The influence of large-scale confluence and difluence on pressure drag

Few studies exist on the effect of synoptic-scale horizontal variations in the wind speed on mountain waves and gravity wave drag. In this presentation we examine the influence of large-scale horizontal confluence and difluence on the pressure drag exerted on the atmosphere by topography. We examine the drag in a series of steady-state and time-evolving simulations for various horizontally varying large-scale flow patterns over a 1.5 km high elongated mountain ridge. It is found that confluence increases drag relative to that generated by a uniform cross-mountain current. More dramatic declines in the drag result when the large scale flow is difluent.

The causes for these changes in pressure drag are investigated. In particular, the effect of the confluence and difluence on the mountain wave amplitude and wavelength are examined, as well as the relationship to wake effects such as downslope winds and lee vortices. Impacts on large-scale flow deceleration are also discussed.