Observations of Small-Scale Turbulence and Energy Dissipation Rates in the Cloudy Boundary Layer

Abstract

Tethered balloon?borne measurements with a resolution in the order of 10 cm in a cloudy boundary layer are presented. Two examples sampled under different conditions concerning the clouds' stage of life are discussed. The hypothesis tested here is that basic ideas of classical turbulence theory in boundary layer clouds are valid even to the decimeter scale. Power spectral densities S(?f?) of air temperature, liquid water content, and wind velocity components show an inertial subrange behavior down to ≈20 cm. The mean energy dissipation rates are ?10?3 m2 s?3 for both datasets. Estimated Taylor Reynolds numbers (Re?) are ?104, which indicates the turbulence is fully developed. The ratios between longitudinal and transversal S(?f?) converge to a value close to 4/3, which is predicted by classical turbulence theory for local isotropic conditions. Probability density functions (PDFs) of wind velocity increments ?u are derived. The PDFs show significant deviations from a Gaussian distribution with longer tails typical for an intermittent flow. Local energy dissipation rates ετ are derived from subsequences with a duration of τ = 1 s. With a mean horizontal wind velocity of 8 m s?1, τ corresponds to a spatial scale of 8 m. The PDFs of ετ can be well approximated with a lognormal distribution that agrees with classical theory. Maximum values of ετ ≈ 10?1 m2 s?3 are found in the analyzed clouds. The consequences of this wide range of ετ values for particle?turbulence interaction are discussed.