To predict the weather even a short time ahead, it is essential that we know the current state of the atmosphere to provide starting conditions for the numerical weather prediction (NWP) model. Regular observations on land, at sea (ships and buoys), in the air (radiosondes and aircraft), and from satellites in space provide us with information on pressure, temperature, wind speed and direction, and humidity.

In practice, we find this is not sufficient for an unambiguous representation of the atmosphere as, in addition to the errors we would expect from any set of measurements, we find the geographic and time distribution of observations is rather uneven with some areas and levels in the atmosphere covered poorly or not at all. However, we do have earlier forecasts available to us which will provide full geographic coverage, and we know that the atmosphere evolves subject to well known physical principles.

Bringing the observational and forecast data together in a mathematically rigorous fashion and ensuring the component fields are in physical balance is a process known as data assimilation: the estimate of the state of the atmosphere obtained is called the analysis.

It is clear that data assimilation is an essential component of the numerical weather prediction capabilities of the Met Office. Our efforts are principally directed towards providing and maintaining systems that can be used for operational purposes but we also devote significant resources towards research.

• Observation monitoring and processing
• Operational assimilation system
• Regional and surface data assimilation
• Middle atmosphere data assimilation and modelling
• Forecast monitoring and Investigation
• Data assimilation research

Close links are maintained with other groups in the Met Office and internationally to further the development and use of data assimilation in NWP.

THORPEX

THORPEX is a ten-year WMO research programme to accelerate improvements in the accuracy of one-day to two-week high-impact weather forecasts for the benefit of society, the economy and the environment.

Joint Centre for Mesoscale Meteorology (JCMM)

The Joint Centre for Mesoscale Meteorology (JCMM) is a research Centre in which the Met Office and University of Reading collaborate. It is located within the Department of Meteorology at the University.

Satellite applications

The Satellite Applications section of the Met Office improves the use of available satellite observations and makes preperations for the use of new measurements from space.

We publish the results of our work internally to the Met Office - a list of data assimilation-related NWP Technical Reports is available - and in international peer-reviewed journals. We also present and discuss our work at conferences and workshops.