Weather satellite

A weather satellite is an Earth observation satellite, which is the observation of meteorological processes, ie physical and chemical processes in the atmosphere of the earth. Especially in areas where no observation on site is possible or very expensive ( eg oceans ), data from weather satellites are indispensable. With the evaluation and use of meteorological satellites are a branch of meteorology, satellite meteorology is concerned. It uses the data primarily for weather forecasting and climatology, the two best known branches of meteorology.

History of meteorological earth observation

Geostationary weather satellites

Geostationary satellites flying at an altitude of 35,800 km above the equator. As they rotate at the same angular velocity about the earth as the earth rotates around itself ( " Earth rotation " ), they are at a fixed point above the earth. The Meteosat satellites must be altered to rotate around its own axis to stabilize.

Geostationary satellites have the advantage of high temporal resolution; obtained approximately every 5 to 30 minutes a new image and can assess the temporal evolution of weather systems well. A major advantage lies in the fact that every time you shoot the same image section is detected. One can create satellite films, so-called loops; these show that the satellite Recorded in fast motion. All well -known from the media (weather) satellites films come from geostationary satellites. The spatial resolution is in the kilometer range (about 1-5 km in the subsatellite point, ie the point of the earth's surface directly below the satellite ). A geostationary satellite ' sees ' about 2 /5 of the earth's surface. Mathematically, one can thus observe almost completely with three satellite earth. At all four edges of the image towards the resolution is always worse ( at the top of the screen you can see the Arctic; Antarctica at the bottom ) because there is no vertical supervision by the satellite is possible.

Beginning of 2010 were among others the following geostationary satellites in use:

• Two Meteosat satellites of EUMETSAT ( Meteosat -9 at 0 ° west longitude and Meteosat -7 at 57 ° east longitude )
• Two GOES satellites of the NOAA National Weather ( GOES -12 at 75 ° and GOES -11 at 135 ° )
• A third satellite of the Meteosat series ( Meteosat -8 ) operates in the ' Rapid Scan Service ' (RSS) at 9.5 ° east longitude, and also serves as a reserve for Meteosat -9
• Addition also satellite of the Japanese MSAS, the Chinese Fengyun and the Indian Insat series for meteorological purposes are in use

Polar orbiting weather satellites

Polar orbiting weather satellites fly in a polar, sun-synchronous train in about 800 km (see also sun- synchronous orbit, SSO). A round lasts about 100 minutes. Thus, the earth's surface in 12 hours completely scanned once. The disadvantage of the low refresh rate is the advantage of good spatial resolution (100 to 1000 m, in the area of ​​Earth's poles ) over.

Together with the geostationary satellites, the earth can be so consistently observed.

Polar orbiting weather satellites operated by EUMETSAT ( MetOp satellites ), the USA (NOAA - type ), China ( FY -1C satellite) and Russia ( Meteor ).

Tasks of weather satellites

• Analysis of the current weather conditions ( synoptic meteorology ), especially in difficult or impossible to access or sparsely populated areas, so that meteorologists get an accurate overview of the weather effective action (ie, inter alia, pressure areas and cloud shapes ); Thus, the spatial resolution of the images must be sufficiently large at low temporal resolution.
• Use as input to weather forecast models ( assimilation ) and verification of the accuracy of weather forecasts
• Determination of vertical gradients of different sizes, such as the temperature, especially over oceans and other areas with no or little ground measurements
• And Atmospheric Research ( meteorology, climatology, Aerology ) because this is what always less money for their own systems available

Mode of operation and data analysis

Weather satellites are as a payload image-receiving sensors ( radiometer ). This measure radiation in different spectral bands ( the so-called channels ), mainly in the visible and infrared regions, and occasionally in the microwave range. For the correct interpretation of the data is necessary to apply the radiation laws of physics. In the infrared range, the earth radiates at an average temperature of 15 degrees Celsius.

Most weather satellites can measure the electromagnetic radiation from the earth's surface and the atmosphere., The radiation is only detected, it is called the passive instruments, as opposed to active instrument, in which the radar beams or laser beams are transmitted and the reflected signal is measured.

The visible ( solar) channel of weather satellites ( abbreviated Vis for engl. Visible ) only measures the light reflected by the Earth and atmosphere sunlight. As clouds of water droplets reflect particularly strong, they appear very bright in the Vis - channels, in contrast to clouds of ice crystals, the most absorbing in the near infrared and therefore appear dark in these channels. Thus, the different types of clouds can be distinguished. By combining the data from the different infrared channels can be close to the different vertical cloud layers. From clouds displacements in successive frames the wind direction can be determined.

Since the reflectivity of the earth's surface from each soil type ( the so-called albedo) depends on the surface can be identified by comparing the spectra of different Vis - channels, is an excellent method to distinguish among the various vegetations, which in recent Meteosat Generation (MSG ) is employed. If there are no interfering clouds present, can be determined by means of the black-body radiation, the temperature of the soil or the sea surface, which is also important for the production of weather forecasts.

Since radiation emitted by satellite radar beams are scattered by water waves and thus leads to a change in the frequency of the radar beam due to the Doppler effect, and wave motions can be measured by weather satellites.

The weather forecast for the shortest-term range ( one to three hours ), the so-called nowcasting is obtained directly from the satellite images. For further predictions, a time series of time- sequentially recorded images will be created and expanded development in the future. Because the nowcasting makes clear reliable predictions, the power of data for a reliable prognosis must be very closely, which is why you used for the high resolution weather satellites.

Furthermore, facilities for communication on the meteorological satellites, such as the reception of weather reports from automatic weather stations and to broadcast the captured satellite images (weather -satellite service ).