The spectral density technique for the determination of CO₂ flux over the ocean

The spectral density technique is investigated as a basis for measuring turbulent fluxes of CO₂ and water vapour over the sea using observations obtained on a well-designed pier of Kyoto University during September 20-22, 2000. By comparing the turbulent fluxes measured by the spectral density technique with those determined by the eddy correlation technique, a set of parameters of the non-dimensional gradient (φ) and Kolmogorov constant (α) was determined. The following parameters are the optimal selection under unstable conditions for the spectral density technique: φ_c = φ_q = (1 - 16z/L)^-1/2; α_c = 0.78 and α_q = 0.76 Here, subscripts c and q refer to CO₂ and water vapour; z is height and L is the Obukhov stability length. The present results demonstrate that with rugged sonic anemometer-thermometer sensors and computers, the spectral density technique applied over the sea has an accuracy comparable to that of the eddy correlation technique.