Observations of stratified atmospheric shear-driven turbulence at low and high Richardson numbers

We analyze turbulence data from mast-based experiments. The data is organized using the gradient Richardson number, Ri, as dimensionless stability parameter, rather than the widely used Monin-Obukhov normalized height, z/L. Ri has the advantage over z/L of being free of artificial correlations with the data we wish to present.

We used data from six different experiments, including CASES-99, SHEBA, CME04, FLOSS, FLOSSII and a Swedish experiment performed on the coast of the island Östergarnsholm in the Baltic sea. This means that we sample a vast variety of physical environments: Grassland, Arctic sea ice, mountain forest canopy, partially snow covered land and coastal conditions. The total amount is the equivalent of five and a half years worth of stably stratified turbulence data.

Important non-dimensional properties of the turbulence field are presented including the turbulent fluxes of momentum, heat and moisture. These are non-dimensionalized using the turbulent variance. We find that two important regimes exist, the weakly- and very stratified regimes. We find that at Ri<0.1 non-dimensional fluxes are almost constant. At 0.11, the momentum flux is still finite while fluxes of heat and moisture cannot be distinguished from zero.