Estimations from geostrophic trajectories of horizontal diffusivity in the mid‐latitude troposphere and lower stratosphere

Abstract

Abstract Zonal and meridional diffusivities K xx and K yy have been found at mid‐latitudes from the spreads of sets of geostrophic trajectories at levels between 700 mb and 30 mb during each season of 1965. Values have been calculated using the relationship R = e −PT cos qr for the form of the corresponding Lagrangian autocorrelograms. In addition a similar set of Eulerian data has been analysed to obtain autocorrelograms and by comparison with the trajectory data Eulerian‐Lagrangian scale relationships have been obtained. The results show values of K xx ranging from 0·2.10 6 to about 20.10 6 and K yy from 0·2. 10 6 to about 6.10 6 m 2 sec −1 . In both cases the lowest values are found in the summer stratosphere and the highest in the spring stratosphere and in the upper troposphere in winter. Their distributions principally follow that of the wind variances. Integral time scales are mainly between 5 and 30 hours meridionally and 10 and 50 hours zonally and are greatest in the stratosphere and lowest in the mid‐troposphere. The contributions of damping and harmonic effects to these time scales are discussed. Comparison of the Lagrangian and Eulerian time scales indicates a mean value β of about 0·6 for their ratio with the meridional values smaller than the zonal values.