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  • Author or Editor: John R. Gyakum x
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John R. Gyakum and Ming Cai

Abstract

The phenomenon of strong vertical wind sheer (SWS) over the North American continent is documented with conventional rawinsonde data for the period 1 October 1983 through 31 March 1984. One of the objectives of this work is to describe the three-dimensional distribution of frontal zones. Since it is not possible to use the coarse horizontal resolution of the sounding network to document fully the existence of frontal zones, we instead use the excellent vertical resolution of the winds to infer the existence of these strong baroclinic zones through use of the thermal wind relation.

SWS, defined as a shear exceeding that associated with a horizontal temperature gradient of 1 5.5°C/500 km, is most frequently observed in the lower and upper troposphere, though secondary frequency peaks are observed in the middle troposphere. The SWS phenomenon is preferentially located in the middle latitudes where the strongest baroclinity is typically found. Monthly fields of SWS show the frequency maxima to he closely associated with the mean jet stream and strong baroclinity.

An examination of an exceptionally active five-day period of SWS activity shows a very strong baroclinic zone throughout the United States. The most active region of SWS in the eastern area is located in a zone of cyclonic wind shear downstream of a time-averaged though. This particular region of SWS is associated with explosive surface cylogenesis. A study of a corresponding period of exceptionally weak SWS activity reveals a markedly different synoptic pattern in which relatively weak wind maxima and baroclinity are shifted poleward into Canada; significantly weaker surface anticylones and no explosive cyclogenesis occurs. These results suggest that the phenomenon of SWS Phenomenon of SWS can be related to synoptic-scale weather patterns.

We demonstrate that SWS phenomenon is preferentially associated with Richardson numbers of less than 1, suggesting a strong association with turbulence. SWS events are associated preferentially with more stable lapse rates than are found otherwise. We also find that SWS events at 700 mb are associated approximately 65% of the time with relative humidities of 75% or less. This result suggests that much of the turbulence observed during these SWS events occurs in cloud-free conditions.

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Marco L. Carrera, John R. Gyakum, and Charles A. Lin

Abstract

The presence of orography can lead to thermally and dynamically induced mesoscale wind fields. The phenomenon of channeling refers to the tendency for the winds within a valley to blow more or less parallel to the valley axis for a variety of wind directions above ridge height. Channeling of surface winds has been observed in several regions of the world, including the upper Rhine Valley of Germany, the mountainous terrain near Basel, Switzerland, and the Tennessee and Hudson River Valleys in the United States. The St. Lawrence River valley (SLRV) is a primary topographic feature of eastern Canada, extending in a southwest–northeast direction from Lake Ontario, past Montreal (YUL) and Quebec City (YQB), and terminating in the Gulf of St. Lawrence. In this study the authors examine the long-term surface wind climatology of the SLRV and Lake Champlain Valley (LCV) as represented by hourly surface winds at Montreal, Quebec City, and Burlington, Vermont (BTV). Surface wind channeling is found to be prominent at all three locations with strong bidirectionalities that vary seasonally. To assess the importance of the various channeling mechanisms the authors compared the joint frequency distributions of surface wind directions versus 925-hPa geostrophic wind directions with those obtained from conceptual models. At YUL, downward momentum transport is important for geostrophic wind directions ranging from 240° to 340°. Pressure-driven channeling is the dominant mechanism producing northeasterly surface winds at YUL. These northeasterlies are most prominent in the winter, spring, and autumn seasons. At YQB, pressure-driven channeling is the dominant physical mechanism producing channeling of surface winds throughout all seasons. Of particular importance, both YUL and YQB exhibit countercurrents whereby the velocity component of the wind within the valley is opposite to the component above the valley. Forced channeling was found to be prominent at BTV, with evidence of diurnal thermal forcing during the summer season. Reasons for the predominance of pressure-driven channeling at YUL and YQB and forced channeling at BTV are discussed.

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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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