resolution
almost-photographic images produced by synthetic aperture radar (SAR),
for Drinkwater’s studies it has several advantages.
“This project was born out of the frustration of not knowing what was
happening between the sporadic images you might get from SAR,” admits
Drinkwater. “If you can only access infrequent irregularly-spaced
images, it’s very hard to study change over time, particularly in
high-latitude regions where the surface can change dramatically over
a short period of time. Now we can combine the two and use the SAR
imagery to zoom in on specific events and locations in our global time
series.”
Scatterometer data, with a much wider field of view than SAR, provides
a global picture of the land surface every few days with ERS-1 and -2,
and with seawinds on Quikscat daily.” Because the orbits converge
over the poles, at high latitudes we can do even better,” explains
Drinkwater, “and start to study the melting and refreezing of ice and
snow, for example, through the course of each day.”
The scatterometer echo can, in effect, peer a little way beneath the
surface, detecting the type of vegetation, the amount of water in the
soil, or the melting and refreezing of seasonal permafrost in the
tundra. “We’re getting a week-to-week, month-to-month and year-to-year
perspective of how the surface is changing,” comments Drinkwater.
“In a study led by Prof. Wolfgang Wagner of Vienna University of
Technology, ERS Scatterometer data has been used to track the ‘green
wave’ in Africa — the explosion of plant growth in semi-arid terrain
as the rainy season comes. The scatterometer first picks up the
increase in topsoil humidity and plant-available water, followed by
the change in the vegetation cover as leafy plants spring up and bloom.
The results of this work are being evaluated in connection with crop
yield assessments by the Food and Agriculture Organisation of the
United Nations. In addition, the method has been extended to Asia,
where snow melt, and the freezing and thawing of the permafrost can
be monitored in addition to the soil moisture”.
The extent of permafrost, for example, is thought to be a particularly
sensitive indicator of climate variability. Understanding the changes
of such key global climate change indicators is the main justification
for the work of Drinkwater and his colleagues in compiling this new
data source.
“The scatterometer is extremely well calibrated and its long-term
performance well-characterised” explains Drinkwater, “so it is very
sensitive to small changes over time. This sensitivity, combined with
our ability to put together a series of snapshots covering the globe
every few days over a period of decades, adds up to a uniquely capable
resource for monitoring the effects of climate change — which could
be the most important environmental issue facing our planet and all
the people on it.”
Further reading on ESA-funded scatterometer research:
* Land Surface Observations using the ERS Wind Scatterometers
http://www.ifremer.fr/cersat/FICHES/DIVERS/IFARS/E_IFARS.HTM
* Evaluation of Operational Land Applications of Scatterometers
http://www.ifars.de/eolas/scatapps.htm
* Detecting Soil Thawing in Siberia
http://esapub.esrin.esa.it/eoq/eoq52/boe52.htm
Related news
* Scatterometers, sea ice and climate change
http://www.esa.int/export/esaCP/GGGTECNZ0GC_FeatureWeek_0.html
Related links
* Technical University of Vienna
http://www.tuwien.ac.at/welcome_eng.html
* Radar Remote Sensing Vienna University of Technology
http://www.ipf.tuwien.ac.at/radar/ers-scat/home.htm
* Brigham Young University Center for Remote Sensing
* ERS
IMAGE CAPTIONS:
[Image 1:
http://www.esa.int/export/esaCP/ESAMWB8VTTC_index_1.html]
One of the many uses of scatterometers on board satellites is to
provide information on the amount of water available in the soil.
This image shows the monthly mean PAW (plant available water in
percentage volumetric soil moisture) available for the period
1992-2000 for Eurasia. Photo: Vienna University of Technology,
Austria
[Image 2:
http://www.esa.int/export/esaCP/ESAMWB8VTTC_index_1.html#subhead1]
QuikScat SeaWinds v-polarised radar backscatter image of Europe for
the 3-day period of days 201-204, 2000. Grey tones represent the
intensity of the radar backscatter coefficient (symbol: sigma-
superscripted ‘oh’) with white indicating a value of -5 dB and black
a value of -20 dB. Image: Scatterometer Climate Record Pathfinder
study and Brigham Young University
[Image 3:
http://www.esa.int/export/esaCP/ESAMWB8VTTC_index_1.html#subhead2]
QuikScat SeaWinds v-polarised radar backscatter image of North
Africa for the 3-day period of days 201-204, 2000. Grey tones
represent the intensity of the radar backscatter coefficient
(symbol: sigma-superscripted ‘oh’) with white indicating a value
of -5 dB and black a value of -20 dB. Image: Scatterometer Climate
Record Pathfinder study and Brigham Young University
[Image 4:
http://www.esa.int/export/esaCP/ESAMWB8VTTC_index_1.html#subhead3]
QuikScat SeaWinds v-polarised radar backscatter image of South
Africa for the 3-day period of days 201-204, 2000. Grey tones
represent the intensity of the radar backscatter coefficient
(symbol: sigma-superscripted ‘oh’) with white indicating a value
of -5 dB and black a value of -20 dB. Image: Scatterometer Climate
Record Pathfinder study and Brigham Young University
