Effect of different fine wire thermocouple design on high frequency temperature measurement

Analysis of experimental data gathered within and above several types of surfaces showed that repeated, organized features in the turbulence (coherent structures) are responsible for a majority of scalar exchanges between the canopies and atmosphere (Gao et al., 1989; Shaw et al., 1989). These coherent structures are related to the ramp-like shapes of high frequency temperature traces (Paw U et al., 1992). Under unstable conditions, the ramps exhibit a slow rise in temperature followed by a sudden drop. Under stable conditions, when there is warm advection and at night, the temperature ramps show a slow decrease in air temperature followed by a sudden rise. In both stability conditions, the temperature ramps can be used to estimate sensible heat flux density (H) using Surface Renewal (SR) method (Paw U and Brunet, 1991; Paw U et al., 1995; Snyder et al., 1996; Spano et al., 1997). Mean values for ramp amplitude (a) and inverse ramp frequency (l + s) during half-hour intervals are used to estimate H. A structure function and statistical moments from Van Atta (1977) are used to determine a and (l + s). The objective of this study was to evaluate the effect of thermocouple characteristics on SR analysis. We found that the size and design of sensors used in high frequency measurements do affect temperature traces and SR results.