The initial goal of the WGNE Drag Project was to compare parametrizations associated with surface drag, i.e. the schemes currently employed by NWP and climate models to compute subgrid-scale surface stresses and their impact on the atmospheric flow. This exercise represented a first step towards a broader goal, namely to draw the attention of the scientific community to the importance of, and the challenges in correctly representing momentum transfer in atmospheric models.

The project was initially motivated by two observations made during recent WGNE meetings. On the one hand, some WGNE members have reported how adjustments to momentum-related schemes (e.g. orographic blocking and boundary layer schemes) led to significant improvements to large-scale forecasts in operational centres. On the other hand, it was noted that the majority of inter-comparison projects and international initiatives to develop physical parametrizations tended to focus on thermodynamic and/or microphysics processes, while very few have involved momentum-related parametrizations and processes.

In this presentation, the design of and the preliminary results from the Drag Project will be described. An overview of momentum-related parametrizations commonly used in the operational centres and their impact on forecasts will be presented, as well as examples of the uncertainties – and the challenges that modelers face to constrain the uncertainties – in those parametrizations.

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*email: ayrton.zadra@ec.gc.ca
*Preference: Oral