Evaluation and optimizaton of orographic drag in the IFS

In this study we validate and optimize the orographic low-level flow blocking and gravity wave drag parametrizations used in the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF), across a range of horizontal resolutions (10 to 100 km). We make use of km-scale simulations, that are able to largely resolve these orographic drag processes, building upon the protocol developed for the WGNE/GASS COORDE (Constraining Orographic Drag Effects) model comparison project. Km-scale simulations over the Middle East mountain chains were used in COORDE to evaluate the performance of the flow blocking and gravity wave drag parametrizations in a variety of models including the IFS. COORDE revealed, for example, that the IFS, along with several other models, underestimates orographic drag due to gravity wave breaking, particularly in the lower stratosphere. By constraining orographic drag associated with mountains with scales larger than 5 km, the COORDE km-scale simulations also helped to constrain the partition between low-level drag processes.

The insights gained from COORDE and from recent work using km-scale simulations at the UK MetOffice and ECMWF were used to revise the representation of orographic drag processes in the IFS. This consisted of a revision of the subgrid orography fields, and a repartitioning of the contributions of the low-level orographic drag processes (turbulent orographic form drag and flow blocking) to the surface drag. The revised subgrid orography fields now represent orographic features with scales smaller than the effective orographic resolution of the model rather than the grid-length. Additionally, the strength of the turbulent orographic form drag was decreased while low-level flow blocking drag was enhanced. With these revisions, the impact of the parametrized orographic drag on the zonal winds becomes more similar to the impact of the explicitly resolved drag from the kmscale simulations over the Middle East region, compared to the current version of the IFS. Moreover, with the new subgrid orographic fields and the retuned orographic drag parametrizations, the IFS exhibits a zonal momentum budget that is consistent across a range of resolutions. The impact of the revised representation of orographic drag on forecast performance was evaluated extensively across various applications used at ECMWF from data assimilation and medium-range forecasts to seasonal prediction. Improvements were found in key aspects of the Northern Hemisphere winter circulation at all timescales and resolutions used for applications at ECMWF.