Influence of the Low-Level Jet on Marine Boundary Layer Winds During Cold Air Advection

The marine boundary layer behaves very differently from the terrestrial boundary layer during significant cold air advection. Strong post-frontal winds aloft mix more efficiently to the surface in marine areas due to the more unstable temperature profile and turbulence, while the terrestrial boundary layer typically becomes more decoupled from higher momentum aloft, especially when nocturnal, surface-based inversions can develop. During these cold air advection events surface wind speeds over the water can exceed wind speeds over land by a considerable margin. These stronger winds can create hazardous conditions for marine navigation. This study linked cold season, post-frontal cold air advection surface wind observations from buoys located near the coast and within Atlantic coastal waters adjacent to South Carolina and Georgia to the presence of a post-frontal, low-level jet (LLJ). The primary goals of the study were determining the coupling efficiency of the LLJ to the marine boundary layer and determining which characteristics of the LLJ are most beneficial for predicting surface wind speed across marine areas. The study found that coupling efficiency varied according to distance from shore, with the greatest efficiency noted farther from the coastline. Of the variables considered, speed and height of the LLJ displayed the strongest relationship to surface winds. Future work could include evaluating a possible LLJ speed threshold for efficient coupling and developing a forecast tool that accounts for LLJ properties thus improving marine forecasts during cold air advection events.