The benefits and challenges of using dense Mode‐ <scp>S</scp> aircraft winds at <scp>ECMWF</scp>

Abstract At the European Centre for Medium‐Range Weather Forecasts dense Mode‐S aircraft winds over Europe were introduced operationally in July 2020, earlier than planned because of loss of other observations during the COVID pandemic. Their use was switched off in November 2022, because we found that locally the density was too high, making the forecasts worse, despite the data being good quality. Diagnostic maps of observation fit to analysis were used to detect this and another problem. The ‘track thinning’ used for aircraft observations was not sufficient with the many hundreds of flights over Europe. Instead ‘box thinning’ developed for satellite data was adapted for aircraft use as a pragmatic solution. After extensive testing with the new thinning Mode‐S winds were switched on again in November 2023. The short‐ and medium‐range benefits of reintroducing Mode‐S winds with the thinning change are clear. At T + 12 the benefit is largest (up to 6% or 7%) for mid/upper‐tropospheric wind against European radiosondes but is also substantial for temperatures over Europe.