Assimilating FY-3E MWHS-2 obs, and assessing all-sky humidity sounder thinning scales

Assimilating FY-3E MWHS-2 obs, and assessing all-sky humidity sounder thinning scales
Date Published
EUMETSAT/ECMWF Fellowship Programme Research Report
Document Number
Abstract Microwave observations sensitive to temperature and water vapour are one of the most important observation types in the four-dimensional variational assimilation scheme used at ECMWF. In this report we assess the forecast benefits of assimilating early-morning data from the Micro-Wave Humidity Sounder-2 (MWHS-2) instrument aboard the Fengyun-3E (FY-3E) satellite, and also investigate the optimal spatial thinning scale for microwave humidity sounder observations. These investigations will help to inform on how to extract the most benefit from both current and future microwave sounder instruments, such as those on the upcoming Metop second generation satellites and the EPS-Sterna constellation. We find that assimilating the FY-3E MWHS-2 data leads to short- and medium-range forecast improvements, particularly in the southern hemisphere, which are statistically-significant out to day 3. While some of the forecast improvement is due to lower noise compared to previous incarnations of the same instrument, dedicated assimilation experiments demonstrate that the early morning orbit of FY-3E, which complements the local times of other assimilated data, also plays a role. Due to the observed forecast improvements, the FY-3E MWHS-2 instrument became operationally active on 22 February 2023. We also find that the current spatial thinning scale for humidity sounding observations of 111 km can be reduced to ∼55 km. This results in less data being discarded and gives an improvement to forecasts out to day 3. Thinning scales smaller than ∼55 km result in degraded forecasts, likely due to neglected spatial observation error correlations.
DOI 10.21957/f42a9d9542