The utility of microwave sounding for numerical weather prediction (NWP) has been well established
over many years, with these instruments comprising a reliable and indispensable part of the
global observing system. But the volume of sounder data assimilated in NWP systems may change
substantially in upcoming years, as there is an ageing constellation of satellites but also the prospect
of proliferating small satellite constellations. This study examines the addition of temperature and
humidity sounders from a baseline that includes no microwave sounders at all, aiming to elucidate
the incremental benefit gained from adding sounders to the assimilation system. Framed as a series
of observing system experiments (OSEs), large improvements in forecast skill and background fits to
independent observations are gained from the addition of the first sounder. Significant further benefit
is observed from additional sounders, with no saturation apparent in the maximal setup.
In the study’s second part, the conclusion regarding lack of saturation from sounders is tested in the
full observing system with the most recently activated microwave sounder at ECMWF, the humidity
sounder on FY-3D. First, an assessment of data quality is given for both microwave sounders on FY-
3D, MWTS-2 and MWHS-2, followed by analysis of forecast impact from the humidity sounder in
research experiments prior to its activation in operations, which began December 2019. The success
of adding FY-3D MWHS-2 to the current suite of microwave sounders shows that additional sounder
observations still translate into improved forecasts, a finding that reinforces the conclusions of the
paper’s first half.