A study of air particle motions during a stratospheric warming and their influence on photochemistry

Q. 1. R. Meteorol. SOC. (1989), 115, pp. 841-866 551.510.532:551.510.41:551.524.735

A study of air particle motions during a stratospheric warming and their influence on photochemistry

By JOHN AUSTIN* and NEAL BUTCHART Boundary Layer and Atmospheric Chemhtry Branch, Meteorological Oftice, Bracknell

(Recsivcd 22 March 1988; revised 21 December 1988)

SUMMARY

Middle stratospheric ten-day trajectories are calculated using satellite data for air particles initially situated on the edge of the circumpolar vortex during the minor warming of January 1979. The trajectory behaviour is consistent with the observed evolution of the isentropic distribution of potential vorticity. Further, from an analysis of a large number of trajectories calculated from closely spaced initial positions it is argued that the trajectories are realistic indicators of transport. In particular, the trajectories divide naturally into a limited number of distinct categories representing either vortex erosion or the continued advection of air around the distorted vortex edge. None of the categories indicate horizontal transport of air into the vortex.

A comprehensive photochemical model is integrated along the trajectories and the results are compared with similar integrations along trajectories calculated for a suitably chosen axially symmetric reference atmosphere. In this way it is established that, for the late January warming, the distortion of the vortex from zonal symmetry led to substantial decreases in the destruction of ozone and in the production of nitric acid on the edge of the vortex. Once the particles are stripped from the vortex by the wave breaking, they experience only small changes in photochemistry if the particles remain in middle latitudes, or much larger changes if they are transported as far as the subtropics. Results from the photochemical model are also compared with co- located satellite observations of the constituents ozone, water vapour and nitric acid for the different categories of trajectories. In many cases there are statistically significant discrepancies between observed and modelled concentrations. It is suggested that these discrepancies are due to the presence of other particles transported along trajectories from different locations, and therefore with different chemical properties, contaminating the coarse resolution co-located observations. The findings are entirely consistent with the view that outside the vortex the wave breaking will mix or generate small unresolvable scales in the tracer fields. Some implications of the results for the photochemical modelling of the stratosphere are discussed.

1. INTRODUCTION

It is generally recognized (WMO 1986 and references therein) that a complete explanation of the observed distributions of chemical species in the middle atmosphere requires an understanding of the complex interplay between dynamical, chemical and radiative processes. The system is further complicated by the presence below of a troposphere which acts as a source and sink for many of the minor constituents and, perhaps more significantly, as a source of wave activity. This is especially true of the northern hemisphere winter stratosphere, where planetary-scale Rossby waves are believed to be responsible for most of the horizontal transport of constituents.

A clearer idea of how the planetary waves might affect the rapid meridional transport of constituents has emerged recently from the numerous studies (e.g. McIntyre and Palmer 1984; Clough et af. 1985; Dunkerton and Delisi 1986; O’Neill and Pope 1987) of isentropic maps of Ertel’s potential vorticity, Q. Briefly, in the extratropical middle stratosphere Q is an approximate material tracer and its redistribution on an isentropic surface provides a broad picture of the quasi-horizontal fluid motions. For the winter hemisphere many of the maps show the main circumpolar vortex of high Q being eroded at its edge by the drawing out of narrow tongues of high-potential-vorticity air. These so-called ‘Rossby-wave-breaking’ events appear to happen almost continually throughout the northern winter (Butchart and Remsberg 1986) but are most prominent during major

*Current address: Department of Atmospheric Sciences AK-40, University of Washington, Seattle, WA 98195, U.S.A.

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842 J. AUSTIN and N. BUTCHART

and minor warmings. A most spectacular example, which has been well documented in the literature, is the minor warming of January 1979 (e.g. see McIntyre and Palmer 1983, 1984). This event was associated with both an irreversible shrinking of the circumpolar vortex (Butchart and Remsberg 1986) and some rather prominent but reversible fluctuations in its shape and position (Dunkerton and Delisi 1986). A summary of the evolution of the 850 K isentropic distribution of potential vorticity during the warming is provided by the four ‘IPV maps’ presented in Fig. 1 (see figure legend for details). The warming was also of special interest as it took place during the LIMS (Limb Infrared Monitor of the Stratosphere; see Gille and Russell (1984)) experiment when the distributions of several stratospheric trace gases were being monitored from the Nimbus 7 space vehicle (see section 3).

