Flux attenuation due to Sensor Displacement over Sea

The eddy correlation method has become a common way of measuring turbulent scalar fluxes, from measurements of vertical velocity and a displaced fast response sensor for the scalar of interest. The two instruments are usually separated to avoid flow distortion, but this displacement unfortunately causes the resulting flux to be attenuated. In most previous papers the loss of flux has been investigated through measurements of temperature and vertical velocity over land. Horst (2006) used measurements from the Horizontal Array Turbulence Study (HATS) and the purpose of this study is to investigate the flux loss over sea using data from the related experiment Ocean Horizontal Array Turbulence Study (OHATS). The flux loss due to sensor separation has been quantified for different conditions over sea. We do here focus the attention on measured flux attenuation with the purpose of finding approximate and convenient functions describing the flux loss for typical separation distances rather than characterizing the complexities found in atmospheric cospectras over sea.

The measurements are from the field campaign OHATS carried out at the Air-Sea Interaction Tower (ASIT). The location of the ASIT tower is south of the coast of Martha's Vineyard on the east coast of the United States. The tower is standing in water at a depth of about 15 m and carries a specially configured rack consisting of twin radio tower sections designed and built to hold two horizontal booms. On each boom 9 sonic anemometers were mounted with a separation distance of 0.58 m. For OHATS the two horizontal booms were deployed at a nominal height of 5 m and 5.58 m above the mean sea surface. The data analysed here consists of 192 30-min periods covering a wide range of atmospheric stability conditions −1.0 < z/L < 1.4, with wind speed varying between 2.5 and 12.8 m/s.

Analysis of the data from OHATS reveals that the flux loss has a dependence on wind direction for large sensor displacements r, but for shorter separation distances more typically used for measuring scalar fluxes no clear dependence is observed. Placing an x-axis in the mean wind direction and the y-axis perpendicular to the mean wind direction, we denote streamwise sensor displacement as r_x and crosswind sensor displacement as r_y. Two exponential functions of the non-dimensional numbers r_x/z and r_y/z were used to fit the OHATS data, and predictions from these functions are shown as the full lines in Figure 1. The blue lines correspond to a typical sensor separation of 0.3 m at a measuring height of 10 m above actual sea level. For this value of r_y/z the model from Horst (2006) for crosswind sensor separation show comparable estimates of attenuation for unstable to neutral stratification. For stable stratification the estimates differ significantly, suggesting that the error due to sensor separation is less in the OHATS dataset over ocean as compared to the HATS dataset over land.

Figure 1: Estimated flux loss for r_x/z and r_y/z equal to 0.03, 0.115 and 0.225 using two exponential functions fitted to the OHATS data is shown as the full blue(upper), black(middle) and red(lower) lines respectively. For crosswind displacements estimated flux loss using the model from Horst (2006) is also shown as dashed lines for the same values of r_y/z. The black crosses and red circles corresponds to bin-averaged measured values for half hours with r_x/z and r_y/z in the intervals 0.08-0.15 and 0.15-0.30 respectively. The error bars denote one standard deviation from the means for each binned group of values.

References: Horst, T., 2006: Attenuation of scalar fluxes measured with horizontally-displaced sensors. 17th AMS Symposium on Boundary Layers and Turbulence, May 22-26, San Diego.