Rapid 500-mb Cyclogenesis and Anticyclogenesis

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.