The aim of this paper is to investigate the coupling between air particle motions and radiative and photochemical processes at the peak of the January 1979 warming. The purpose is to quantify the different effects that vortex erosion and fluctuations in its shape and position have on an individual air particle’s photochemistry. Attention is focused on particles initially situated near the edge of the circumpolar vortex and therefore likely to be involved in the wave breaking. A trajectory analysis (Austin and Tuck 1985) performed on a large collection of these particles, indicates that over a period of ten days or so, most executed large meridional excursions which significantly perturbed their radiative and photochemical properties. Quantitative estimates of the changes in the concentrations of constituents of each particle are obtained by integrating a photochemical model along the trajectories (Austin et al. 1987). For convenience this model is hereafter referred to as ‘the photochemical trajectory model’. The results of these calculations are compared with those from control integrations for an axially symmetric ‘reference’ stratosphere (see section 5 for a description of this reference state) in order to provide a meaningful measure of the extent to which the planetary waves perturb the photochemistry of the different particles. A further comparison with co- located satellite observations provides an indication of the relative importance of dynamical and radiative/photochemical processes in determining the observed distributions of constituents, and forms the basis for a reassessment of the use of the trajectory technique in dynamically active situations.

Trajectories calculated using both the isentropic and quasi-isentropic techniques developed by Austin and Tuck (1985) are presented in section 2 together with a discussion of their representative nature. Section 3 contains a brief description of satellite observations of ozone, water vapour, nitric acid and nitrogen dioxide during late January and early February 1979. An outline of the photochemical model and its initialization is supplied in section 4. Results of integrating the model along the trajectories described in section 2 are presented in section 5 together with the photochemical model results for the control integrations. These results are contrasted, in section 5, to provide the first quantitative estimates of the different effects vortex distortion and erosion have on air particle photochemistry. The model results are then compared with observations in section 6. Finally, in section 7, there is a discussion of the results together with some concluding remarks about their implications for the trajectory and other photochemical modelling techniques used for studying the stratosphere.

2. TRAJECTORY CALCULATIONS

(a) Isentropic trajectories In this section an isentropic trajectory analysis is used to establish more firmly the

horizontal transport inferred from the 850 K IPV maps (see introduction). As the paper

PHOTOCHEMISTRY DURING A STRATOSPHERIC WARMING 843

is principally concerned with the photochemical fate of air particles in the vicinity of the edge of the vortex and therefore involved in the wave breaking process of January 1979 it was decided, first, to calculate trajectories for a large number of particles initially densely packed within the 'material' area lying between the 2.5 and 3.5 contours in Fig. l(a) (lightly shaded region) for 27 January. Furthermore, for the purpose of establishing the sensitivity of the trajectory calculations to small horizontal displacements in initial particle position it was considered sufficient to restrict attention to a small segment of this area, provided it was large enough to contain more than one observational grid point. The segment was chosen on the basis of some preliminary calculations and the evolution of the IPV maps which suggested that a proportion of the particles within it would be subsequently transported away from the vortex as a result of the wave breaking. The initial positions of 6 rows of 21 particles placed within this segment on 27 January are indicated by the dots in Fig. 2(a). Also shown are the positions of the 2.5 and 3.5 Q contours (cf. Fig. l(a)). For each of these particles 10-day isentropic trajectories were calculated using horizontal wind fields derived from SSU (Stratospheric Sounding Unit) data, as described more fully in Austin and Tuck (1985). Also, to improve the accuracy of the calculations in regions of large horizontal wind shear, it was found

29 JAN 79

I \ \ I

2 FEB 79

Figure 1. Ertel's potential vorticity, Q, on the 850 K isentropic surface as calculated from SSU (Stratospheric Sounding Unit) data in units of 10-4Km2kg-'s-'. The lightly shaded regions correspond to values between 2.5 and 3.5, and the heavily shaded regions correspond to values greater than 3.5, for the days (a) 27 January 1979, (b) 29 January 1979, (c) 31 January 1979, (d) 2 February 1979. The projection is polar stereographic and the most northerly latitude circle is at W N . The Greenwich meridian is in the lower half of each diagram. The sequence of maps shows an off-centred circumpolar vortex of high Q with an additional tongue of high values emerging from the western edge of the vortex on the 27 January. On the 29 January this tongue has apparently broken away from the vortex leaving remnants of high-Q air in the western hemisphere. The maps for 31 January and 2 February then show the development of a second tongue, again representing material being dragged out from the main cyclonic vortex. According to McIntyre and Palmer (1984) the isentropic mixing associated with 'wave-breaking' events of this kind is likely to be responsible for eroding the main winter polar

vortex, to produce the region of uniform potential vorticity (or 'surf zone') that surrounds it.

