Assimilation of Global Precipitation Mission (GPM) Microwave Imager (GMI) in all-sky conditions

This memo describes the assimilation of the Global Precipitation Measurement (GPM) microwave imager (GMI) in all-sky conditions at ECMWF. GMI is a well-calibrated microwave imager that provides a good reference for evaluating the quality of simulated brightness temperatures. Biases between GMI observations and ECMWF simulations are all within the expected calibration uncertainty of 0.8 K, except in the 89 GHz channels. Previous microwave imagers have shown much larger biases (order 2K to 4 K) so this is promising for the accuracy of the ECMWF model fields and observation operator, though more investigation is needed before final conclusions can be made. The assimilation of the GMI microwave imager channels in all-sky conditions was activated operationally in August 2015, and gives beneficial reductions in short and medium-range forecast errors of around 0.5%. Microwave imagers like GMI provide information on cloud and water vapour in the lower troposphere that is currently unique in the ECMWF analyses. The use of very long experiments has also given sufficient statistical power to confirm a small degradation of around 0.02% in the quality of dynamical forecasts in the lower-stratosphere, which further highlights a link between stratospheric dynamics and the assimilation of cloud and precipitation in the troposphere, most likely through tropical waves, although background error correlations cannot yet be ruled out. GMI also has humidity sounding channels, sensitive to the mid-troposphere. These will be activated in Cycle 43r3 in mid 2017, and their impact is beneficial but small, partly due to the great quantity of such data that is already being assimilated from sensors such as MHS.