All-sky assimilation of Megha-Tropiques/SAPHIR radiances in the ECMWF numerical weather prediction system

The Megha-Tropiques mission is an Indo-French satellite, on a low-inclination orbit, which is dedicated to the study of tropical water and energy cycles. This mission carries a suite of payloads among which is the SAPHIR microwave humidity sounder. This instrument sounds the atmosphere with six channels around the 183.31GHz water vapour absorption band. The present study assesses the impact of SAPHIR radiance assimilation within the IFS 4D-Var data assimilation system, and it is performed within the all-sky framework which offers the unique capability of directly assimilating observations affected by clouds and precipitation. A first part of the study consists of specifying observation errors for this instrument, which provides a different set of channels compared to other microwave humidity sounders assimilated in the all-sky system. The impact of SAPHIR on ECMWF forecasts is evaluated over several periods of time and for two different IFS cycles, and is also compared to the impact of the four all-sky MHS sounders (which is evaluated by removing these observations in the band 30^oN to 30^oS). Analysis-based verification is difficult to interpret in the tropics, so the evaluation is based on fits to observations, including precipitation observations, for which a confidence test was derived. By most measures, SAPHIR produced similar benefit to activating the four MHS sounders in the tropics. Neither instrument type could significantly improve precipitation forecasts on its own, but assimilated together they improve short-range tropical precipitation skill by around 1%. When considering the activation of SAPHIR on top of the already very dense observing system including MHS and many other all-sky instruments, there was still beneficial impact on humidity fields. This shows that the impact of adding new all-sky microwave humidity sounding information has not yet saturated.