Dynamical interactions between a mid-tropospheric closed cyclone, a low-level jet, and cyclogenesis

Abstract

Output from the Eta Model was used to examine the development of a cutoff cyclone, a low-level jet (LLJ), cyclogenesis, and the interaction of these phenomena during the period 16-18 October 1994. Formation of an upper-level low followed descent of potential vorticity from the stratosphere along an upper-tropospheric front. The cutoff cyclone became established by 1200 UTC 16 October 1994, as stratospheric potential vorticity values descended to levels below 400 hPa. During this episode, a trough developed in the lee of the Rockies, and rapid cyclogenesis occurred following the movement of the upper-level low into the northern Great Plains. To study this event, the Eta Model was chosen. The high spatial and temporal resolution of the Eta Model made it suitable for studying the development of the cutoff cyclone and the LLJ. A statistical analysis of the Eta Model results indicated that the model provided an accurate representation of the atmospheric situation. The formation of a cutoff low followed the descent of high potential vorticity air into the middle troposphere with the cyclone cutting off from the westerly current when the potential vorticity maximum became isolated in the base of the trough. The upper-level cyclone interacted with the low-level baroclinic zone initiating the development of a lower-level cyclone. A LLJ developed over the central United States in association with a deepening trough in the lee of the Rockies and an upper-level jet (ULJ) streak propagating to the northeastward from the southwestern United States. The LLJ was coupled to the return branch of the indirect circulation in the exit region of the ULJ; Q-vector convergence indicated that the LLJ was driven by large-scale forcing. A schematic diagram was constructed using existing theories concerning the development of a surface cyclone, interaction with an upper-level cyclone, the LLJ and the ULJ. The rapid development of the surface cyclone was related to the interactions of the upper-and lower-level cyclonic systems and the LLJ and ULJ.