The Stable Internal Boundary Layer over a Coastal Sea. Part I: Airborne Measurements of the Mean and Turbulence Structure

Abstract

It has been recognized that progress toward understanding the mesoscale structure of the ocean requires more knowledge of the small-scale structure of the marine atmosphere and the processes that couple the air and the sea. The coastal ocean is characterized by large variations in sea surface temperature and surface roughness that affect the structure of the atmosphere. Stable layers are frequent features of the coastal marine atmospheric boundary layer. Their main effect is the formation of a discontinuity between the sea surface and the upper part of the boundary layer that supports gravity waves, wind speed jets, and large wind shear. The general structure of a stable internal boundary layer (IBL) that forms over the sea, downstream of a warm landmass, is discussed. Aircraft data are presented from the Internal Boundary Layer Experiment (IBLEX) conducted over the Irish Sea in 1990. With the airflow from the land to the sea, thermodynamic profiles were obtained perpendicular to the coast to investigate the modification of the boundary layer. Horizontal legs were flown parallel to the coast to obtain information about the turbulence structure at the upwind and downwind ends of the research area. Despite the large horizontal inhomogeneity in the IBL, local similarity scaling applies throughout the IBL below LL, where LL, is the local similarity length scale. The turbulence parameters, which are nondimensionalized with the local scales, are generally constant with respect to height. The IBL is characterized by large temperature and moisture gradients and a large wind shear that maintains a Richardson number close to its critical value. Turbulence appears to be continuous, maintained by the strong wind speed shear against the stabilizing effect of the downward-directed heat flux. The turbulence fluxes indicate generally cogradient heat fluxes in the IBL and large momentum fluxes due to the strong shear and high winds. Small-scale countergradient beat fluxes are observed that may be the result of the breakup of large, cogradient turbulent eddies near the sea surface and the return of the air to buoyant equilibrium at smaller spatial scales.