An Observational Study of Strong Vertical Wind Shear over North America during the 1983/84 Cold Season

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.