Momentum fluxes and turbulence structure of the marine Atmospheric Boundary Layer

In the frame of the CBLAST-Low project the main characteristics of the vertical turbulent structure of the Marine Atmospheric Boundary Layer (MABL) up to a height of 600m was examined using both remote sensing (Acoustic Sounder) and in situ instrumentation at the southern coast of the Nantucket Island, MA, USA, during summer 2003. The ability of Acoustic Sounders (SODARS) to measure the mean and the turbulent components of the wind speed as well as the thermal structure of the ABL is well known. With high time and space resolution, especially in the vertical direction, it is sometimes more desirable compared to single-level turbulence measurements in certain research areas of the ABL.

Our measurement site was at a distance of 90m from the waterfront where the land surface was relatively flat and a suite of instruments designed to characterize the changing structure of the MABL was deployed. The SODAR system was measuring the vertical profiles of the horizontal wind speed and direction, the vertical (w) and the two horizontal wind components u (north-westward) and v (north-eastward), the standard deviations of the three wind components, the momentum fluxes (u'w' and v'w' ) and the atmospheric stability class at 30 minutes intervals, with a vertical resolution of 40 m and a range up to the height of 600m. The total vertical momentum fluxes, the mechanical production term (P) of the turbulence energy budget and the turbulent kinetic energy (TKE) were also calculated. On a 20m meteorological mast, there were two levels (10 and 20 m) of high-rate sampling sonic anemometers, a fast hygrometer at 20 m and slow response sensors measure the mean wind, temperature, and relative humidity at 5, 10, and 20m height respectively at 10 minutes intervals. The high-rate measurements yield to the estimation of momentum, sensible heat and latent heat fluxes through the eddy correlation method. It should be mentioned that the values of momentum, heat and humidity fluxes and the stability parameter z/L, calculated at 20m height by the fast sensors, were used as reference surface values of the MABL, while the 10m height ones were excluded from this study since they were influenced by the developed Internal Boundary Layer (IBL). In addition, rawinsondes were launched at the experimental site every four to six hours per day.

Study of the vertical profiles of the echo strength (analogue to the temperature structure parameter CT2) and stability class from the SODAR data has shown that the MABL is characterized by very stable atmospheric conditions at the first 150-200m followed by slight stable to neutral conditions at higher levels. The vertical profiles of the wind speed indicated very frequently, under medium to high wind speeds, the development of a Low-Level Jet (LLJ) with a maximum corresponding to the top of the ground based stable layer. The estimated profiles of the potential temperature, specific humidity, wind speed and direction and bulk Richardson number (Ri), from the rawinsondes launches, confirmed the mentioned above vertical structure of the MABL where the intense ground based inversion, the different layering above and the developed LLJ were also observed. A possible cause for the development of the LLJ, which is associated with the strong stability of the MABL lower layer, is the development of an inertial oscillation due to the frictional decoupling over the sea. This plausible explanation is in agreement with the calculated hodographs of the wind, estimated from SODAR data, at the LLJ level where the relative increase of the wind speed and the wind direction turning in relation to the geostrophic wind, given by the rawinsondes data, were evident. The profiles of the total vertical momentum fluxes give high values above the wind maximum height and much lower ones close to the surface. It should be mentioned that the vertical profiles of the momentum fluxes u'w' give strong positive values above the wind maximum height while the corresponding ones of v'w' (v component is facing the wind) exhibit relatively high negative values above the wind maximum. Large values of the TKE exist at heights above the LLJ which is probably associated with the shear forcing near the developed wind maximum. The estimated profiles of the mechanical production term (P) exhibit high values below and above the height of the LLJ. Results regarding the vertical structure of the momentum transport and the TKE, using SODAR observations and other relevant measurements from this experimental campaign, under different meteorological conditions, will be presented and discussed.