A Numerical Modeling Study of Warm Offshore Flow over Cool Water

Abstract

Numerical simulations of boundary layer evolution in offshore flow of warm air over cool water are conducted and compared with aircraft observations of mean and turbulent fields made at Duck, North Carolina. Two models are used: a two-dimensional, high-resolution mesoscale model with a turbulent kinetic energy closure scheme, and a three-dimensional large-eddy simulation (LES) model that explicitly resolves the largest turbulent scales. Both models simulate general aspects of the decoupling of the weakly convective boundary layer from the surface, as it is advected offshore, and the formation of an internal boundary layer over the cool water. Two sets of experiments are performed, which indicate that complexities in upstream surface conditions play an important role in controlling the observed structure. The first (land–sea) experiments examine the transition from a rough surface having the same temperature as the ambient lower atmosphere, to a smooth ocean surface that is 5°C cooler. In the second (barrier island) experiment, a 4-km strip along the coastline having surface temperature 5°C warmer than the ambient atmosphere is introduced, to represent a narrow, heated barrier island present at the Duck site. In the land–sea case, it is found that the mesoscale model overpredicts turbulent intensity in the upper half of the boundary layer, forcing a deeper boundary layer. Both the mesoscale and LES models produce only a small change in the boundary layer shear and tend to decrease the momentum flux near the surface much more rapidly than the observations. Results from the barrier-island case are more in line with the observed momentum and turbulence structure, but still have a reduced momentum flux in the lower boundary layer in comparison with the observations. The authors find that turbulence in the LES model generated by convection over the heated land surface is stronger than in the mesoscale model, and tends to persist offshore for greater distances because of greater shear in the upper boundary layer winds. Analysis of the mesoscale model results suggests that better estimation of the mixing length could improve the turbulence closure in regions where the surface fluxes are changing rapidly.

On the Coupling of Wind Stress and Sea Surface Temperature

“…The model (Skyllingstad 2003;Skillingstad et al 2005) is based on the equations of motion given in Deardorff (1980), with the addition of an open boundary condition on the downwind boundary allowing for changes in the boundary layer structure. The domain contained an upstream region with uniform SST, a transition region with an SST gradient, and a downstream region with uniform SST that differed from the upstream SST.…”

Bromoform in tropical Atlantic air from 25°N to 25°S

“…Further, the marine boundary layer may be substantially shallower than the 1 km assumed. As warm offshore air is advected over a cool, smooth ocean surface, the weakly convective boundary layer may be decoupled from the surface resulting in a stable internal boundary layer (IBL) [Skyllingstad et al, 2005]. Turbulence decreases at the surface, and a surface stratified layer may be formed.…”

“…Turbulence decreases at the surface, and a surface stratified layer may be formed. In the land-sea mesoscale simulation of Skyllingstad et al [2005], the upstream conditions after 6 h consist of a weakly convective, 300 m deep boundary layer. Consistent with these results, the majority of the land-sea trajectories (3 -6) calculated by the HYSPLIT model predicted a 300 m deep boundary layer at the coast increasing to $400 m at the position of the ship.…”

Meteorological factors controlling low-level continental pollutant outflow across a coast

“…These physical effects have a strong impact on the reactive nitrogen budget and therefore indirectly on ozone through the photochemistry. Furthermore, polluted air masses above the MBL can be decoupled from the surface by the inversion at the top of the MBL (Vickers et al, 2001), and are subject to higher wind speeds than tracer exported below H MBL (Skyllingstad et al, 2005).…”

“…Turbulence was greatly reduced in this layer of the atmosphere. Skyllingstad et al (2005) performed a large eddy simulation and showed that turbulence was damped from the surface upwards while a maximum in turbulence remained at the top of the MBL for 20 km offshore. The decoupling from the surface occurred very quickly after air flowed over the cool sea, allowing pollutants exported by coastal outflow to become isolated from the surface flow.…”