| Introduction to weather satellites |
Weather forecast models use a rich variety of observations
from which to analyse the current state of the atmosphere. Since
the launch of the first weather satellite in 1960 global observations
have been possible, even in the remotest areas. It was not until
1969 that the first temperature profile information estimated
from satellite measurement were introduced to an NWP model. Even
in those early days the new satellite measurements improved forecasts
in the southern hemisphere.
During the 1970s and 1980s a wide range of satellite missions
have been launched from which many different meteorological quantities
could be estimated. Some satellite instruments allowed improved
estimation of moisture, cloud and rainfall. Others allowed estimation
of wind velocity by tracking features (e.g. clouds) visible in
the imagery or surface wind vectors from microwave backscatter.
However, it was only in the 1990s that the measurements made
by the satellite instruments began to be used by NWP models in
their raw form. Data assimilation systems for NWP models had advanced
to the point where a measurement of the earth's radiance in narrow
spectral bands could be directly assimilated just like an in situ
measurement of atmospheric temperature. This advance, alongside
other improvements in the quality and range of satellite observations
and our understanding of how to model them, has led to a situation
where satellite measurements are a vital and integral part of
the global observing system in all regions, not just those where
other observations are sparse.
The Satellite Applications Section of the Met Office improves
the use of available satellite observations and makes preparations
for the use of new measurements from space. In addition to the
assimilation of satellite data into NWP models, the Section produces:
new imagery products for forecasters, such as microwave precipitation
imagery; environmental products (such as volcanic ash monitoring);
products for climate monitoring, such as sea-surface temperature.
The Satellite Application Section conducts research of several
different aspects of the exploitation of satellite data:
| Satellite imagery
(visible, infrared and microwave) |
The most basic form of satellite imagery provides pictures of the
current cloud conditions. This is a familiar sight on TV weather
forecasts. However, satellite imagery can also undergo various types
of quantitative processing to obtain information on important meteorological
variables such as wind speed and direction, cloud height and cloud
amount, surface temperature, sea ice cover, vegetation cover, precipitation,
etc.
More
about satellite image applications
| Satellite sounding
of the atmosphere (infrared and microwave) |
Satellite sounding instruments measure radiation at infrared or
microwave wavelengths that has been emitted by the atmosphere itself,
and thus provide information on the temperature and composition
(e.g. humidity, ozone amount) of the atmosphere over a range of
altitudes. Over the last few years, much improvement in forecast
accuracy has come from use of data from the Advanced Microwave Sounding
Unit on the NOAA series of satellites. Work continues to improve
the exploitation of these data and also to make use of data from
a new generation of advanced infrared sounding instruments (spectrometers
and interferometers). In order to make best use of these data, it
has proved important to develop fast radiative transfer models to
allow the radiance data to be assimilated into the NWP models.
More
about infrared sounding
More
about microwave sounding
In active remote sensing, man-made sources of energy (e.g. microwave,
radio wave or light) from a radar, lidar or other transmitting device,
and which has interacted with the earth's surface or atmosphere
is detected. From the measurement, meteorological information may
be derived for eventual use in numerical weather prediction models.
Quantities that can be derived include near-surface wind speed and
direction over the oceans, vertical profiles of horizontal wind
speed, profiles of temperature and humidity and aerosols, vertically
integrated water vapour and precipitation rates.
More
about satellite active sensing
| Radiative transfer modelling |
In order to simulate the upwelling radiance measured by a satellite
instrument (e.g. for a near nadir viewing sounder like ATOVS),
the electromagnetic radiation emitted or absorbed by gases along
the viewing path and from the surface and/or cloud must be computed.
For radiance assimilation in a NWP model, this calculation must
be made in a few milliseconds for all the radiance channels in
order not to delay the production of the forecast model. Hence
the development of fast radiative transfer (RT) models is an important
activity of the satellite infrared sensing group to enable assimilation
of satellite radiances within a NWP model.
More
about radiative transfer modelling
| Further information
and links |
The Education section of the Met Office has prepared a leaflet
with more information on the basics of weather satellites aimed
at school-aged children and information is also available on the
Metlink site.
Other useful links are:
|
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