HARMONIE-AROME 40h1.1 Upgrade

The operational Numerical Weather Prediction suite at Met Éireann was upgraded on the 1st of May 2018. Cycle 40h1.1 of the HARMONIE-AROME model was introduced, replacing the previous operational cycle 37h1.1. The major changes with the upgrade were an enlarged model domain and the introduction of 3D-Var data assimilation with 3-hour cycling. In preparation for this upgrade, extensive testing and validation of cycle 40h1.1 was carried out. Forecasts of representative periods were compared with the operational model. Initial tests with the default set-up revealed a significant cold bias in near-surface temperature forecasts. To reduce this bias, a number of modifications to the cycle 40h1.1 configuration were made. Changes to the turbulence scheme and the surface analysis improved the verification scores for 2 m temperature. These, however, had a negative impact on the accuracy of the near-surface wind-speed forecasts. This was addressed by tuning the surface drag effects. Pre-operational testing with month-long experiments produced an overall neutral or positive impact on the verification scores, when compared with the operational cycle 37h1.1. In addition, analysis of a number of case studies showed no particular disadvantages to the new set-up. The new operational cycle 40h1.1 was initially run in parallel with cycle 37h1.1. Performance scores for May 2018 were broadly neutral, showing both slight improvements in some fields, and some slightly negative effects in others.