Effects of spatial variability of SST on the marine boundary layer

A series of experiments are performed using large-eddy simulation to examine how spatial variations in SST affect turbulence in the marine boundary layer. Two sets of simulations are presented; one representing conditions that were observed during the CBLAST pilot and 2003 experiments, with a second set examining the sensitivity of turbulence to the scales of SST variability.

In the first set of experiments we compare the output of idealized LES cases with observed turbulence parameters such as the turbulence kinetic energy and vertical momentum flux. These experiments show that conditions of warm air moving over cooler SST regions can cause a decoupled boundary layer structure. Turbulence in the decoupled layer decreases mostly because of dissipation, but with some loss due to buoyancy as cooler air in the near surface boundary layer is advected upward. In cases with weaker SST changes, turbulence decreases, but reaches an equilibrium, suggesting that the boundary layer remains coupled to the surface.

Sensitivity experiments provide a mechanism for estimating what scales are important in controlling changes in the boundary layer turbulence structure. For example, we test to see how small regions of much colder water match up against large regions of slightly colder water when the total flux is held constant. The goal of these experiments is to determine if an average SST can be used to accurately represent the boundary layer response to subgrid scale variability in mesoscale models.