Satellite sounding of the atmosphere — microwave

Satellite sounding instruments measure radiation at microwave and infrared wavelengths that have primarily been emitted by the atmosphere itself. The different wavelengths provide information on the temperature and composition (e.g. humidity, ozone amount) of the atmosphere, over a range of altitudes. Microwave sounding instruments provide good quality information, even in the presence of clouds, but not heavy rain. However, they can not provide the very high vertical resolution which the infrared sounders can deliver.

The technology is well-established, but work continues to improve the exploitation of the data provided, particularly the information on clouds and temperature and water vapour in the lowest few kilometres of the atmosphere and over land.

Microwave sounding radiometers

The Advanced Microwave Sounding Unit (AMSU), a 20-channel radiometer, provides information on the temperature and water vapour structure of the atmosphere. Together with the infrared instrument HIRS it forms part of the ATOVS sounding system.
AMSU is really three radiometers:

• AMSU-A1 which provides temperature information
• AMSU-A2 which provides information on clouds and water vapour
• AMSU-B which provides information on water vapour. On more recent satellites the AMSU-B has been replaced by the MHS instrument, but the capability remains very similar.

The microwave sounders are an important and well-established part of the observation network for weather forecasting.

Microwave sounding — technical details

The microwave region of the spectrum used for atmospheric remote sensing ranges from one to 200 GHz in frequency

An oxygen (O2) spectral band composed of many individual spectral lines between 50 and 60 GHz gives us information on the temperature structure of the atmosphere

Information on water vapour content can be gained from two water vapour (H2O) lines at 22.225 and 183.31 GHz.

Information on the Earth's surface (e.g. sea-surface winds and temperature) and rain is obtained at frequencies which are not close to absorption lines.

For lower frequencies the majority of clouds are essentially transparent but at frequencies above 20 GHz cloud effects have to be considered.

Information about cloud liquid water path can be obtained from the higher frequencies where cloud absorption and scattering becomes significant.