Enhancements to the assimilation of ATMS at ECMWF: Observation error update and addition of NOAA-20

The assimilation of microwave radiances from the Advanced Technology Microwave Sounder (ATMS) has been enhanced. Firstly by accounting for inter-channel error correlations and tuning the error variances for the ATMS onboard Suomi National Polar-orbiting Partnership (Suomi-NPP) satellite and secondly by additionally assimilating ATMS radiances from the recently launched NOAA-20 satellite.

Results from accounting for correlated errors show significant improvements to the first guess fits to independent observations including temperature sensitive observations (e.g. AMSU-A, radiosonde temperature), humidity sensitive observations (e.g. MHS, radiosonde humidity) and wind observations. This indicates the change is improving the accuracy of short range temperature, humidity and wind forecasts throughout the atmosphere. Also, extra-tropical geopotential height, wind and temperature forecasts are improved out to 5 days. These positive results led to this change being recommended for implementation with ECMWF cycle 46r1.

A thorough data quality assessment of the NOAA-20 ATMS instrument has been carried out and results indicate that the data before antenna pattern correction is of very good quality. Indications are that the biases are comparable and the newer instrument has lower noise than the Suomi-NPP ATMS for most channels. In addition, the striping present when looking at maps of first guess departures of Suomi-NPP ATMS observations is significantly reduced for NOAA-20 ATMS. Initial assimilation experiments indicate that assimilating the additional NOAA-20 ATMS observations on top of a system already assimilating Suomi-NPP ATMS observations leads to small incremental benefits to temperature, humidity and geopotential height forecasts, particularly in the stratosphere. These results led to the operational introduction of the assimilation of NOAA-20 ATMS data on 22nd May 2018.

The data after antenna pattern correction initially suffered from some asymmetry across the scan. This was caused by some parameters being inherited from Suomi-NPP ATMS whereas the NOAA-20 ATMS instrument turned out to have slightly different characteristics. Recently the antenna pattern correction has been updated and the data after this update look to be of good quality with smaller and more symmetric scan biases than Suomi-NPP ATMS.