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Stephen J. Colucci and J. Clay Davenport

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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Timothy L. Alberta, Stephen J. Colucci, and J. Clay Davenport

Abstract

Rapid intensification of cyclones and anticyclones in National Meteorological Center 500-mb height analyses is investigated during the 1983–84 through 1986–87 cool seasons (September through May). Rapid intensification is defined by analyzed height decreases (increases) of at least 10(5) dam in 24 h at the centers of 500-mb cyclones (anticyclones), defined here by closed contours at 6-dam intervals around low (high) 500-mb heights. The threshold 24-h central height changes for rapidly intensifying 500-mb cyclones (anticyclones) are two standard deviations lower (higher) than the mean central height changes for all 500-mb cyclones (anticyclones) during the 1983–84 cool season. Rapid 500-mb cyclogenesis is climatologically preferred over the northwestern Pacific Ocean and northeastern North America, while anticyclogenesis is favored in high latitudes with little longitudinal preference, but with some concentration in frequency near Alaska and Greenland. Both phenomena are observed preferentially north of the 546-dam contour at 500 mb; thus, most rapidly intensifying cyclones (anticyclones) apparently are not (are) cut off from the 500-mb westerlies. On the average, rapidly intensifying 500-mb cyclones (anticyclones) are associated with intensifying surface cyclones (anticyclones). The surface cyclones associated with rapid 500-mb cyclogenesis are more likely to be rapidly developing than is expected climatologically. In a limited sample it is seen that, on the average, rapid 500-mb cyclone and anticyclone developments are underpredicted by operational forecast models. A case of nearly simultaneous 500-mb rapid cyclogenesis and anticyclogenesis over the Atlantic Ocean during, January 1985 is diagnosed with a quasi-geostrophic model. Neither system is cut off from the westerlies, and both are associated with rapidly intensifying surface counterparts. The 500-mb anticyclone in turn is preceded by (nearly rapid) surface cyclognesis upstream and evolves into a blocking pattern. The 500-mb height tendencies at the center of the rapidly intensifying 500-mb cyclone are forced primarily by lower-tropospheric cold air advection decreasing upward in magnitude beneath the system. On the other hand, the quasi-geostrophic tendencies underestimate those analyzed at the anticyclone center, suggesting the importance of non-quasi-geostrophic processes in rapid 500-mb anticyclone development.

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