A Case Study Characterizing the Marine Boundary Evolution During a Wind Regime Change

In this presentation we highlight a case study of a rapidly evolving Marine Boundary Layer caused by the interaction between a downslope wind descending upon a pre-existing coastal Low-Level Jet wind regime.  Three separate regions are analyzed and defined by 1) the initial stage of interaction, 2) the breakdown of the Low-Level Jet wind regime, and 3) the establishment of the downslope wind wind regime.  The first region, or region 1, analyzes, specifically, the wave stresses of the wind field to investigate shear-generated waves produced by the interaction between wind regimes.  Results show that at least two stable wave periods of identical amplitude are produced at a duration of 14 minutes before significant growth is observed.  The transition to wave growth was found to lead to a wave breaking event that subsequently resulted in the underlying wind regime—the Low-Level Jet—to breakdown.  During the jet breakdown, in region 2, wave-turbulence interactions occurred and resulted in an energy exchange between the wave and turbulent scales defined by the interaction term that joins the wave kinetic energy and turbulent kinetic energy budget equations.  Results show that turbulence from aloft deposits energy into an otherwise isolated system below that is defined by the Low-Level Jet wind regime.  Following the deposition of turbulent energy, the energy naturally dissipates to smaller scales, but then is followed by additional deposition of turbulent energy at later time periods.  This occurs cyclically until the Low-Level Jet dissipates and descends towards the surface.  The third region, or region 3, assesses the erosion and establishment of the downslope wind regime, which approaches the surface at an angle.  The progression of the downward shifted Low-Level jet nose initiates a density current-like structure that generates waves akin to bore waves.  After an hour, the waves gradually dissipate and the downslope wind regime is established. 

The importance of this case study is linked to wind energy applications and climate studies.  For wind energy, most assume a simple logarithmic relationship in offshore environments that is relatively consistent from day to day.  This study highlights the complexity of the wind field over the short-term by showing winds in an evolving boundary layer.  Little is known outside the results from this dataset in the offshore environment along the east coast, and it is the hopes the presenter to reveal the need to consider more advanced techniques for wind resource assessment.  As for climate studies, the stable boundary layer and marine boundary layer is relatively difficult to quantify since scaling relationships often breakdown.  The predictability and the energy exchange associated with generated waves is important for both scalar transport and weather forecasting.  Therefore no knowing with certainty, the affect of wave generation and/or nocturnal convection, can significantly pose problems for assessing the dispersion of pollutants and precipitation events.