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 rx and crosswind sensor displacement as ry. Two exponential functions of the non-dimensional numbers rx/z and ry/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 ry/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 rx/z and ry/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 ry/z. The black crosses and red circles corresponds to bin-averaged measured values for half hours with rx/z and ry/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.