Evolution of a Marine Boundary-Layer Jet

Abstract

High-resolution profiles of temperature and wind-speed measurements were made with a tethered baloon in and above the marine boundary layer at San Nicolas Island (SNI) during a period when the cloud-free boundary layer grew from near the sea surface to 450 m in approximately 12 h. Measurements showed the formation of a low-level jet which remained centered at the temperature inversion as the boundary layer grew. The upper limit of the jet coincided with the top of a temperature transition layer that extended from the sharp temperature jump at the inversion to the free atmosphere above. The experimental evidence suggested that the jet was caused by thermal wind resulting from a specific sea surface temperature gradient, and from horizontal temperature gradients caused by a sloped inversion and the transition layer. Production of mechanical turbulence by wind shear in the jet caused rapid entrainment into the mixed layer of warmer air from above, and the fast growth of the boundary layer. A quasi-two dimensional (2D) model including turbulence parameterized in terms of turbulent kinetic energy (TKE) and dissipation rate was able to reproduce the main features of the evolving boundary-layer jet and temperature field. The predicted shape, location, and intensity of the jet and the growth of the boundary layer were similar to the observations. The model also predicted realistic heat and momentum fluxes and TKE budgets as judged by comparisons with aircraft measurements by Brost et al. in a similar case off the West Coast. Using a variety of initial conditions, the model further showed that the jet was likely caused by the combined effects of the inertial acceleration of the wind field, the specific temperature gradients, and the sloping inversion.