Synoptic and mesoscale processes supporting vertical superposition of the polar and subtropical jets in two contrasting cases

Abstract

Observational studies have shown that the tropopause characteristically exhibits a three‐step Pole‐to‐Equator structure, with each break between steps in the tropopause height associated with a jet stream. While the two jet streams, the polar and subtropical jets, typically occupy different latitude bands, their separation can occasionally vanish, resulting in a relatively rare vertical superposition of the two jets. A cursory examination of several historical and recent high‐impact weather events over N orth A merica and the N orth A tlantic suggests that superposed jets are a component of their evolution. This study examines the processes that support the production of a polar/subtropical jet superposition during two such events; the 18–20 D ecember 2009 M id‐ A tlantic B lizzard and the 1–3 M ay 2010 N ashville F lood. Given that ageostrophic transverse circulations and convection have both been shown to be capable of restructuring the tropopause within a single jet environment, the analysis focuses on the role these same processes play within the more complex double‐jet environment. The results demonstrate that ageostrophic transverse circulations play a primary role in the production of a superposition during the D ecember 2009 case by placing subsidence, and a downward protrusion of high potential vorticity ( PV ) air, between the two jet cores, thereby contributing to the production of the single, steep tropopause wall characteristic of the superposed jet environment. Furthermore, convection fundamentally influences the existence and structure of the subtropical jet stream in both cases, through its associated latent heat release and irrotational outflow as well as the geostrophic adjustment process that responds to upper‐tropospheric mass deposition from convection on the anticyclonic shear side of the jet.