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Rapid Surface Anticyclogenesis: Synoptic Climatology and Attendant Large-Scale Circulation Changes

Stephen J. ColucciDepartment of Environmental Sciences, University of Virginia, Charlottesville, VA 22903

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J. Clay DavenportDepartment of Environmental Sciences, University of Virginia, Charlottesville, VA 22903

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Abstract

The synoptic climatology of rapid surface anticyclogenesis, defined by a surface anticyclone pressure increase of at least 5 mb per 24 h, is investigated for the 1984 calendar year over the western portion of the Northern Hemisphere. In this sample, the phenomenon occurs preferentially during the cool season and over land, especially over northwestern North America and southeastern Canada. The northwestern North American events are associated with cold anticyclones downstream of amplifying 500-mb ridges, and most are followed by 500-mb trough amplification and cold air outbreaks over North America. Most of the southeastern Canadian events are each linked with a relatively warm anticyclone intensifying between a mobile upstream 500-mb trough and a stationary downstream 500-mb cutoff low, which is displaced downstream during local warming. A diagnosis of one such event reveals that large quasi-geostrophic height rises are observed at 500 mb near the rapidly intensifying surface anticyclone as the 500-mb cutoff low is ejected downstream. Comparison of this case with a similar example during which large height rises and rapid surface anticyclogenesis are not observed suggests the approaching trough may have to be at the same latitude and nearly the same size as the cutoff low in order for rapid surface anticyclogenesis and attendant cutoff displacement to be observed.

Abstract

The synoptic climatology of rapid surface anticyclogenesis, defined by a surface anticyclone pressure increase of at least 5 mb per 24 h, is investigated for the 1984 calendar year over the western portion of the Northern Hemisphere. In this sample, the phenomenon occurs preferentially during the cool season and over land, especially over northwestern North America and southeastern Canada. The northwestern North American events are associated with cold anticyclones downstream of amplifying 500-mb ridges, and most are followed by 500-mb trough amplification and cold air outbreaks over North America. Most of the southeastern Canadian events are each linked with a relatively warm anticyclone intensifying between a mobile upstream 500-mb trough and a stationary downstream 500-mb cutoff low, which is displaced downstream during local warming. A diagnosis of one such event reveals that large quasi-geostrophic height rises are observed at 500 mb near the rapidly intensifying surface anticyclone as the 500-mb cutoff low is ejected downstream. Comparison of this case with a similar example during which large height rises and rapid surface anticyclogenesis are not observed suggests the approaching trough may have to be at the same latitude and nearly the same size as the cutoff low in order for rapid surface anticyclogenesis and attendant cutoff displacement to be observed.

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