Investigations of flux-profile relationships in the marine atmospheric surface layer during CBLAST

Marine meteorologists and oceanographers have long relied on flux-profile relationships that relate the turbulence fluxes of momentum, heat and moisture (or mass) to their respective profiles of velocity, temperature, and water vapor (or other gases). The flux-profile or gradient-flux relationships are used extensively in numerical models to provide lower boundary conditions and to “close” the model by approximating higher order terms from low order variables. The most commonly used flux-profile relationships are based on Monin-Obukhov (MO) similarity theory. MO similarity is expected to hold and the derived parameterizations are expected to be universal as long as the assumptions that govern the MO similarity laws are valid: (1) a combination of mechanical and thermal forcing drive the turbulent exchange such that the relative strength of these two forces determines the characteristics of the near surface turbulence, (2) the scaling variables are independent of height in the surface layer, and (3) the turbulence statistics are stationary and horizontally homogeneous. The constant flux layer constraint is generally assumed to be valid in the lowest 10% of the unstable atmospheric boundary layer.

Near the ocean surface, however, wave induced forcing is expected to influenced the characteristics of the near surface flow. This presentation will provide examples of wave related processes and their impact on the air-sea exchange and the vertical structure of the near surface flow over the ocean using data taken during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) Experiment. The data was collected from the Air-Sea Interaction Tower (ASIT) located 3.2 km south of Martha’s Vineyard in the Atlantic Ocean. The ASIT stands in a water depth of 15-m and extends 22-m into the marine atmosphere. In the summer of 2003, the atmospheric side of ASIT was instrumented with sensors to measure velocity, temperature, pressure, humidity, solar and infrared radiation, precipitation; and sensors to remotely measure the wave field and sea surface temperature. Turbulence sensors were deployed at 6 levels to directly measure the fluxes of momentum, kinetic energy, temperature variance and sensible heat. The lowest 4 levels included sensors to measure the moisture variance and latent heat flux, while 2 levels were instrumented to measure the static pressure flux. A separate mast was deployed to support a moving package that measured the mean profiles of velocity, temperature and humidity.

The data is being used to investigate the relationship between the momentum, sensible heat, and latent heat fluxes and their associated mean profiles of velocity, potential temperature, and moisture, respectively. These investigations examine the validity of MO similarity theory as well as the departure from MOS due to the influence of the underlying wave field and other surface layer phenomena such as fog. The TKE and scalar budget equations are used to investigate the physical processes that cause departure from classical law-of-the-wall behavior due to, e.g., stratification and energy flux to the wave field. The investigation hopes to provide improved parameterization of these processes for inclusion in numerical models of the marine atmospheric boundary layer.