Structure and dynamics of the low-level jet

Abstract

The low-level jet (LLJ), evolution and related circulations were examined through quasi-geostrophic (Q-G) diagnostics for five strong cases of LLJ occurring in 1991. Operational upper-air data on isobaric and isentropic coordinates for the first time provide a detailed description of the ageostrophic circulation in the exit region of an upper-level jet streak. The LLJ develops primarily beneath the exit region of an upperlevel jet streak and is associated with a frontal system. The ageostrophic circulation along the jet-front system is characterized by a thermally indirect circulation transverse to the flow in the exit region of the streak. This cell shows intense rising motion on the northern side of the LLJ and weak sinking motion on the southern side of the LLJ. Horizontally, the lower branch of the ageostrophic cell shows the stronger ageostrophic wind pointing northward and coinciding with the LLJ; the closing upper branch shows the stronger ageostrophic wind pointing southward and coinciding with the upper-level jet stream. This study showed that LLJ development is related to lee-side troughing, extratropical cyclogenesis and frontogenesis, such that the intensification of the LLJ is part of the geostrophic adjustment process that is related to the deepening of a lee cyclone. Because of the Q-G nature of the process, the development of southerly flow associated with the LLJ in extratropical cyclones is successfully simulated by a one- layer barotropic model. This model uses a realistic basic state for January, valid at 850 hPa, and is forced by a locally applied sink of mass over the Great Plains in a two day run. The forcing imitates the baroclinic development of a cyclone. The prominent flow in the southeastern quadrant of the surface cyclone reproduced the basic properties of the LLJ observed in the case studies.