Variability of the Structure Parameters of Temperature and Humidity Observed in the Atmospheric Surface Layer Under Unstable Conditions

The structure parameters of temperature and humidity are important in scintillometry as they determine the structure parameter of the refractive index of air, the primary atmospheric variable obtained with scintillometers. In this study, we investigate the variability of the logarithm of the Monin-Obukhov-scaled structure parameters (denoted as log(2s )) of temperature and humidity. We use observations from eddy-covariance systems operated at three heights (2.5, 50, and 90 m) within the atmospheric surface layer under unstable conditions. The variability of log(C2 s ) depends on instability and on the size of the averaging window over which log(C2 s ) is calculated. If instability increases, differences in log(C2s ) between upward motions (large C2 s ) and downward motions (small C2 s ) increase. The differences are, however, not sufficiently large to result in a bimodal probability density function. If the averaging window size increases, the variances of log(C2 s ) decrease. A linear regression of the variances of log(C2 s ) versus the averaging window size for various stability classes shows an increase of both the offset and slope (in absolute sense) with increasing instability. For temperature, data from the three heights show comparable results. For humidity, in contrast, the offset and slope are larger at 50 and 90 m than at 2.5 m. In the end we discuss how these findings could be used to assess whether observed differences in C2 s along a scintillometer path or aircraft flight leg are just within the range of local variability in C2 s or could be attributed to surface heterogeneity. This is important for the interpretation of data measured above a heterogeneous surface.