What is the role of satellite observations in weather forecasting?
The European Centre for Medium-Range Weather Forecasts (ECMWF) uses satellite data for climate monitoring, atmospheric composition monitoring and forecasts as part of the European Commission’s Copernicus programme. Satellite data also help to monitor the quality of forecasts and to identify and remedy deficiencies in Earth system models. As indicated by ECMWF, satellite observations make a crucial contribution to the quality of today’s weather forecasts. Their global coverage means that they provide information on the atmosphere, the land surface, the ocean and sea ice that cannot be provided by in situ measurements. Through a process called data assimilation, they help to produce the best possible estimate of the current state of the Earth system. That estimate is used as the initial condition on which weather forecasts are based.
What are the main areas of research in satellite data assimilation at ECMWF?
Active areas of research include the development of techniques to fully exploit hyperspectral infrared instruments, which have many channels: all-sky assimilation for infrared radiances, the use of more of the information that satellites provide on the wider Earth system, such as surface conditions, the ocean and atmospheric composition, and the treatment of random and systematic errors in satellite observations. In addition, the Centre works on new generations of instruments with partners such as the European Space Agency (ESA) and EUMETSAT in Europe and similar agencies in China, Japan and the United States. ECMWF has, for example, begun to assimilate Doppler wind light detection and ranging (LiDAR) observations from the ground-breaking Aeolus satellite, and it is preparing the ground for the assimilation of cloud profile observations from the EarthCARE mission planned by ESA and the Japan Aerospace Exploration Agency (JAXA).
What types of satellite observations are there?
Some satellites carry passive instruments, which measure radiation emitted naturally, and other satellites use active instruments, which send out signals and measure the backscatter. Natural radiation contains information, for instance, on temperature, humidity, clouds and surface conditions. It can also provide information on winds by tracing motions of humidity or cloud features in successive observations. Active instruments use radar or LiDAR to probe the surface, clouds and winds. Radio occultation observations are unique in that they involve sending signals from one satellite to another. The bending angle of such signals crossing the troposphere or stratosphere depends on temperature and humidity.
How has the use of satellite observations at ECMWF evolved?
The use of satellite data has greatly increased over the last few decades. Today such data collectively make the biggest contribution to forecast quality at ECMWF compared to other types of observations, such as those made with weather stations or aircraft data. Satellite observations do not measure variables such as temperature or ocean wave height directly. Instead, they measure quantities linked to these variables, such as radiances or radar echoes. ECMWF has come to play a leading role in extracting the maximum amount of information from such observations.
How are radiances assimilated into the Earth system model used by ECMWF?
Currently most satellite data used at ECMWF come from passive instruments measuring infrared or microwave radiances. Such radiances typically reflect conditions in a rather deep layer of the atmosphere and could be the result of any number of atmospheric states. To be able to compare the observations with the state of the atmosphere in the forecasting model, it is necessary to know what radiances would be observed if the model correctly stated the atmosphere.
