Abstract. Many long-lived stratospheric chemical constituents enter the stratosphere through the tropical tropopause, are transported throughout the stratosphere by the Brewer-Dobson circulation, and are photochemically destroyed in the upper stratosphere. These chemical constituents, or "tracers," can be used to track mixing and transport by the stratospheric winds. Much of our understanding about the stratospheric circulation is based on large-scale gradients and other spatial features in tracer fields constructed from satellite measurements. The point of view presented in this paper is different, but complementary, in that transport is described in terms of tracer probability distribution functions. The probability distribution function is computed from the measurements and is proportional to the area occupied by tracer values in a given range. The flavor of this paper is tutorial, and the ideas are illustrated with several examples of transport-related phenomena, annotated with remarks that summarize the main point or suggest new directions. The examples illustrate how physically based statistical analysis can shed some light on aspects of stratospheric transport and dynamics that may not be obvious or quantifiable with other types of analyses. The dependence of the statistics on location and time is also shown to be important for practical problems related to statistical robustness and satellite sampling. An important motivation for the work presented here is the need for synthesis of the large and growing database of observations of the atmosphere and output generated by atmospheric models. INTRODUCTIONFor a comprehensive view of how the atmosphere works and how it responds to anthropogenic forcing over a range of length scales and timescales, we must look at both observational and model data from many different perspectives, using a variety of physically relevant analysis tools. Common among these are field maps that emphasize the patterns in field contours, the zonal mean structure of the field, and mean vertical profiles (see, for example, "Evaluation of the UARS Data," a special issue of Journal of Geophysical Research, 101 (D6), 9539-10, 476, 1996). There is no doubt about the value of these pictures in developing an understanding about stratospheric transport, particularly the spatial structure of a "tracer," a dynamical or chemical quantity that is conserved on the transport timescales of interest.Tracer-rich air enters the stratosphere in the tropics, where it is lofted to higher altitudes by the upwelling branch of the circulation. At high altitudes the air becomes depleted of tracer through photochemical loss, and tracer-poor air is then carried poleward and downward in the extratropics by the downwelling branch of •Also at NASA Goddard Space Flight Center, Greenbelt, There are also practical problems related to the collection and analysis of observations of the atmosphere and their comparison with models. Since models cannot be expected to reproduce all of the structural details in a tracer fiel...

Abstract. The global modeling initiative (GMI) science team is developing a three-dimensional chemistry and transport model (CTM) for use in assessment of the atmospheric effects of aviation. This model must be documented, be validated against observations, use a realistic atmospheric circulation, and contain numerical transport and photochemical modules representing atmospheric processes. The model must retain computational eificiency for multiple scenarios and sensitivity studies. To meet these requirements, a facility model concept was developed in which the different components of the CTM are evaluated separately. The assessment of the impact on the stratosphere of the exhaust of supersonic aircraft will depend strongly on the meteorological fields used by the CTM. Three data sets for the stratosphere were considered: the National Center for Atmospheric Research Community Climate Model (CCM2), the Goddard Earth ObservingSystem data assimilation system, and the Goddard Institute for Space Studies general circulation model. Objective criteria were developed to identify the data set that provides the best representation of the stratosphere. Simulations of gases with simple chemical control were chosen to test various aspects of model transport. The data sets were evaluated and graded on their performance on these tests. The CCM2 meteorological data set has the highest score and was selected for GMI. This objective model evaluation establishes a physical basis for interpretation of differences between models and observations. Further, the method provides a quantitative basis for defining model errors, for discriminating between different models, and for ready reevaluation of improved models. This will lead to higher confidence in assessment calculations.

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...

[1] It is well-established that the air in the tropical stratosphere remains somewhat isolated from that in the midlatitudes over relatively long time scales. We examine the seasonal and interannual variability of the subtropical tracer gradients that mark the boundaries between tropical and extratropical air. We use probability distribution functions (PDFs) of satellite measurements of long-lived tracers to identify the air in the transition region between the tropics and the extratropics in each hemisphere. This region is commonly called the subtropical ''edge'' region, and it appears as a valley in the tracer PDF. We provide a method to identify a single characteristic latitude for these edge regions as well as, when possible, their areal extent. We identify the subtropical edges in more than six years of measurements at eight pressure levels on quasi-monthly time scales. We correlate the variability of the subtropical edges with variability in several transport parameters and thus shed light on the mechanisms of tropical isolation from a diagnostic standpoint. We also discuss the implications of our results in terms of large-scale tracer transport in the context of simple models of tracer budgets.

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