Extra aircraft data including humidity: ECMWF evaluation of quality and impact

Assimilation of temperature and wind from aircraft is very well established, but about 10% of the commercial aircraft involved are now providing humidity data - mainly over North America so far but with some reports over Europe. Two different systems are available: 1) WVSS-II - a laser diode measurement, available in some AMDAR/ACARS reports, mainly from long-haul aircraft, 2) capacitive humidity measurements are available with most TAMDAR reports, mainly from short-haul aircraft with more ascent/descent profiles but less data in the upper troposphere (TAMDAR reports are proprietary and are currently being evaluated but they are not used operationally at ECMWF). Capacitive sensors react more slowly at low temperatures (i.e. upper troposphere).

Separate winter and summer trials (three months each) were performed at ECMWF assimilating quality controlled AMDAR humidity and/or TAMDAR temperature/wind/humidity in addition to the data used operationally. Observation minus background statistics were compared between AMDAR, TAMDAR and radiosonde reports over North America. The results suggest that after quality control the accuracy of aircraft humidity is similar to that of the radiosondes - slightly better in the case of AMDARs. The extra data had rather minor impact on large scale synoptic forecast fields but some benefit at short range when compared to the US NEXRAD radar precipitation fields.