Probability density functions of turbulent velocity and temperature in the atmospheric surface layer

The probability density functions (pdf's) for the longitudinal and vertical velocities, temperature, their derivatives, and momentum and sensible heat fluxes were measured in the atmospheric surface layer for a wide range of atmospheric stability conditions. The measured pdf's for both the velocity and the temperature fluctuations are near-Gaussian and consistent with corresponding laboratory measurements for near-neutral and stable stability conditions. Hence the first- and second-order moments are sufficient to predict the heat and momentum flux pdf's. The lower-order moments can be estimated from mean meteorological conditions using surface layer similarity theory. For unstable conditions the pdf for temperature is non-Gaussian and is strongly skewed due to local convective thermal plumes. For near-neutral and stable conditions the pdf's for the velocity and temperature longitudinal gradients have long exponential tails, in agreement with findings in laboratory experiments and numerical simulations.