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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.

Satellite Applications

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

Satellite active sensing

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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