Downslope Windstorms in Howe Sound: Two Case Studies

Howe Sound is a north-south oriented fjord located near Vancouver in southwest British Columbia, Canada. Prevailing wind in the sound is either northerly outflow or southerly inflow, driven mainly by synoptic-scale pressure gradients and strongly channelled by the topography. Occasionally, strong cross-channel southeast winds are observed. Although these rare windstorms could be predicted based on some rules of thumb, their detailed structures and underlying mechanisms remain poorly understood.

This work presents case studies of two southeast windstorms in Howe Sound, occurred on 18 January and 12 February 2010, respectively. The synoptic flow pattern in both cases is characterized by strong southeast winds in the boundary layer veering to southwesterlies in the lower troposphere. Dynamically, this flow pattern favours gravity wave breaking, leading to severe downslope windstorms into Howe Sound. Analysis of high-resolution numerical weather prediction model data confirms the development of a classical “bull's-eye” pattern of maximum winds along the mountain slope in the presence of large-amplitude gravity waves ducting in the lower troposphere. As the dry, warm downslope flow spread over the water, strong convection was triggered in the middle of the sound, leading to a positive feedback to the windstorm.

Downslope windstorms in Howe Sound not only cause severe marine damage over the water, but also have an impact on public safety along the Sea-to-Sky Highway across the mountain slope. Based on the results of these two case studies, a conceptual model is proposed to facilitate operational prediction of this kind of windstorms in Howe Sound and other similar coastal fjords.