Investigation of a sea breeze front in an urban environment

Abstract

AbstractThe dynamics of a sea breeze front interacting with the heavily urbanized New York City area are examined. In addition, we investigate the impact of the urban‐influenced sea breeze front on transport and diffusion of simulated passive tracer plumes. We employ the U. S. Navy's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®—a registered trademark of the Naval Research Laboratory) to perform a nested simulation with data assimilation for the sea breeze event of 9 August 2004. Available surface and upper‐air observations are used to validate the simulation. We also perform a sensitivity study in which the urban influence is removed (no‐urban).The sea breeze front has characteristics of a density current, including an elevated head at the leading edge. The density current moves slowly and unevenly across the city. Kelvin–Helmholtz billows form in the region of the density current head, and the results show evidence of the occurrence of Kelvin–Helmholtz instability (KHI). The density current head is greatly elevated owing to the enhanced surface drag of the urban area. This urban influence is further explored in the no‐urban simulation, in which the head of the density current is not elevated to the same degree and KHI does not occur.The sea breeze/density current has a large impact on transport and diffusion of simulated tracer plumes, not only changing the direction of plume motion due to the wind shift but also redistributing tracer material in the vertical so as to produce dramatic, rapid changes in near‐surface concentration as the front passes. In particular, large upward vertical velocity at the head of the density current advects tracer material to large elevations, greatly reducing near‐surface concentration. After passage of the front, tracer is released into the shallow density current and confined near the surface, enhancing near‐surface concentrations. KHI results in turbulent mixing at the upper surface of the plume, allowing for a small reduction in near‐surface concentration. Published in 2007 by John Wiley & Sons, Ltd.