Observations of cross‐ridge flows across steep terrain

A field experiment, Gaudex, has been conducted to address the need for quantitative measurements of turbulence in the vicinity of steep terrain. A dense network of automatic weather stations and turbulence towers was deployed along cross‐ridge transects over Gaudergrat, a steep triangular cross‐section ridge in eastern Switzerland. A new feature, whereby ridge‐normal cross‐ridge flows develop at the crest even when the flow in each valley is oriented parallel to the ridge axis, is identified. This occurs independently of whether the flow is thermally or synoptically driven. Pressure measurements across the ridge show that this flow is driven by a cross‐ridge pressure gradient. Two mechanisms for generating cross‐ridge flows have been identified from measurements of pressure, wind and temperature. In most cases the pressure gradient arises from a difference in flow speeds between the two sides of the ridge, caused by different valley geometries. Less commonly, the cross‐ridge flow is explained by linear speed‐up of the ridge‐normal flow.