A Lagrangian view of stratospheric trace gas distributions

Abstract

Abstract. As a result of photochemistry, some relationship between the stratospheric age of air or mean age and the amount of tracer contained within an air sample is expected. The existence of such a relationship allows inferences about transport history to be made from observations of chemical tracers. This paper lays down the conceptual foundations for the relationship between age and tracer amount for long-lived tracers, developed within a Lagrangian framework. Although the photochemical loss depends not only on the age of the parcel but also on its path, we show that under the "average path approximation" that the path variations are less important than parcel age. The average path approximation then allows us to develop a formal relationship between the age spectrum and the tracer distribution. Using this relationship, tracer-tracer correlations can be interpreted as the result of mixing which connects parts of the "single-path photochemistry curve," a universal path-independent curve that describes the photochemical loss in terms of the total photon exposure. This geometric interpretation of mixing gives rise to constraints on trace gas correlation curves as can be seen in the atmospheric trace molecule spectroscopy observations. IntroductionAs air enters the tropical stratosphere, it moves upward and toward the extra tropics roughly along the streamlines of the Brewer-Dobson circulation. Mass balance requires that most of the air rising into the tropical stratosphere will immediately move poleward and subsequently descend into the middle world (see Holto• et al., [1995] for discussion of the middle world region of the stratosphere). However, some air continues to rise into the stratosphere and mix with the environment. Understanding the movement, residence time, and mixing of air within the stratosphere is key to assessing the impact of pollutants on the ozone layer.Thc measurement of nearly inert trace gases, which have known time-dependent sources, can be used to estimate the time elapsed since the air first entered the stratosphere. This transit time is commonly called the mean age, and it gives important clues about the stratospheric circulation. The mean age depends not only on the timescale of the mean circulation but also on fluctuations around the mean transport; that is, it depends on the mixing. As we discuss below, the sampled air can have a complex time history; thus the transit time from the tropopause to some point within the stratosphere is not, in general, well-characterized by a single timescale.The realization that the character of a sample of air is determined by both the transport and mixing history has led to the idea that the transit time is more aptly described by a distribution of timescales [Kida, 1983;Hall and Plumb, 1994]. In this distribution, each element corresponds to a single path connecting the source and observation or sample point. Thus an air sample can be considered to consist of a large number of 0148-0227/00/1999JD900787509.00 "irreducible parcels," each of which h...

Methane as a diagnostic tracer of changes in the Brewer–Dobson circulation of the stratosphere

“…2 that L(r, t ) is spatially inhomogeneous in both the vertical and meridional directions. One cannot easily sort out the components of changes in the mean AoA from changes in the path spectra (Schoeberl et al, 2000). However, to first approximation, both the path and loss function L can be assumed stationary, such that CH 4 in the stratosphere is a fraction (f ) of the tropospheric CH 4 and where f is a function of latitude/height and season (e.g., Fueglistaler, 2012).…”

Investigating lower stratospheric model transport: Lagrangian calculations of mean age and age spectra in the GCM ECHAM4

“…A disadvantage of the Lagrangian method is that a large number of trajectories have to be computed, because (1) the trajectory density, which is proportional to the air density, decreases exponentially with height, and (2) the number of trajectories has to be large enough to produce statistically stable results (cf. Schoeberl et al 2000). On the other hand, the Lagrangian method takes into account the non-stationarity of transport, reducing the computational costs compared to the Eulerian calculation (see above).…”

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“…6 which shows two parcels with same transit time from the tropopause but different pathways. As a result the two parcels have different photochemical exposure (Schoeberl et al, 2000) and Cl y .…”