Evaluation and impact of aircraft humidity data in ECMWF's NWP system

Assimilation of temperature and wind data from aircraft is very well established, but about 10% of the commercial aircraft reports now include humidity data - mainly over North America so far but with some reports over Europe. This report primarily evaluates the quality and impact of aircraft humidity data on global NWP. Two different systems are available: 1) laser diode measurements, available in some AMDAR (Aircraft Meteorological DAta Relay) reports, mainly from long-haul aircraft), 2) TAMDAR (Tropospheric Airborne Meteorological Data Reporting) capacitive humidity measurements, mainly from short-haul aircraft with more ascent/descent profiles but less data in the upper troposphere.
Separate winter and summer experiments were performed at ECMWF assimilating quality controlled AMDAR humidity data and/or TAMDAR temperature/wind/humidity data 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 synoptic scale forecasts but some benefit at short range when compared to the US precipitation fields based on NEXRAD radar and surface observations. ECMWF has assimilated AMDAR humidity operationally since March 2016.