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J. AUSTIN and N. BUTCHART

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27 JAN 79

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28 JAN 79 I

30 JAN 79

Figure 2. Positions of the air particles corresponding to a set of 126 ten-day isentropic trajectories on the 850 K surface super- imposed on the corresponding daily map of Ertel's potential vorticity at the same level. For clarity only potential vorticity contours 2.5 and 3.5 x 10-4Km2kg-1s-1 are shown. (a) Initial positions of the particles arranged in 6 rows of 21 at OOGMT on 27 January 1979; (b)-(k) subsequent positions of the particles at daily intervals. The projection is the same

as in Fig. 1.

necessary to increase the number of timesteps per day from the 100 used by Austin and Tuck to 800 for the calculations described in this paper. Positions of all the particles, and also the two Q contours, for the subsequent 10 days are shown in Figs. 2(b)-(k).

During the first two days of the integration (Figs. 2(b) and (c)) most of the particles have become spread out along the region between the two Q contours, though the particles have tended to move closer to the inner contour. By 29 January (Fig. 2(c)) the fastest moving particles have travelled half way round the vortex, about twice as far as the slowest moving particles. Note also in Fig. 2(c) the three particles which have become completely separated from the main ensemble and which, until 3 February, clearly followed the small cutoff region enclosed by the 2.5 contour (see Figs. 2(c)-(h)). The other particles continued to be advected around the vortex, mostly remaining within the region between the two Q contours. However, by 30 January (Fig. 2(d)) the leading particles have moved inside the 3-5 contour and the particles in the trailing group have drifted outside the 2.5 contour. Nevertheless, the first significant movement of particles away from the region enclosed by the two Q contours did not occur until 31 January near


4 FEE 79

Figure 2 (continued)

120"E (Fig. 2(e)). Particles from this location eventually moved towards the subtropics but remained as a fairly coherent group up to and including 4 February (Fig. 2(i)). Other particles continued in their path around the vortex before splitting into two groups on 2 February (Fig. 2(g)). One group subsequently embarked on a second circuit of the vortex while the rest moved into the tongue formed by the drawing out of the 2-5-Q contour (see Figs. 2(h)-(k)). By the 6 February (Fig. 2(k)) separate groups of particles were no longer clearly distinguishable and although the particles had become well spread out, most were still within or close to the region enclosed by the two Q contours. The three particles that were originally stripped from the main group on 29 January (Fig. 2(c)) appeared eventually to rejoin those particles circulating the vortex when the small cutoff region they were following lost its separate identity on 4 February (Fig. 2(i)). Those particles that left the vortex near 120"E on 31 January and were transported into the subtropics drifted back towards the two Q contours (see Figs. 2(j)-(k)). It is also interesting to note that none of the particles have penetrated the innermost parts of the vortex.


Figure 3. The trajectories of the particles in Fig. 2 regrouped into different categories of motion. (a) Trajectories of particles which peeled from the vortex; (b) trajectories of particles which were transported around the Aleutian high; (c) trajectories of particles which remained on the vortex edge; (d) trajectories of particles which were transported into the subtropics. The number of trajectories in each category is indicated

in the top left-hand corner of each diagram. The projection is the same as in Fig. 1.

The coherent behaviour of different groups of particles seen in Fig. 2 suggests that this may be a natural way of classifying particle motions. This is best illustrated by separating the actual particle trajectories into the four diagrams shown in Fig. 3. The number in the top left-hand corner of each diagram denotes the number of trajectories in each category. From a photochemical perspective one of the most significant aspects of each of these trajectories is the range of latitudes covered. Later, in section 5 , the effects of these meridional motions on the chemistry of the particles will be quantified using a comprehensive photochemical model.

