A new diagnostic approach, piecewise tendency diagnosis (PTD), is employed to study the dynamics of weather regime transitions occuring during Northern Hemisphere Winter. Developed by Nielsen-Gammon and Lefevre (1996), PTD extends piecewise potential vorticity inversion by first partitioning the local time tendency of quasi-geostrophic potential vorticity (QGPV) into an array of advective and nonconservative forcing terms. These forcing terms are then inverted separately to obtain the associated three-dimensional height tendency. The diagnostic is based upon the QGPV balance and conservation relations and is used to isolate individual mechanistic influences upon the atmospheric height tendency. This provides a quantitative methodology for assessing the contribution of individual dynamic and physical sources towards local height tendency. In this study PTD is extended to include the effects of diabatic heating, ageostrophic forcings, and spherical geometry.
In our study PTD is applied to study both the composite and individual case dynamics of the formation of persistent cyclonic flow anomalies over the North Pacific. Our results indicate that the calculated height tendency patterns matches the observed tendency of the individual case and composite very well. In both situations, linear geostrophic advective terms provide the largest dynamic forcing. In particular, both the individual and composite cases develop largely from linear baroclinic processes, with spatial deformation processes providing a smaller first-order contribution. PTD analyses of the composite events provide results that are consistent with previous synoptic and diagnostic studies. For both the composite and case analyses, the QG non-linear and non-QG advection terms played a secondary but nonnegligible role in intensification. The non-QG vertical advection of stratification anomalies generally opposes development, but is weak. The effect of diabatic heating was small in both cases, acting to intensify the low in the individual case and slow development in the composite.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics