Midtropospheric Closed Cyclone Formation over the Southwestern United States, the Eastern United States, and the Alps

Abstract

Observational composites of midtropospheric closed cyclone formation are constructed and diagnosed for three regions: the southwestern United States, the eastern United States, and the southern lee of the Alps. The spatial scales upon which closed cyclone formation occurs are then 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 scale (zonal waves 10-25). This analysis leads to a description of closed 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 cyclogenesis process is contained in the relative positioning of, and interaction between, preexisting jets and waves. In all regions the precursor wave pattern is characterized by a broad trough over the impending cyclone region, with the strongest meridional flow and implied geostrophic vorticity maximum located upstream of this trough axis. This flow configuration is associated with sustained cyclonic vorticity advection into the amplifying trough axis, and also provides a conduit by which intensifying transient short-wave trough-jet streak features can propagate into the downstream trough. A closed circulation then develops as the geostrophic wind speed maximum moves into the base of the trough and cyclonic vorticity becomes concentrated within the trough axis. This evolution also occurs coincident with the movement of the transient trough feature directly into the amplifying long-wave trough axis. In the southwestern United States and Alps cases, the favorable northwesterly flow configuration is initiated two days prior to closed cyclone formation by vigorous upstream wave amplification and by the rapid eastward movement of the upstream ridge axis relative to the downstream trough axis. Downstream of the cyclogenesis region, relatively modest anticyclogenesis, and modest mid- and lower-tropospheric thermal advection, is observed in these cases. In contrast, the favorable northwesterly flow configuration in the eastern United States cases is already established two days prior to closed cyclone formation. These cases are also characterized by vigorous downstream planetary-scale ridge amplification and a well-defined pattern of mid- and lower-tropospheric thermal advection.