MIDTROPOSPHERIC CLOSED CYCLONE FORMATION OVER THE SOUTHWESTERN UNITED-STATES, THE EASTERN UNITED-STATES, AND THE ALPS

Observational composites of midtropospheric closed cyclone formation a rc constructed and diagnosed for three regions: the southwestern Unite d States, the eastern United States, and the southern lee of the Alps. The spatial scales upon which closed cyclone formation occurs are the n examined by zonally decomposing the composite 500-hPa height fields into three distinct wave groups: the planetary scale (zonal waves 1-3) , the large synoptic scale (zonal waves 4-9), and the small synoptic s cale (zonal waves 10-25). This analysis leads to a description of clos ed cyclogenesis as a combined wave interaction and wave superposition process involving both wave groups 4-9 and 10-25, which is intimately linked to preexisting along-stream speed variations and flow curvature . This description is inconsistent with modal and nonmodal analytical instability theories of cyclogenesis. The essence of the closed cyclog enesis process is contained in the relative positioning of, and intera ction between, preexisting jets and waves. In all regions the precurso r wave pattern is characterized by a broad trough over the impending c yclone region, with the strongest meridional flow and implied geostrop hic vorticity MaXimum located upstream of this trough axis. This flow configuration is associated with sustained cyclonic vorticity advectio n into the amplifying trough axis, and also provides a conduit by whic h intensifying transient short-wave trough-jet streak features can pro pagate into the downstream trough. A closed circulation then develops as the geostrophic wind spect maximum moves into the base of the troug h and cyclonic vorticity becomes concentrated within the trough axis. This evolution also occurs coincident with the movement of the transie nt trough feature directly into the amplifying long-wave trough axis. In the southwestern United States and Alps cases, the favorable northw esterly flow configuration is initiated two days prior to closed cyclo ne formation by vigorous upstream wave amplification and by the rapid eastward movement of the upstream ridge axis relative to the downstrea m trough axis. Downstream of the cyclogenesis region, relatively modes t anticyclogenesis, and modest mid- and lower-tropospheric thermal adv ection, is observed in these cases. In contrast, the favorable northwe sterly flow configuration in the eastern United States cases is alread y established two days prior to closed cyclone formation. These cases are also characterized by vigorous downstream planetary-scale ridge am plification and a well-defined pattern of mid- and lower-tropospheric thermal advection.