Remote sensing

The main task for the Section for Remote Sensing is to improve existing methods and develop new ones to make the institute better equipped to exploit the remotely sensed data in our analysis, short-term forecasts and environmental surveillance.

Remotely sensed data from weather radars and satellites are an important source of information in the analysis of the current weather situation. These data usually have a higher spatial and temporal resolution than conventional observations. They can provide information on the conditions on the earth’s surface and the atmosphere and are important data for the atmospheric models and the ocean models.

Satellite data from the weather satellites METEOSAT and NOAA POES are downloaded and processed at the Norwegian Meteorological Institute’s ground station in Oslo (Blindern). Improving the basic processing of these data is a continuous process.

The data are used for the purpose of gaining knowledge about snow- and ice coverage, sea temperatures, surface radiation and various weather phenomena such as fog and precipitation. The data downloaded from the satellites are based on so-called passive instruments, hence they measure radiation reflected by land and ocean surfaces. From satellite data such as these, information about vertical profiles of atmospheric temperature and humidity can be derived. This information can then be utilised as access data for numeric weather forecast models.

Data from experimental satellites, i.e. wind scatterometers and altimeters, are used to gain information about wind, wave conditions and ice from the ocean surface. Data from ERS-scatterometers and altimeters are used in the institute’s numerical models for forecasting weather and wave conditions.

Currently, the section is aiming to use scatterometer data from the American Quick-SCAT-satellite.

Remote sensing techniques

Remote sensing techniques allow us to gain knowledge of a meteorological object without being in direct contact with it. Clouds, precipitation, atmospheric gasses like water vapour and the earth and ocean surfaces are meteorological objects. All meteorological objects absorb, scatter and emit electromagnetic radiation which can be utilized in remote sensing techniques.

The information gained by remote sensing is vital to the operational forecaster. These data have high spatial and temporal resolution, a feature essential to short term forecasting (0-12 hours), also known as nowcasting. The remotely sensed observations are available from areas with only few conventional observations as for instance large ocean areas.

Remote sensing instruments can be placed in one of two groups:

  1. Passive instruments measure the electromagnetic radiation emitted by an object.
  2. Active instruments emit a signal and compare it with the back scattered or reflected signal.

Passive instruments

These instruments measure the electromagnetic radiation emitted by the earth's surface and atmosphere, and are commonly used on operational weather satellites (e.g. NOAA POES, METEOSAT). The observations are utilized in several ways, e.g. to create images for weather monitoring, for classification of weather and surface types, to extract information on vertical atmospheric profiles of temperature and humidity and to estimate the surface radiation budget.

Passive instruments used operationally for meteorological purposes measure visible light (0.3-4.0 micrometer wavelength), infrared radiation (4-100 micrometre) or microwaves (0.1-10 centimetre). Observations in the visible and infrared channels are commonly used to create images that are subjectively interpreted by the forecaster. However, they can also be used in objective interpretation.

Active instruments

These instruments measure electromagnetic radiation at frequencies where natural emission from meteorological objects are insignificant. An example is the low frequency part of the microwaves.

"Active" instruments mean that these instruments emit a signal themselves that is reflected or backscattered by the object. By comparing the emitted and the retuned signal, information about the object is achieved. The most common active instrument used in meteorology is RADAR (Radio Detection And Ranging) which operates at microwave wavelengths. An example is the wind scatterometer. This is a radar emitting microwaves and measuring the intensity of backscatter from the sea surface. This intensity is dependent on the sea surface wind.

LIDAR (LIght Detection And Ranging) is less commonly used in operational forecasting, but is frequently used for experiments. SODAR (SOund Detection And Ranging) can be used in local studies of for instance wind conditions at airports.
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