Mode-S aircraft data now provide high-density coverage of wind and temperature over large parts of Europe (see the figure in the preceding article). The Royal Netherlands Meteorological Institute (KNMI) has been working on Mode-S data for about ten years and the UK Met Office for almost as long. In recent years, a wider, more coordinated approach has been supported by EUMETNET and, partly funded by the EU, KNMI set up the European Meteorological Aircraft Derived Data Centre (EMADDC). EMADDC has an operational product centred on the Netherlands and a test product covering much of Europe. When the numbers of flights over Europe reduced dramatically in March and April, EMADDC consulted with its partners, suppliers and EUMETNET and decided to make the test product available to users for now. For the same reasons, ECMWF brought forward plans to process and assess the data.
What is Mode-S?
In some Air Traffic Control (ATC) regions, Mode-S Enhanced Surveillance (Mode-S EHS) reports are broadcast by aircraft in response to interrogation from ATC radar. These can be processed to derive wind and temperature. The quality of the wind data is similar to that of other aircraft reports, the temperature quality is somewhat worse because of the indirect reporting. The obvious feature of Mode-S data is the high data density: locally there can be about one hundred times more Mode-S than AMDAR reports. This is partly because of the high frequency of reports (up to every four seconds) and partly because almost all commercial aircraft have to respond, but only a subset provide AMDAR reports. Mode-S winds have been used in several limited-area forecasting systems.
Testing at ECMWF
ECMWF started to archive the data on 25 April and has since carried out testing and assimilation experiments after making various minor software changes. After using the standard thinning routine, about 4.5% of Mode-S reports were assimilated. One test assimilated the winds only (ModeS:UV), another assimilated the temperatures as well (ModeS:UVT). The largest impact is in the upper troposphere, as expected. Twelve-hour wind forecasts are improved by up to 6% compared to European radiosondes and up to 3 or 4% for temperatures. Most of the impact comes from Mode-S winds, while assimilation of Mode-S temperatures has very little effect on wind forecast scores but gives a slight improvement in upper tropospheric temperatures. We hope to start operational assimilation of Mode-S winds at ECMWF in July and to work on other aspects, such as the weighting and use of the temperatures, in the near future. As the number of aircraft flights recovers slowly, the numbers of both AMDAR and Mode-S reports over Europe will increase and the balance between them will be kept under review. There is also the prospect of acquiring Mode-S data from some other parts of the world, such as Australia.