The broad agreement between the advection of a large number of particles and the observed movement of regions of constant Q on the 850K isentropic surface is further confirmation that Q is approximately conserved on the isentropic surface for periods up to ten days (Clough et al. 1985). The only particles which appeared to suggest any significant non-conservation of Q were those which left the vortex in the eastern hemisphere. However, when the IPV maps were analysed using a finer contour interval (not shown) it was possible to detect some evidence that high Q was also being eroded from the vortex in that region, but details of the process were probably lost because of inadequate resolution of the observations (see also discussion in section 7) . Nevertheless, the advection that was tentatively inferred from the IPV maps was consistent with the behaviour of the trajectories. Moreover, it is known from theoretical considerations (McIntyre and Palmer 1984; Hoskins et af. 1985; also section 7 of this paper) that small scales in the Q field are likely to be generated by the action of the large-scale wind field such as that used in the trajectory calculations. For this reason the trajectory technique


is a powerful tool for validating features seen in the IPV maps, particularly where resolution is a limiting factor (McIntyre and Palmer 1984; Dunkerton and Delisi 1986; Juckes and McIntyre 1987). In addition, the comparison with the observed advection of Q provides an important check on the reliability of the trajectories themselves (Austin and Tuck 1985). Another key point, which suggests that the trajectories are not only reliable, but also useful, indicators of the main transport processes, is the small number of trajectory types (four) obtained from such a large number of particles. These were originally densely packed, yet no single trajectory displayed a unique behaviour that might have been considered spurious. It would therefore be reasonable to assume that trajectories calculated for particles initially more widely spaced in the whole of the ‘material area’ bounded by the 2.5- and 3.5-Q contours will be broadly representative of different categories of tracer motion near the edge of the vortex. Further, in considering the whole annular area the number of possible categories would be anticipated to increase. On the other hand it should also be recognized that with more coarsely spaced initial particle positions certain categories of trajectories may be inadvertently omitted.

(b ) Quasi-isentropic trajectories In order to perform a more rigorous analysis of air particle motions and their

photochemical evolution, the vertical displacement of the particles from a fixed isentropic surface needs to be incorporated into the trajectory calculations. This is particularly important for calculating photochemical effects since the results are rather sensitive to air density and hence height. In this study, observed ozone and temperature data are used in conjunction with a modern radiation algorithm to calculate diabatic heating and cooling rates at discrete points along each trajectory. The net heating rate is used to adjust the isentropic level and hence vertical position at which the horizontal winds used in the trajectory calculation are determined. Austin and Tuck (1985) referred to these trajectories as ‘quasi-isentropic’ since the displacements of particles from their original isentropic surface are in some sense small and the trajectories generally maintain the characteristics of those calculated isentropically. This last point strongly suggests that if the analysis of section 2(a) had actually been based on the computationally more expensive quasi-isentropic trajectories similar conclusions would have been obtained, and hence justifies using a fairly limited number of particles to investigate the photochemical fate of air that was originally at the edge of the vortex on the 850K surface on 27 January 1979. The particles chosen for this investigation were, therefore, initially spread completely around the Q contour labelled 2.5 in Fig. l(a) for 27 January. A small number of additional particles at a limited range of coordinates slightly deeper in the vortex, near the 3.0-Q contour in Fig. l(a), provided an indication of the sensitivity of the calculations to horizontal wind shear. Initial coordinates for all these particles are given in Table 1. The ten-day quasi-isentropic trajectories calculated for these particles then give a broad indication of the transport of air from and around the vortex edge. As with the isentropic results the trajectories were found to divide naturally into separate categories, four of which could be identified with those shown in Fig. 3, and a 5th category consisting of particles which were transported to mid-latitudes. In addition it is possible that other categories existed as discussed at the end of section 2(a). Nevertheless that should not affect the conclusion that the categories found are likely to be reliable indicators of at least some, and probably most, of the transport which occurred at the edge of the vortex.

Added confidence in the trajectory behaviour is provided by the fact that similar results were also obtained when winds derived from LIMS data were used to calculate the trajectories, provided initialization was based on the LIMS-derived IPV distribution.


TABLE 1. PHOTOCHEMICAL MODEL CALCULATIONS ALONG QUASI-ISENTROPIC TRAJECKNUES

Trajectory starting Model d([03]/[M])/dt Model d([HNOJ/[M])/dt Number coordinates ( PPm/daY) (PPb/daY)

(a) Particles which peeled from the vortex after one circuit 1 83.0N 276.7E -0.042 0.090 2 86.1N 241.2E -0.034 0.095 3 84.2N 151-5E -0.002 0.027 4 79.5N 129.5E -0.010 0.006 5 73.0N 120.3E -0.020 (control -0.066) 0.052 (control 0.084) 6 69.ON 116.2E 0.050 0.123

Mean -0.010 0.066 (b) Particles which were transported around the Aleutian high 7 44N 225E -0.012 -0.077 8 45N 242E -0.021 -0.oOO 9 55N 254E -0-030 0.042

10 67N 248E -0.031 (control -0.023) -0.024 (control -0.031) 11 81N 248E -0.021 0.062

Mean -0.023 0.001 (c) Particles which remained on the vortex edge 12 49N 93E -0.031 0.084 13 49N 78E -0.028 (control -0.050) 0.091 (control 0.158) 14 48N 55E -0.015 -0.022 15 47N 37E -0.040 0.053 16 45N 20E -0.016 0.066

Mean -0426 0.054 (d) Particles which were transported to the subtropics 17 81N 140E 0.040 0.051 18 75N 129.5E 0-070 0-057 19 69N 119E 0460 (control -0.048) 0.091 (control 0.124) 20 62.5N 114.5E 0.056 0.028 21 56N llOE 0.039 0.039

Mean 0.053 (e) Particles which were transported to middle latitudes 22 42N 5E -0.060 23 37N 353E -0.055 24 34N 332E -0.084 25 36N 322E -0-093 26 32N 309E -0.147 (control -0.151) 27 34N 2%E -0.110 28 39N 283E -0.127 29 32N 271E -0.088 30 29N 259E -0427

0.053

0.164 0.184 0.179 0.219

0.220 (control 0.225) 0.175 0.184 0.252 0.190

Mean -0.088 0.196

The main characteristics of the motion indicated by the trajectories were unchanged despite the different data sources used.

Figure 4 shows the trajectories of air particles which peeled from the vortex after just one circuit, with a typical example, number 5 , shown bold with time markers every day. The horizontal coordinates are very similar indeed to the isentropic results presented in Fig. 3(a). Figure 5 shows the trajectories of air particles which were transported around the Aleutian high. The larger range of starting coordinates for the quasi-isentropic trajectories gives rise to a wider range of tracks than for the isentropic trajectories (Fig. 3(b)) but otherwise the results are again very similar. Figure 6 shows the trajectories


Figure 4. Quasi-isentropic trajectories starting on the 850 K isentropic surface on 27 January 1979; trajectories of particles which peeled from the vortex (see Table 1). Trajectory number 5 is indicated in bold, with time markers every day. The projection is

the same as in Fig. 1.

Figure 5 . As Fig. 4 but for trajectories of particles which are transported around the Aleutian high (see Table 1); trajectory number 10 is marked in bold, with

time markers every day.

Figure 6. As Fig. 4 but for trajectories of particles which remained on the vortex edge (see Table 1); trajectory number 13 is marked in bold, with time

markers every day.

of air particles which continued on the edge of the vortex. Qualitatively, the trajectories are again very similar to the isentropic results (Fig. 3(c)). Figure 7 shows the trajectories of particles which did not quite complete one circuit of the vortex before being transported to the subtropics in the eastern hemisphere. These trajectories are similar in character to those shown in Fig. 3(d), but in contrast to most of the isentropic trajectories, none completed a circuit of the mid-latitude anticyclone in the 10 days. Figure 8 shows the trajectories of air particles which left the vortex and were transported to mid-latitudes. The starting coordinates of this group of trajectories lie almost entirely within the quadrant 0-9O"W at the edge of the vortex, a range of coordinates not considered in section 2(a). Although trajectories in this last group are similar qualitatively to the trajectories shown in Fig. 7 the photochemical changes along them are quite different, as will be shown in section 5 .


Figure 7. As Fig. 4 but for trajectories of particles which were transported to the subtropics (see Table 1); trajectory number 19 is marked in bold, with time

markers every day.

Figure 8. As Fig. 4 but for trajectories of particles which were transported to middle latitudes (see Table 1); trajectory number 26 is marked in bold, with time

markers every day.

