The Structure of Temperature and Humidity Turbulent Fluctuations in the Stable Surface Layer.

Most of the information regarding the behavior of scalars in the turbulent atmospheric boundary layer is derived from temperature measurements. It is generally assumed that other scalars behave "in the same way", that they have the same statistical behavior and turbulent transport properties as temperature. Both theoretical and experimental evidence has been presented in the literature suggesting that this simple picture might not be valid under stable stratification conditions. The evidence, however, is often contradictory (for instance, the ratio of the eddy diffusivities of heat and water vapor has been found to be both larger and smaller than one in different experiments). Under the validity of Monin-Obukhov similarity conditions (stationarity and surface uniformity), the budgets for the turbulent temperature and humidity variances and covariance are used to show that they indeed have a similar behavior, with very small discrepancies due to their different molecular diffusivities. Besides explicitly assessing the molecular effects, it is also shown how earlier theoretical analysis can be reconciled. Analysis of atmospheric turbulence data measured during nocturnal periods confirms this similarity, except in one night when large-scale advective processes associated with frontal activity cannot be ruled out. It is also shown that the dimensionless temperature and humidity statistics are essentially constant with stability, validating the hypothesis of vertical homogeneity under stable conditions. Analogous results are obtained from spectral analysis, which also shows how part of the experimental discrepancies may be attributed to the spatial separation of the temperature and humidity sensors. Higher-order scalar cospectra (whose integral yields the third moment) are calculated for the first time, and shown to follow a Kolmogorov-Corrsin type of power law in the wavenumber, with a -2 exponent. Radiative effects on the temperature spectra are also studied; it is shown that under common conditions close to the surface, radiation has a very minor effect; non-dimensionalization of the temperature spectral budget on the other hand discloses a simple dimensionless parameter which can be readily used to estimate the importance of radiation on an individual basis.