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  • Author or Editor: Eyad Atallah x
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David Small, Eyad Atallah, and John Gyakum

Abstract

The community of Tuktoyaktuk (Northwest Territories, Canada) along the Beaufort Sea experiences dramatic shoreline erosion during storm surge events that tend to occur during persistent northwesterly wind events in the late summer months (July–September) when the sea ice coverage of the Beaufort Sea reaches its annual minimum. This study compiles the climatology of hourly surface wind, low-level geostrophic wind, and static stability to investigate the physical mechanisms responsible for the high frequency of northwesterly winds observed at Tuktoyaktuk during the late summer. The results link the prevalence of westerly to northwesterly winds at the surface to the high frequency of northwesterly geostrophic winds and a tendency for low static stability. With an environment that favors strong northwesterly geostrophic wind and suggests lower static stability, the high frequency of strong northwesterlies observed at the surface appears to be associated with momentum mixing by turbulent eddies. A composite analysis indicates that persistently strong northwesterly winds are associated with anomalously low pressure northeast of Tuktoyaktuk and high pressure over the Bering Sea and eastern Siberia. The high pressure anomalies over the Bering Sea also extend well to the east along the northern edge of the Brooks Range. An apparent topographic modification of the sea level pressure (SLP) field by cold air trapped to the north of mountains produces the pressure gradient favorable for strong westerly to northwesterly geostrophic winds at Tuktoyaktuk. The results suggest that cold-air damming contributes to the wind regime at Tuktoyaktuk by altering the pressure gradient along the Beaufort coast.

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Alissa Razy, Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

Abstract

Orographic wind channeling, defined as dynamically and thermally induced processes that force wind to blow along the axis of a valley, is a common occurrence along the St. Lawrence River Valley (SLRV) in Quebec, Canada, and produces substantial observed weather impacts at stations along the valley, including Montreal (CYUL). Cold-season observed north-northeast (n = 55) and south-southeast (n = 16) surface wind events at CYUL are identified from 1979 to 2002. The authors partition the north-northeast wind events into four groups using manual synoptic typing. Types A and D (“inland cyclone” and “northwestern cyclone”) are associated with strong lower-tropospheric geostrophic warm-air advection and near-surface pressure-driven channeling of cold air from the north-northeast, along the axis of the SLRV. Type C (“anticyclone”) shows no evidence of a surface cyclone and thus is the least associated with inclement weather at CYUL, whereas type B (“coastal cyclone”) is associated with predominantly forced wind channeling along the SLRV. Type D of the north-northeast wind events and all south-southeast wind events exhibit similar sea level pressure patterns. The respective magnitudes of the pressure gradients in the Lake Champlain Valley south of CYUL and the SLRV play a large role in determining the favored wind direction. Soundings of the various event types illustrate substantial differences in temperature structure, with a large near-surface temperature inversion particularly prevalent in north-northeast events. The results of this study may provide guidance in forecasting winds, temperatures, and observed weather in and around the SLRV, given certain synoptic-scale regimes.

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