Satellite sounding of the atmosphere

This area of research focuses on improving the way we use passive satellite data from microwave and infrared sensors to improve our forecasting capabilities, primarily via the assimilation of these data into our Met Office Numerical Weather Prediction models.

Passive remote sensing uses the natural radiation signature emitted by the atmosphere or surface. It uses the fact that the radiation reaching the satellite will vary from wavelength to wavelength depending upon the temperature structure of the intervening atmosphere and the presence of any absorbing gases and clouds. The method contrasts with satellite active sensing, where an instrument measures artificially-generated radiation emitted by a satellite, and received by either the same instrument or another receiver.

Microwave sounding is crucially important to our ability to derive accurate global analyses of temperature and humidity. This is because, especially for the lower frequencies used, the majority of ice clouds are essentially transparent in the microwave spectral region, allowing information on the vertical profiles of temperature and humidity to be assimilated into the Unified Model. Information about cloud liquid water can also be obtained from microwave channels, as cloud absorption is significant, as well as information about the Earth's surface.

In contrast, clouds tend to be opaque in the infrared spectral region, and infrared sounding channels, therefore, provide little or no information below cloud. However, the infrared sounding instruments, especially the newer generation of advanced sounders such as AIRS, IASI and CrIS, provide significantly higher vertical resolution than the microwave sounders in cloud-free areas and in the air above clouds.

Key aims

• To improve forecast accuracy through better assimilation of microwave and advanced infrared sounder data into the Unified Model.

• To assess the potential for using new instruments and techniques for future improvements to our satellite sounding capabilities.

Current projects

• Improved use of microwave and infrared sounding data over land and sea-ice.

• Improved assimilation of high-resolution advanced infrared sounder data.

• Improved characterisation of biases and correlated errors in sounder data.

• Improved analysis of cloud and precipitation fields from microwave and infrared sounder data.

• Improved analysis of trace gases.

• Assessment of potential future sounding instruments (e.g. polarimetric radiometry, sub-millimetre sounding).