ABSTRACT

Deformation of the tropopause plays a fundamental role in the development of a variety of weather systems. Among the physical and structural mechanisms known to promote local deformation and steepening of the tropopause are the development of upper-level jet/front systems (ULJFs), the superposition of polar and subtropical ULJFs, and, more recently, the encroachment of coherent tropopause disturbances (CTDs) upon pre-existing polar or subtropical gradients of potential temperature on the dynamic tropopause. When viewed from any of these perspectives, tropopause steepening necessarily involves structural and dynamical evolution of the lower stratospheric frontal zone above the jet core. It is hypothesized that such changes have consequences for the local deformation of the tropopause and, accordingly, for the development of lower tropospheric weather systems.

This conjecture, coupled with a nearly complete absence of prior research focus on these features, motivates the research in which the structure, evolution, and life cycle of the lower stratospheric frontal zones associated with upper-level jet/front systems (ULJFs), as well as their influence on local tropopause deformation, extratropical development, stratosphere/troposphere exchange and their relationship to coherent tropopause disturbances (CTDs) will be examined. The investigation will employ both synoptic-climatological and case study approaches using the National Center for Environmental Prediction's Global Final Analysis (FNL) data as well as output from fine-scale numerical simulations of selected events performed using the National Center for Atmospheric Research's Weather Research and Forecasting (WRF) model.

The FNL analysis will be used to both examine the relationship between extreme surface cyclogenesis and tropopause steepness as well as to investigate the vertical structure of strong and extreme CTDs which is hypothesized to have an important influence on lower stratospheric frontal structure and tropopause slope. Employing output from fine-scale WRF simulations of selected cases of ULJF life cycles, the influence of lower stratospheric frontal circulations on local deformation of the tropopause above the jet core will be examined. This analysis will be augmented by a piecewise potential vorticity (PV) inversion, performed using the same model output, that will be used to examine the circulations associated with the perturbation PV of the upper tropospheric and lower stratospheric frontal zones of the ULJF, respectively. Isolation of these separate circulations will lend insight into the nature of the separate frontal developments, the influence each evolving structure has on the other, on tropopause deformation above and below the jet core, as well as on surface cyclogenesis. The WRF-Chemsitry model will be used to examine the influence of varying lower stratospheric frontal structures and their associated circulations on the degree of stratosphere/troposphere exchange.

By involving both graduate and undergraduate students, the research will advance discovery, learning, teaching, training and diversity within this subfield. The results of the research will be disseminated widely through scholarly publications and theses. The resulting increased understanding of the structure, evolution and dynamics of the lower stratospheric frontal zones associated with ULJFs will provide new insights into the role of lower stratospheric processes in deforming the tropopause and, consequently, in driving the sensible weather in the mid-latitudes. Thus, the research represents first steps towards understanding of an understudied problem with direct impacts on the life cycles of extratropical cyclones. As such, its pursuit potentially will provide a benefit to society by enhancing the ability to forecast weather that is directly and indirectly associated with these disturbances.