Lagrangian Velocity Correlations and Absolute Dispersion in the Midlatitude Troposphere

Employing daily wind data from the ECMWF, we perform passive particle advection to estimate the Lagrangian velocity correlation functions (LVCF) associated with the midlatitude tropospheric flow. In particular we decompose the velocity field into time mean and transient (or eddy) components to better understand the nature of the LVCF's. A closely related quantity, the absolute dispersion (AD) is also examined. Given the anisotropy of the flow, meridional and zonal characteristics are considered separately. The zonal LVCF is seen to be non-exponential. In fact, for intermediate timescales it can either be interpreted as a power law of the form $\tau^{-\alpha}$ with $0<\alpha<1$ or as the sum of exponentials with differing timescales - both interpretations being equivalent. More importantly the long time correlations in the zonal flow result in a superdiffusive zonal AD regime. On the other hand, the meridional LVCF decays rapidly to zero. Before approaching zero the meridional LVCF shows a region of negative correlation - a consequence of the presence of planetary scale Rossby waves. As a result the meridional AD, apart from showing the classical asymptotic ballistic and diffusive regimes, displays transient subdiffusive behaviour.