A case study of the low-level jet during an episode of spring convection

Abstract

Circulation and transport of the low-level jet (LLJ) were examined for a case of severe convection that occurred in Spring 1995. Radiosonde observations and Eta model output on constant pressure and isentropic surfaces allowed a detailed examination of the relationship between the LLJ and a subtropical jet (STJ) streak. Height falls associated with a lee cyclone caused an ageostrophic and isallobalic wind that strengthened the LLJ. The smaller horizontal resolution of Eta model giids and hourly forecasts allowed a detailed examination of the LLJ evolution and transport methods. The model's accurate placement of terrain provided realistic plots of the LLJ at the commonly used 850 mb level. The radiosonde observations, when interpolated to grid points, yielded suspicious results over higher terrain. The hourly forecast of transport variables revealed a step-by-step progression of convection along ridges of equivalent potential temperature advection. The evolution of the LLJ appeared to be influenced by a thermally indirect circulation (TIC) induced in the exit region of a STJ streak. A low-level isaflobaric wind responding to a leeside trough and cyclogenesis also effected the LLJ. The TIC is part of the ageostrophic circulation in the exit region of the streak. The low-level isallobaric wind coincided with a trough moving east over the northern Great Plains and a deepening trough in the lee of the Rockies. Results of the case study for the LLJ evolution showed 1) The LLJ developed beneath a STJ streak exit region as the lower branch of an indirect circulation. 2) A strong cross-contour ageostrophic wind in the lower troposphere aided in accelerating the LLJ. 3) The LLJ strengthened in response to a isaflobaric wind associated with lee cyclogenesis. 4) LLJ speed and magnitude decreased during a transition between the large scale forcing mechanisms as the ageostrophic circulation, cross-contour ageostrophic wind, and isabobaiic wind all weakened. 5) Eta model hourly forecasts fairly well predicted heat and water vapor transport, offering an hourly look at convective movement.