On the Role of Upper Tropospheric Jet Streaks and Leeside Cyclogenesis in the Development of Low-Level Jets in the Great Plains

Abstract

A review of 15 cases of low-level jets (LLJ's) which developed in the Great Plains and which have been previously discussed in the literature is presented. The review notes that boundary-layer processes have been emphasized as causative factors in the development of the LLJ while upper tropospheric features were not considered and the importance of synoptic-scale processes was generally minimized. For 12 out of the 15 cases, a systematic upper level flow pattern is isolated which includes the existence of a trough over the southwest United States and the propagation of upper level jet streaks from the Rocky Mountains toward the Great Plains. This flow pattern is responsible for leeside cyclogenesis or leeside troughing that produces the pressure gradients needed for the development of LLJ'S. For the other three cases, a blocking ridge exists over the Great Plains and the upper level flow is relatively weak. It is during these situations that the ?classic,? diurnally oscillating LLJ is observed. A more detailed review of four case studies indicates that the subsynoptic-scale processes associated with the upper level jet streak's forcing of leeside cyclogenesis could, at times, be an important factor in the development of LLJ's in the Great Plains. The review questions 1) the notion that a retrogression of the subtropical high provides the increased pressure gradient force needed for the development of a LLJ in the Great Plains region, and 2) the assumption of using a constant or diurnally oscillating pressure gradient force which has been used for studying the total evolution of the LLJ. Changes in the pressure gradient force related to leeside cyclogenesis and leeside troughing and the isallobaric wind response to these changes seem to be an integral part of the process that leads to the development of LLJ's observed in the Great Plains.