Sonic anemometer (co)sine response and flux measurement: I. The potential for (co)sine error to affect sonic anemometer-based flux measurements

The potential for sonic anemometer (co)sine errors to affect eddy flux measurements is investigated. Similarity theory is used to show that the standard deviation of the instantaneous angle between the wind vector and the horizontal (the angle of attack), depends on surface roughness, measurement height and atmospheric stability. Three days of data from a peat bog and a pine forest are used to test this dependency. The normalised flux-angle distributions are derived for these two sites and it is shown that for the peat bog the sonic anemometer operates outside its manufacturer’s specified angle-acceptance envelope for 20% of the daytime fluxes; for the forest some 50% of the daytime fluxes were measured under these conditions. The highest values of flux-angle were observed at the peat bog site during two 30 min periods when CO2, respired the previous night, was flushed out during the dawn breakdown of the nocturnal boundary layer. The implications for the design of eddy flux measuring systems is discussed.