An example of coupling between an upper-level and a low-level jet–front system is analyzed using the mesoscale hydrostatic model SALSA. The case study chosen is the cold front sampled during the intensive observation period 2 of the Mesoscale Frontal Dynamic Project FRONTS 87 experiment (11 November 1987). Two prominent features of the cold front are a well-developed undulation and a frontal band ahead of the undulation.
Dynamic and diabatic processes of the coupling are investigated through the results of two numerical simulations: full physics and adiabatic. In particular, the roles of ageostrophic circulations are investigated through the decomposition of the ageostrophic wind into its isentropic components: the advective inertial and the diabatic inertial components, and the isallobaric component.
In both simulations it is shown that the low-level branch of the indirect and transverse ageostrophic circulation, associated with the exit region of the upper-level jet streak, is the origin of the cold-front undulation beneath the upper-level jet axis. Other effects of the coupling processes are diagnosed in the full-physics simulation: the intensification of the low-level jet, the formation of the frontal band ahead of the cold front on the left side of the low-level jet, the intensification of the convective system close to the ascending branch of the transverse indirect circulation, and the kinetic energy generation in the exit region of the upper-level jet streak.
An examination of the isentropic components of the ageostrophic wind shows that the upper-level branch of the indirect circulation is the inertial advective component of the ageostrophic wind, whereas the low-level branch is mainly isallobaric. Being caught up in its eastward motion by the faster propagating low-level jet, the low-level branch of the indirect circulation reinforces, by its 7 m s−1 ageostrophic component, the advection of heat and moisture toward the convective system close to the ascending branch of the indirect circulation. In that region, the diabatic inertial component of the ageostrophic wind grows at a rate smaller than that needed for mass adjustment between the thermal and wind fields, thereby leading to kinetic energy generation through the inertial rotation mechanism downstream and on the cyclonic side of the upper-level jet streak.
Dynamical and diabatic processes are shown to contribute to the blocking-in phase and to the development of synergistic interactions between the upper- and the low-level jet-front systems. The results of this study are summarized in a conceptual scheme of the coupling mechanism, including an explanation for the formation of the frontal band.