Interaction of a stable marine atmospheric boundary layer with a low-level jet

Low-level jets (LLJ) are commonly observed in the warm sectors of extratropical cyclones in advance of surface cold fronts. In this study, we examine simulations of a strong LLJ and an intense cold front which transited a region of low sea-surface temperature (SST) south of Newfoundland on 16 February 1997 during FASTEX IOP-16. Simulations are performed using the Naval Research Laboratory’s nonhydrostatic COAMPS model. The cold SST (<4 C) and associated strong downward heat flux (~-250 w m-2) have a dramatic impact on both the LLJ and front. Below the core of the jet, a strongly stable marine atmospheric boundary layer (MABL) forms at the leading edge of front. In this region, turbulent mixing of momentum is inhibited and the jet accelerates to greater than 50 m s-1. Significant turbulence exists within the stable MABL due to strong vertical wind shear, however, this turbulence is confined to low levels due to strong buoyant destruction and does not extend to the elevation of the core of the LLJ. As the front continues to move rapidly to the SE, warmer SST’s are encountered and the MABL transitions from a shallow, stable state to a deep, well-mixed structure. Turbulent mixing increases and the jet decelerates significantly. Results of sensitivity studies and analyses of the temporal evolution of the turbulent kinetic energy, boundary layer depth, surface heat flux, and jet core speed will be presented.