A Diagnostic Study of Jet Streaks: Kinematic Signatures and Relationship to Coherent Tropopause Disturbances

Pyle, Matthew E.; Keyser, Daniel; Bosart, Lance F.
January 2004
Monthly Weather Review;Jan2004, Vol. 132 Issue 1, p297
Academic Journal
A diagnostic study is conducted of the kinematics and evolution of upper-level jet streaks representative of three (of the four) phases in the Shapiro conceptual model of a jet streak progressing through a synoptic-scale baroclinic wave over North America. The three phases selected for consideration apply to those segments of the wave pattern where jet streaks are relatively straight. The 1200 UTC 2 December 1991 case (trough-over-ridge) considers a strong jet streak located over eastern North America and constitutes the bulk of the study; the other two cases, which also concern jet streaks over North America, are from 0000 UTC 11 November 1995 (northwesterly flow) and 0000 UTC 28 October 1995 (southwesterly flow). Kinematic signatures consistent with the classic four-quadrant conceptual model for a straight jet streak are evident in all three cases, although flow curvature and thermal advection lead to significant departures from this conceptual model. The position of the jet streak within the synoptic-scale flow pattern also is shown to have a discernible influence on the kinematic fields, adding to the more localized effects of flow curvature and thermal advection in causing observed jet-streak kinematic signatures to depart from the four-quadrant conceptual model. The investigation of the evolution of the trough-over-ridge jet streak focuses on a vortexlike feature situated on the cyclonic-shear side of the jet streak and manifested as a localized mesoscale depression in the height of the dynamic tropopause (DT), corresponding to a local maximum of pressure on the DT. Complementary signatures of this vortexlike feature, referred to as a coherent tropopause disturbance (CTD), are a local minimum in potential temperature on the DT and a maximum in potential vorticity (PV) on tropopause-intersecting isentropic surfaces. In the trough-over-ridge case, a CTD is tracked for 17.5 days during which time it influences not only the jet streak considered for kinematic study but also one additional jet streak. The evolutions of the northwesterly and southwesterly flow jet streaks are also evaluated in relation to their association or lack thereof with CTDs. The northwesterly flow jet streak intensifies in the absence of a CTD, whereas the southwesterly flow jet streak is associated with a CTD that is tracked for 11.5 days and that participates in the intensification of one additional jet streak. In all three cases, the jet streaks coincide with large horizontal gradients of pressure and potential temperature on the DT and of PV on tropopause-intersecting isentropic surfaces. In the two cases involving CTDs, their role is to enhance these respective gradients over a mesoscale region; this enhancement appears to focus and strengthen jet-streak winds over the same region, suggesting the importance of CTDs in jet-streak evolution.


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