PROBABILITY DENSITY-FUNCTIONS OF TURBULENT VELOCITY AND TEMPERATURE IN THE ATMOSPHERIC SURFACE-LAYER

The probability density functions (pdf's) for the longitudinal and ver tical velocities, temperature, their derivatives, and momentum and sen sible heat fluxes were measured in the atmospheric surface layer for a wide range of atmospheric stability conditions. The measured pdf's fo r both the velocity and the temperature fluctuations are near-Gaussian and consistent with corresponding laboratory measurements for near-ne utral and stable stability conditions. Hence the first- and second-ord er moments are sufficient to predict the heat and momentum flux pdf's. The lower-order moments can be estimated from mean meteorological con ditions 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 conditio ns the pdf's for the velocity and temperature longitudinal gradients h ave long exponential tails, in agreement with findings in laboratory e xperiments and numerical simulations.