Climatological features of orographic low‐level jets over frobisher bay

Abstract

In this study, based on an analysis of operational hourly surface and 12-hourly upper air data, the seasonal and diurnal climatological features of orographic low-level jets and associated vertical wind shear over Frobisher Bay, southern Baffin Island are discussed. These strong winds are a common phenomenon and frequently lead to hazardous flying conditions and hardship on the ground. The influence of the surrounding topography causes significant channelling of the surface wind at Iqaluit, with prevailing wind directions consequently being from the northwest and southeast. Shifts between the dominant wind directions occur with surface geostrophic winds roughly perpendicular to the valley axis, indicating the controlling influence of large-scale pressure gradients and vertical momentum transfer, in combination with local channelling effects, on the forcing of weak and strong surface winds. Following seasonal shifts in the large-scale surface pressure distribution, there is a shift in prevailing wind direction from northwest in winter to southeast in summer. The importance of the topography for the forcing of boundary-layer flows is also evident in the presence of speed maxima at the typical ridge-top level and below, associated with strong along-valley surface winds. The adjustment of the channelled boundary-layer flow to the large-scale flow conditions above the topography frequently leads to strong vertical wind shear and low-level turbulence. Distinct characteristics of weak and strong surface winds from different directions at Iqaluit are identified, including differences in vertical wind shear and the relationship of these winds to the surface geostrophic wind. RÉSUMÉ [Traduit par la rédaction] Dans cette étude, basée sur une analyse des données opérationnelles de surface horaires et aérologiques aux 12 heures, nous discutons des caractéristiques climatologiques saisonnières et journalières des courants-jets orographiques à basse altitude et du cisaillement vertical du vent qu'ils produisent dans la baie Frobisher, dans le sud de l'île de Baffin. Ces vents forts sont un phénomène courant et occasionnent fréquemment des conditions de vol dangereuses et des difficultés au sol. La topographie environnante est souvent à l'origine d'un important effet de canal sur le vent de surface à Iqaluit et c'est pourquoi les directions dominantes du vent sont le nord-ouest et le sud-est. Les sautes entre les directions dominantes du vent se produisent avec des vents géostrophiques de surface à peu près perpendiculaires à l'axe de la vallée, ce qui révèle l'influence déterminante des gradients de pression à grande échelle et du transfert vertical de quantité de mouvement, conjointement avec l'effet de canalisation local, sur le forçage des vents de surface faibles et forts. Les changements saisonniers dans la distribution de la pression à la surface à grande échelle s'accompagnent d'un changement dans la direction dominante du vent, du nord-ouest en hiver au sud-est en été. L'importance de la topographie pour le forçage de l...

Simulating wind channelling over Frobisher Bay and its interaction with downslope winds during the 7–8 November 2006 wind event

“…1), and they attributed the dominance of the strong northwesterly and north-northwesterly surface winds in winter to topographically induced wind channelling. In agreement with Hudson et al (2001), the analysis of surface winds recorded at Iqaluit over a 21-year period performed by Nawri and stewart (2006) revealed that the northwesterly and southeasterly surface winds together accounted for more than 80% of all hourly observations of strong surface winds (>10 m s -1 ). The highest occurrence of strong winds was in October-March.…”

“…Moreover, Nawri and stewart (2008) found that cyclones provided conditions, through the associated large-scale pressure field, for both sustained and strong surface winds at Iqaluit along either the northwest or southeast direction and sudden shifts between these prevailing directions. Nawri and stewart (2006) also investigated the vertical structure of winds with surface speeds greater than 10 m s -1 using operational 12 h soundings at Iqaluit. The mean vertical profiles corresponding to northwesterly surface winds during the winter (January-March) showed an orographic low-level jet reaching its maximum speed at the surface.…”

“…Whiteman and Doran (1993), for instance, summarized the mechanisms that produce channelled winds, namely, downward momentum transfer, thermally driven, pressure-driven, and forced channelling. These allowed for the explanation of the highly bidirectional wind regimes in valleys from climatological data available at relatively few locations: Frobisher bay (Nawri and stewart, 2006), The Owens Valley (Zhong et al, 2008), and the st. Lawrence River Valley (Carrera et al, 2009). However, the conceptual models of classic flows induced by topography may have limited skill in describing the complex flow structure and dynamics of the low-level winds resulting from various combinations of three-dimensional topography and meteorological conditions.…”

Storm Studies in the Arctic (STAR)

“…Topographic effects can also result in high wind speed (gap and barrier flows) and significant precipitation events (e.g., Martin and Moore 2005; Nawri and Stewart 2006. Occasionally, strong northeasterlies may reach the surface in Iqaluit, or they may generate intense horizontal vortices (rotors) at the sharp rise of the terrain (e.g., Ozawa et al 1998;Hudson et al 2001).…”

“…Climatologically, wind speeds above 90 km h −1 occur fewer than once a year, in either autumn or winter. However, strong low-level jets with hourly wind speeds greater than 60 km h −1 are reported about 25 times a year on average, mainly from autumn to spring and primarily from the northwest Stewart 2006, 2008).…”