Nonlin. Processes Geophys., 15, 33–52, 2008 www.nonlin-processes-geophys.net/15/33/2008/ Nonlinear Processes © Author(s) 2008. This work is licensed in Geophysics under a Creative Commons License. Channelling of high-latitude boundary-layer flow N. Nawri and R. E. Stewart Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Canada Received: 24 August 2007 – Revised: 14 December 2007 – Accepted: 21 December 2007 – Published: 31 January 2008 Abstract. Due to the stability of the boundary-layer 1 Introduction stratification, high-latitude winds over complex terrain are strongly affected by blocking and channelling effects. Con- Strong and variable surface wind conditions are among the sequently, at many low-lying communities in the Canadian main weather hazards in the Arctic. They cause dangerous Archipelago, including Cape Dorset and Iqaluit considered flying conditions, significant low-level visibility reduction in in this study, surface winds for the most part are from two di- blowing snow and, through snow accumulation, may block ametrically opposed directions, following the orientation of roads and bury entire buildings. Near the coast, where most the elevated terrain. Shifts between the two prevailing wind communities are located, strong and variable surface winds directions can be sudden and are associated with geostrophic often have a significant impact on sea state and sea ice con- wind directions within a well defined narrow range. To quan- ditions, affecting again important transport and travel routes. titatively investigate the role of large-scale pressure gradi- Due to these hazardous impacts there are concerns about ents and the quasi-geostrophic overlying flow, an idealised changing prevailing local wind conditions in the context of dynamical system for the evolution of channelled surface global environmental change. Already, vulnerabilities and winds is derived from the basic equations of motion, in which limits to adaptive capacity due to stronger and more variable stability of stationary along-channel wind directions is de- surface wind conditions have been identified in some com- scribed as a function of the geostrophic wind. In comparison munities of Canada’s Nunavut Territory (Ford et al., 2006a; with long-term horizontal wind statistics at the two locations Ford et al., 2006b; Henshaw, 2006; Laidler and Elee, 2006). it is shown that the climatologically prevailing wind direc- The prevailing surface wind, and to some degree general tions can be identified as stationary states of the idealised weather conditions, at many Arctic communities are affected wind model, and that shifts between prevailing wind direc- not only by the proximity to the sea, but also by the surround- tions can be represented as stability transitions between these ing terrain. As a result, orographic modification of low-level stationary states. In that sense, the prevailing local wind con- winds and particularly blocking and channelling effects on ditions can be interpreted as attracting states of the actual stably stratified flow are noticeable in the prevailing wind flow, with observed surface winds adjusting to a new stable conditions at many locations of the Canadian Arctic (Hud- direction as determined by the idealised system within 3– son et al., 2001), such that surface winds for the most part 9 h. Over these time-scales and longer it is therefore advan- are from two diametrically opposed directions, and shifts tageous to determine the relatively slow evolution of the ob- between the two prevailing wind directions can be sudden. servationally well-resolved large-scale pressure distribution, In that context, channelling refers to flow conditions under instead of modelling highly variable surface winds directly. which wind directions are approximately perpendicular to The simplified model also offers a tool for dynamical down- the local elevation gradient. scaling of global climate simulations, and for determining Due to the sparse available data, detailed modelling of the future scenarios for local prevailing wind conditions. In par- wind field at specific high-latitude locations is not practica- ticular, it allows an estimation of the sensitivity of local low- ble on an operational basis, particularly over complex terrain. level winds to changes in the large-scale atmospheric circu- Given the importance of strong and variable surface winds lation. across the Arctic, the goal of this study therefore is to develop a simple conceptual model for stably stratified orographic boundary-layer flow, which can be implemented for short- term local wind forecasting and downscaling of large-scale climate predictions more easily than high-resolution numer- Correspondence to: N. Nawri ical simulations. Thereby, the focus is on large-scale driven ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union and the American Geophysical Union. 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