The Coupling of Upper and Lower Tropospheric Jet Streaks and Implications for the Development of Severe Convective Storms

Louis W. Uccellini Space Science and Engineering Center, University of Wisconsin, Madison 53706

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Donald R. Johnson Space Science and Engineering Center and Department of Meteorology, University of Wisconsin, Madison 53706

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Abstract

Transverse circulations in the exit and entrance regions of jet streaks are investigated through numerical simulation, a case study, and an application of the isallobaric wind equation in isentropic coordinates, to study the interaction between upper and lower tropospheric jets and the development of severe convective storms. A hybrid isentropic-sigma coordinate numerical model is used to simulate the mass and momentum adjustments associated with a jet streak propagating in a zonal channel. The numerical results depict a two-layer mass adjustment in the exit and entrance region of the jet streak. The results also verify that the isallobaric wind on lower isentropic surfaces is a primary component of the return branches of transverse circulations and is foxed by the two-layer mass adjustment accompanying the propagating jet streak. Results from the case study of a severe weather out- break show that 1) a low-level jet (LLJ) beneath the exit region of an upper tropospheric jet streak is embedded in the lower branch of an indirect circulation, 2) intensification of the lower branch and development of the LLJ is largely a result of an increased isallobaric wind component, and 3) the development of the LLJ is coupled to the upper tropospheric jet streak by the two-layer mass adjustment within the exit region of the streak. The isallobaric wind component of the LLJ is the primary reason for the axis of the LLJ being at a significant angle to the upper jet's axis and the resulting veering of the wind with height. In the exit region, the geometry of this adjustment, combined with warm, moist, lower tropospheric air to the right and ahead of the jet streak and cool, dry air at the jet streak level, produced the differential advections that convectively destabilized the atmosphere. Results of the case study support the concept that the development of conditions favorable for severe convective storms can be forced by mass and momentum adjustments which accompany the propagation of an upper tropospheric jet streak.

Abstract

Transverse circulations in the exit and entrance regions of jet streaks are investigated through numerical simulation, a case study, and an application of the isallobaric wind equation in isentropic coordinates, to study the interaction between upper and lower tropospheric jets and the development of severe convective storms. A hybrid isentropic-sigma coordinate numerical model is used to simulate the mass and momentum adjustments associated with a jet streak propagating in a zonal channel. The numerical results depict a two-layer mass adjustment in the exit and entrance region of the jet streak. The results also verify that the isallobaric wind on lower isentropic surfaces is a primary component of the return branches of transverse circulations and is foxed by the two-layer mass adjustment accompanying the propagating jet streak. Results from the case study of a severe weather out- break show that 1) a low-level jet (LLJ) beneath the exit region of an upper tropospheric jet streak is embedded in the lower branch of an indirect circulation, 2) intensification of the lower branch and development of the LLJ is largely a result of an increased isallobaric wind component, and 3) the development of the LLJ is coupled to the upper tropospheric jet streak by the two-layer mass adjustment within the exit region of the streak. The isallobaric wind component of the LLJ is the primary reason for the axis of the LLJ being at a significant angle to the upper jet's axis and the resulting veering of the wind with height. In the exit region, the geometry of this adjustment, combined with warm, moist, lower tropospheric air to the right and ahead of the jet streak and cool, dry air at the jet streak level, produced the differential advections that convectively destabilized the atmosphere. Results of the case study support the concept that the development of conditions favorable for severe convective storms can be forced by mass and momentum adjustments which accompany the propagation of an upper tropospheric jet streak.

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