An Examination of Extreme Cold Air Outbreaks over Eastern North America

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  • 1 Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia
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Abstract

The sequence of development and thermodynamic aspects of two strong cold air outbreaks over eastern North America during January 1977 are described. In the first outbreak, surface cyclogenesis occurs prior to the outbreak onset (local 850 mb temperature decreases). Regions of strong cold air advection and adiabatic warming are found immediately upstream of the cyclone over the cold air outbreak area. Since the two regions are nearly superimposed, the effect of advective cooling is partially opposed by adiabatic warming. In the second outbreak, surface cyclogenesis follows the outbreak onset. In this case, local and advective cooling is observed over a larger region as a cold air pool over central Canada is transported southeastward. Initially, adiabatic warming is weak or replaced by adiabatic cooling over eastern North America as cold air advection dominates the thermodynamic energy balance. As downstream cyclogenesis proceeds during the latter stages of the outbreak, adiabatic warming intensifies over the cold air advection region, reducing the cooling effect. An inspection of 15 other strong cold air outbreaks in a ten-winter sample reveals a qualitatively similar relationship among the size of the cold air advection region, timing of the surface cyclogenesis and areal averaged 850 mg temperatures. It is hypothesized that the intensity of cold air outbreaks over eastern North America is proportional to the areal coverage of the cold air advection region and timing, relative to cold air advection onset, of surface cyclogenesis.

Abstract

The sequence of development and thermodynamic aspects of two strong cold air outbreaks over eastern North America during January 1977 are described. In the first outbreak, surface cyclogenesis occurs prior to the outbreak onset (local 850 mb temperature decreases). Regions of strong cold air advection and adiabatic warming are found immediately upstream of the cyclone over the cold air outbreak area. Since the two regions are nearly superimposed, the effect of advective cooling is partially opposed by adiabatic warming. In the second outbreak, surface cyclogenesis follows the outbreak onset. In this case, local and advective cooling is observed over a larger region as a cold air pool over central Canada is transported southeastward. Initially, adiabatic warming is weak or replaced by adiabatic cooling over eastern North America as cold air advection dominates the thermodynamic energy balance. As downstream cyclogenesis proceeds during the latter stages of the outbreak, adiabatic warming intensifies over the cold air advection region, reducing the cooling effect. An inspection of 15 other strong cold air outbreaks in a ten-winter sample reveals a qualitatively similar relationship among the size of the cold air advection region, timing of the surface cyclogenesis and areal averaged 850 mg temperatures. It is hypothesized that the intensity of cold air outbreaks over eastern North America is proportional to the areal coverage of the cold air advection region and timing, relative to cold air advection onset, of surface cyclogenesis.

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