Professor Lesley Gray

Abstract:

The dominant tracer transport processes in the equatorial and subtropical latitudes of the middle and upper stratosphere are investigated. Distributions of water vapor in Northern Hemisphere winter from the Microwave Limb Sounder onboard the Upper Atmosphere Research Satellite are employed, together with a three-dimensional Stratosphere Mesosphere Model that incorporates a representation of the quasi-biennial oscillation (QBO). The model reproduces the observed tracer distributions, in particular a "staircase" feature that is present in northern winter of 1992 (easterly QBO phase) but not in 1993 (westerly QBO phase). This feature is highly asymmetric about the equator. The model circulation is diagnosed to show that while the induced QBO circulation in the lower stratosphere of the model is relatively symmetric about the equator, in the middle and upper stratosphere it is highly asymmetric and in the correct sense to give rise to the staircase feature. Model experiments are compared in which trajectories are advected by (1) the full three-dimensional circulation and (2) the residual mean circulation only, thereby removing the local effects of isentropic mixing by planetary waves on the trajectory distributions. These confirm the importance of advection by the QBO circulation at equatorial and subtropical latitudes. However, sharpening of the tracer gradients at the subtropical edge of the surf zone by the action of planetary waves is shown to be important in the formation of a subtropical "cliff" between 10 and 20 mbar at 20°-30°N. The model results also suggest that the prominence of the summer subtropical peak in easterly phase years compared with westerly phase years is not entirely due to increased summer upwelling of the large-scale global circulation caused by the stronger planetary wave driving. The depression of the winter half of the equatorial peak by the local asymmetric QBO circulation is also shown to be important. Copyright 2000 by the American Geophysical Union.

Abstract:

On 20th March 1994 at 12Z two dry layers with high ozone mixing ratio were encountered in the free troposphere by an ozonesonde ascent at Aberystwyth (52.4°N, 4.1°W). The layers resembled tropopause folds, but were not associated with any significant synoptic development, and did not have enhanced static stability. We use isentropic trajectory analysis, with winds taken from ECMWF analyses, to look for the origin of the layers. The analysis suggests that the two layers had different source regions. The lower layer spent 10 days over the Atlantic before reaching Northern Europe, and appears to have originated in the break-up of a stratospheric streamer between the 5th and 9th of March. The upper layer seems to have originated over the Western USA where the trajectories passed through a region of low Richardson number above the Rocky Mountains. In both cases, the low-water-vapour/high-ozone air mass had been advected with little mixing in the troposphere for at least 10 days. We argue that the air must be of stratospheric origin, since photo-chemistry cannot generate large amounts of ozone at this time of year and the trajectories do not, in any case, point to a boundary-layer origin. If, as the analysis suggests, the upper layer reached the troposphere by mixing across the jet then this may be evidence for stratosphere-troposphere exchange (STE) taking place other than by folding of the tropopause. The persistence of layers with anomalous chemical content in the troposphere for so long indicates that an accurate model of tropospheric transport and mixing is needed to assess the chemical impact of STE on tropospheric chemistry, in addition to a representation of the dynamical behaviour near to the tropopause. Copyright (C) 2000 Elsevier Science Ltd.