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Satellite imagery
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| Why use satellite imagery? |
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The first meteorological satellite was launched in 1960 by the
USA and provided cloud cover photography. Originally, satellite
images were treated purely as qualitative pictures which were
manually viewed and interpreted by meteorologists. Nowadays though,
satellite imagery undergoes a great deal of mathematical manipulation
and can yield quantitative analyses of atmospheric temperature,
humidity, motion and many more meteorological variables. The major
advantage of satellites is their ability to produce global
coverage, which becomes especially important over oceans and remote,
unpopulated land regions, where other methods of observation are
impracticable. Over large areas of the southern hemisphere, satellites
are the only means of observing the atmosphere and surface. As well as observing
changes in surface features such as snow, vegetation and sea surface
temperature, satellite imagery can also capture the development
of transient features such as clouds of water or ice and plumes
of ash, gases or dust. Research carried out here at the Met Office is
developing new and improved satellite products for assimilation
into NWP models and presentation to the forecasters in the form
of imagery and movie loops. The links to the right give more details
about the different products that we generate from satellite imagery.
| Satellites used for satellite imagery applications |
The imagery used is produced by instruments on board two types
of satellite.
- Polar orbiters are positioned about 900 km above the surface
of the Earth, in a sunsynchronous orbit, which means they see
the same part of the Earth at the same time each day. Polar
orbiters make about 14 orbits a day and can view all parts of
the atmosphere/surface at least twice a day. Although their temporal
resolution is limited, they have high spatial resolution (typically
around 1 km between pixels) since they are relatively close
to the Earth's surface.
- Geostationary satellites are positioned about 36,000 km above
the equator in a geostationary orbit, which means they are always
fixed in position above one part of the Earth. These satellites
scan continuously (hence have high temporal resolution 15-30
minutes), but have limited spatial resolution (typically 3-10
km between pixels). Below is a table listing some details of
the geostationary and polar satellites used by the Met Office.
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The new Meteosat Second Generation satellites
(Meteosat-8 and 9) are now providing a rich source of new data over Europe
and Africa and work is underway to fully exploit
these data. In addition data from the polar orbiters are used, specifically the Advanced Very High
Resolution Radiometer ( AVHRR)
on the NOAA and METOP platforms and MODIS
data on the Terra and Aqua platforms for research. The WMO Satellite
Activities Dept web page gives more international satellite information.
Radiance is measured by the satellite radiometer and stored as digital
values in two-dimensional arrays of pixels, which make up the image. Different
instruments scan at different wavelengths and provide complementary
information about the atmosphere and surface:
- Infrared radiation, particularly around 10-12.5 µm, tells us about
the temperature of emitting bodies, such as cloud tops or the surface
in cloud-free regions. Infrared images are good for viewing
high clouds and surfaces at any time of the day or night.
- Water vapour radiation, centred around 6.7 µm, measures radiation
in the water-vapour absorption band. WV images are good for viewing
water vapour distributions in cloud-free areas, and for viewing clouds.
Most of the radiation sensed is from the 300-600 hPa layer.
- Visible radiation, produced in a wavelength band ~ 0.5-0.9 µm, shows
low and high clouds but only by reflected sunlight, so no images are produced at
night.
Examples of the current infrared satellite imagery can be found on the Met Office web site here and visible imagery here.
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