ECMWF steps up assimilation of aircraft weather data

Bruce Ingleby, Lars Isaksen, Mohamed Dahoui


On 8 March 2016, ECMWF started the operational assimilation of humidity data from commercial aircraft reporting via the AMDAR programme. This was one of the changes included in model cycle 41r2, which was made operational that day. About 11% of AMDAR reports include a usable humidity value, mainly from the US AMDAR programme but also from its European equivalent. The aircraft involved have a diode laser instrument installed to measure atmospheric water vapour.

The AMDAR programme

The Aircraft Meteorological Data Relay (AMDAR) programme is coordinated by the World Meteorological Organization (WMO) and is part of the Global Observing System. It uses predominantly existing aircraft on-board sensors and communication systems to collect, process and format meteorological data and to transmit them to ground stations.

The global AMDAR system produces about 700,000 observations per day of air temperature and wind speed and direction as well as an increasing number of humidity and turbulence measurements.

EUMETNET runs the E-AMDAR service to provide measurements of high-quality upper-air meteorological parameters from aircraft. 

EUMETNET, a grouping of 31 European national meteorological services that cooperate in activities including observing systems, is considering whether to install further humidity sensors. EUMETNET funded ECMWF to perform a study of the usefulness of aircraft humidity data, including a trial covering the period from April to June 2014.

Over North America, the quality of AMDAR humidity data was similar to or slightly better than that of radiosondes when compared with short-range forecasts. In the same region, the assimilation of AMDAR humidity data slightly improved short-range precipitation forecasts verified against radar/surface composite fields. The impact on standard verification scores was small, in part because the extra data are in already well-observed regions. Early in 2016, the European AMDAR humidity sensors were upgraded, and monitoring in ECMWF’s Integrated Forecasting System (IFS) helped in the early diagnosis of problems. The sensors now provide high-quality data.

New routes

Over the last few years, there has been a significant increase in the number of AMDAR reports. This has come primarily from US-based aircraft, but Europe and other regions are also contributing. Earlier this year, work by the WMO, EUMETNET-AMDAR and Météo-France resulted in 70 Air France and 60 British Airways Boeing 777 aircraft providing AMDAR data. In part these are replacing older aircraft, but there are also new routes over several data-sparse areas of the globe. These reports include wind and temperature but not humidity information. To minimise the communication costs involved, these aircraft do not report over Europe, which is already densely observed, but they do report over Africa, the Carribean and ocean areas.

Bias correction

In the IFS, aircraft data are currently the most important in situ data in terms of improving forecasts. This is despite the fact that, for reasons that are not fully understood, aircraft temperature reports are often biased towards higher values (typically by 0.2 to 0.5 degrees). The size of the bias depends on various factors. An improvement to the bias correction used at ECMWF is being prepared for future implementation.

Number of humidity observations
Number of humidity observations. The maps show the number of AMDAR aircraft humidity observations per 2°x2° square used at ECMWF in April 2016 for the upper troposphere (above 400 hPa) (left) and for the lower troposphere (below 700 hPa) (right). One flight route in Southeast Asia with a small number of observations is not shown.

Improvement in forecasts
Improvement in forecasts. The charts shows the normalised percentage difference between radar/weather station (SYNOP) observations and 6-hour accumulated precipitation forecasts without using aircraft humidity observations (left) and using such observations (right), in an experiment covering the period 1 April to 30 June 2014. More grey squares or less intense colours indicate improvements (smaller biases).

Number of wind observations
Number of wind observations. The map shows the number of AMDAR aircraft wind observations in the upper troposphere (above 400 hPa) per 2°x2° square recently made available from Air France and British Airways Boeing 777 aircraft and used at ECMWF between 9 March and 25 April 2016.