Relative dispersion in the atmosphere from reanalysis winds

Abstract

The dispersion of pairs of synthetic particles, advected with ECMWF winds, is examined. The particles were deployed at three latitudes and on three potential temperature surfaces in both hemispheres. Separation statistics are calculated and evaluated in relation to 2D turbulence theory and to Eulerian structure functions calculated directly from the wind data. At the smallest sampled scales (100-1000 km), the pair-separation velocities are correlated, and the dispersion is laterally isotropic, at least at the higher latitudes. At larger scales, the dispersion is zonally anisotropic, and the pair velocities are uncorrelated. In all cases, the dispersion grows exponentially in time, and the second-order Eulerian structure functions consistently increase as separation squared. This implies nonlocal dispersion, which obtains with energy spectra at least as steep as K-3. Regional variations are seen in the parameters however. The e-folding times and the maximum scales for exponential growth are significantly larger on the 430-K surface than on the 315-K surface, and the dispersion is anisotropic at low latitudes, even at the smallest scales. Therefore, 2D homogeneous turbulence theory is applicable at best at subdeformation scales at the higher latitudes.