Initiation and Evolution of an Intense Upper-Level Front

Abstract

Within confluent northwesterly flow of an intensifying baroclinic wave over North America in late October 1963, an intense frontal zone developed in 12 h near the inflection point in the middle and upper troposphere. By 24 h after its initial appearance, the zone extended roughly from 400 to 700 mb and from the inflection point to just beyond the downstream trough. Horizontal gradients of potential temperature reached 15?20 K (100 km)?1. Air within the frontal zone was extremely dry. As the accompanying trough approached the east coast of he United States, surface frontogenesis occurred offshore, remaining distinct from the upper-level front. A region of subsidence, elongated in the direction of the upper-level flow, displayed maximum descent on the warm edge of the frontal zone and played a frontogenetical role through tilting of the isentropic surfaces. Analysis of isentropic potential vorticity showed significant increase of this quantity near the cold base and a probable decrease near the top as the front developed. Turbulent beat flux, associated with reduced Richardson numbers within the frontal zone, was likely responsible for this nonconservation of potential vorticity and for the propagation of the zone to lower colder values of potential temperature. Vertical wind shear through the frontal layer was supergeostrophic in the upper ridge and subgeostrophic in the trough. An inertial oscillation at the top of the layer began as air in the ridge flowed toward lower geopotential, forming a jet streak and then flowing toward higher geopotential near the inflection point, a region of intense individual frontogenesis.