Lagrangian Mean Circulations in the Stratosphere

Abstract

Three-dimensional winds derived from LIMS satellite observations for the 1978/79 winter are used to compute the mean Lagrangian motion in the winter stratosphere. Material tubes of air parcels are initialized every 4 days and followed for periods of 10 days each. The initial positions of the tubes are chosen so that they lie along contours of constant geopotential and potential temperature. Maps of air parcel distributions give a qualitative picture of the degree of deformation of the material tubes with time. In addition, quantitative measures of the Lagrangian mean velocity and dispersion are computed. During quiet periods, when the zonal wind is strong and the vortex is nearly axisymmetric, the air parcel tubes tend to remain coherent for the full 10 days of the integration. When the wave amplitudes are large, many of the tubes break and the parcels disperse. During the observed minor sudden warming, those tubes closest to the vortex center remained coherent with little distortion. In contrast, during the major sudden warming every material tube in the stratosphere was broken, and there was extensive mixing between air parcels from low and high latitudes. Lagrangian mean vertical motion tended to be smaller than the motion in the transformed Eulerian coordinate system, which is sometimes used to represent the mean Lagrangian flow. The horizontal velocities determined from the Lagrangian parcel trajectories do not in general correspond well with the transformed Eulerian velocities. Largest differences in horizontal winds occur for situations during which the tubes underwent extensive deformation and the dispersion of air parcels was large. This suggests that the transformed Eulerian circulation is not capable of representing the horizontal Lagrangian motion when a large part of the latter is due to dispersive motion rather than to net displacements of coherent material tube.