Midtropospheric Cutoff Cyclogenesis.

The process of midtropospheric cutoff cyclone formation in the middle latitudes is investigated observationally. A cutoff cyclone is defined by the 500hPa height field as a trough at 500hpa having at least one closed 30m height contour around a central minimum value. The study is accomplished in three phases. In the first phase, the Northern Hemisphere distributions of cutoff cyclone (and anticyclone) centers are studied from a climatological perspective for the 15-year period between December 1962 and December 1977. The study documents frequency distributions, genesis and lysis distributions, and temporal variability of 500 hpa cutoff cyclone and anticyclone centers for all four seasons. In the second phase, Northern Hemisphere composite analyses spanning two days before to two days after cutoff formation are constructed to diagnose the large scale precursor environment and structural evolution associated with cutoff cyclones forming over three specific regions: the southwest United States, the eastern United States, and the southern lee of the Alps. An analysis of the dominant scale of motions associated with the cutoff events is performed by zonal Fourier decomposition of the composite 500hpa height fields into specific wave groups: the planetary scale (waves 0-3) the large synoptic scale (waves 4-9), and the small synoptic scale (waves 10-25). Interactions between these wave groups are examined through application of the quasi-geostrophic vorticity equation to the 500hpa composite wave group analyses. In the third phase, a case study diagnosis of a cutoff cyclone event occurring over the eastern United States between 17-21 January 1986 is performed. The main purpose of the case study is to examine the midtropospheric potential vorticity evolution associated with the process of cutoff formation in response to propagation of a northwesterly jet into the base of a downstream diffluent trough, as noted in the composite analyses. The quasi-geostrophic potential vorticity equation is utilized to examine the relative influence of upper--versus lower--level forcing on cutoff formation. An Eulerian budget analysis is then performed to diagnose the cutoff cyclogenesis process within the context of curvature and shear potential vorticity.