Observations

Every day, hundreds of thousands of observations are made of the atmosphere around the world, measuring quantities such as pressure, wind, temperature and humidity. The observations are made in many ways — at single sites on land and sea; from the air by weather balloons and aircraft; from space by satellite, and by rainfall radar. To use these observations in an operational weather forecasting system we have to monitor their availability, quality control them, and process them into a form that can be used by the computer models and forecasters.

We develop and maintain software systems which can do all of these tasks for current operational data sources, and which can be readily extended for new observational systems in the future.

Types of observations

Surface data

These include:

• land-based synoptic observations, including cloud type and present weather
• marine surface data from ships and buoys
• wind data from measurements of ocean-backscatter observed from satellites

Surface pressure is usually reduced to a mean sea-level pressure for reporting and plotting purposes, also many stations report pressure at the station altitude and some high-level stations compute a height at 850 hPa.

These are all transformed into p* (the pressure at the model ground surface), in order to simplify the assimilation process. Wind data are taken at 10 m over land, but the height may vary for marine data depending on the observing system. Temperature and moisture are measured at a height of 1-2 m over land, but again the height may vary for marine data.

Radiosondes

A radiosonde is a unit used on weather balloons that measures various atmospheric parameters and transmits them to a fixed receiver. A special feature of the radiosonde processing is vertical averaging. The radiosonde system provides a standard set of measurements of wind speed and direction, temperature and dew-point temperature. The dew-point data provide information on moisture and are usually combined with the temperature data to provide the moisture information as relative humidity.

Radiosonde reports help us build a detailed picture of the atmosphere at that location. In some reports there may be more than 100 levels. The vertical averaging maps these reports on to the computer models. As well as providing the models with information which is consistent with its resolution, this also simplifies the subsequent processing as each report can then be treated in a similar manner regardless of how many levels it originally contained.

Aircraft observations

Observations of temperature and winds are available either from manual or automatic reporting systems. The altitude of the observations has to be converted from a flight level to a pressure before being assimilated in to the Met Office forecast models.

True representation of actual conditions is more of a problem with aircraft reports than some upper-air observation systems, particularly in the vertical. Planes flying in the direction of the wind may seek the core of the jet stream, which is restricted in height, and therefore may be sampling part of the atmosphere which is not fully resolved by the model (where typically the vertical resolution may be only 50 hPa). Automated reports from commercial airliners are provided through the AMDAR (Aircraft Meteorological DAta Relay) programme.

Recently, a new type of automated aircraft report has been developed in the USA, primarily for use by regional airlines, called TAMDAR (Tropospheric Airborne Meteorological DAta Report, or 'Tropospheric AMDAR'). It is envisaged that these two types of report will ultimately replace the manual reports known as AIREPs.

Weather satellites

We continually investigate ways to exploit satellite data, including atmospheric motion and wind reports and satellite sounding information on the temperature and composition of the atmosphere.