Impact of assimilating Mode-S EHS winds in the Met Office's high-resolution NWP model

Aircraft play an important role in providing in-situ upper air observations for use in numerical weather prediction (NWP) models. Aircraft observations of wind, temperature and humidity have been routinely assimilated into NWP models for decades. More recently, the Met Office has installed a network of six receivers to collect Mode-Select Enhanced Surveillance (Mode-S EHS) broadcasts and derive wind and temperature information. The paper focuses on the assimilation of high-frequency (sub-hourly) Mode-S winds into the Met Office's hourly cycling high-resolution NWP model, which employs four-dimensional-variational data assimilation (4D-Var). An observing system experiment (OSE) experiment was carried out to assess the impact of additionally assimilating Mode-S wind data on the model forecasts in both summer and winter. It is shown that assimilating Mode-S winds has a positive benefit on the forecast skill in the first 6 hr wind profiles, with an improvement of the mean bias and root mean square (RMS) errors of up to 0.5 m center dot s(-1) in the first hour and 0.3 m center dot s(-1) in the sixth hour. The assimilation of Mode-S winds is also seen to be beneficial to the near-surface-focused UK NWP composite index, and to the hourly precipitation accumulations in fractions skill score (FSS) for forecasts up to 9 hr ahead.