A climatology of turbulent dispersion in the troposphere

Abstract

The authors present properties of turbulent, meridional mixing along isentropic surfaces within the troposphere. Twice-daily wind fields analyses from the European Centre for Medium-Range Weather Forecasts numerical weather prediction model for 1992 are used to calculate Lagrangian trajectories of large ensembles of particles. The ensemble-averaged rms growth of the meridional relative dispersion over the first 10 days after particle release is used to quantify mixing properties. These properties are considered as a function of height in the atmosphere, season, and geographic region. Results are characterized by release latitude and flow regime and compared with simple theories. All three dispersive regimes-exponential, ballistic, and Richardson-Obukhov-that have been documented in previous studies are found to be important. The extratropics are found to display superdiffusive growth of the relative rms dispersion, consistent with the nonlocal character of midlatitude mixing. The Tropics are characterized by exponential growth of the rms dispersion, consistent with locally constant eddy timescales. Some evidence for zonal inhomogeneity in dispersion growth rates is found.