3. OBSERVATIONS OF CHEMICAL SPECIES

Before going on to present the quantitative results of the photochemical calculations it is useful to review briefly the distributions of water vapour (H,O), ozone (03), nitric acid (HN03), and nitrogen dioxide (NO,) observed by the LIMS instrument for the period of interest. For H20 , O3 and HN03 the distributions are essentially synoptic but the NO, measurements refer to the descending node only (i.e. mostly nighttime measurements, see Gille and Russell (1984) for more details). The mixing ratios for the 850 K level are presented in Figs. 9-12 for the same four days as in Fig. 1. Shaded regions correspond to those similarly shaded in Fig. 1.

One of the most prominent features of the water vapour fields seen in Fig. 9 is the lack of strong horizontal gradients, which tends to make the data appear noisy. Despite this there is strong qualitative agreement between the regions of high Q and high water vapour concentrations (cf. Figs. 1 and 9) which suggests that for periods of a week or so horizontal advection was dominating other processes for these two quantities (Butchart 1987).

Comparison between Figs. 1 and 10 shows that there was an anticorrelation between the ozone distribution and potential vorticity, with the large gradients in ozone mixing ratio clearly following the edge of the high-Q region (see also Leovy et al. 1985). However, the appreciable local maximum in the ozone concentration over the Aleutian Islands did not correspond to any distinctive feature in the IPV maps, possibly because ozone photochemistry and/or diabatic effects were much more important than horizontal transport in that region.

For nitric acid (Fig. 11) the correlation with potential vorticity was much less prominent and the feature corresponding to the vortex structure in the IPV maps was only barely visible in the nitric acid maps. The vortex structure was even less visible in the nitrogen dioxide measurements (Fig. 12) although broadly speaking the distributions of nitric acid and nitrogen dioxide were anticorrelated (cf. Figs. 11 and 12). However, details of the nitrogen dioxide distribution will have depended to a large extent on individual air particle history (Austin et al. 1987).

PHOTOCHEMISTRY DURING A STRATOSPHERIC WARMING 85 1

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Figure 9. LIMS H 2 0 volume mixing ratios (p.p.m.) interpolated to the 850K isentropic surface. The light shading corresponds at the same level to values of Ertel's potential vorticity between 2.5 and 3.5 x 10-'Km2kg-'s-' and the dark shading corresponds to values greater than 3.5 x 10-4Km2kg-1s-1. (a) 27 January 1979; (b) 29 January 1979; (c) 31 January 1979; (2) 2 February 1979. The projection is the same as in

Fig. 1. 27 JAN 79 23 JAN 79

Figure 10. As Fig. 9 but for O3 volume mixing ratios in p.p.m.


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Figure 11. As Fig. 9 but for HNO, volume mixing ratios in p.p.b,

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Figure 12. As Fig. 9 but for NO* volume mixing ratios in p.p.b. from the descending-node data (nighttime measurements) .


4. PHOTOCHEMICAL MODEL DESCRIPTION AND INITIALIZATION

(a) Model description The photochemical model used in this study was adapted from that used by Austin

et al. (1987) and incorporates 72 reactions and 29 chemical species. The model includes the full complexity of the photochemistry apart from bromine, chlorocarbon and chlorofluorocarbon reactions, which are thought to be unimportant on a ten-day timescale at this altitude (see, for example, WMO 1986). Solar fluxes for the photodissociation coefficients were computed using a random band model and spherical geometry was employed, which was particularly important for calculating the solar flux at large zenith angles. The model timestep was 30s during the daytime, 60s at night and 1s at dawn and dusk. Reaction rates are taken from DeMore et al. (1985).

(b ) Initialization of the chemistry Initialization is one of the major problems in using a photochemical trajectory model

because of all the species in the model only four are available from LIMS data and an additional two from the Stratospheric and Mesospheric Sounder (Jones and Pyle 1984). However, near the 850K isentropic level, the only unmeasured species which have a significant impact on the LIMS constituents are NO, N205, and the radicals OH, H 0 2 , O(’D) and O(3P). Since the LIMS species are of primary interest in this work, the problems of initialization are therefore reduced to that of estimating the concentrations of the above six unobserved species as well as taking account of the random errors in the LIMS data.

Perhaps the most convenient way of initializing the LIMS species is to interpolate from the MAT (Mapped Archive Tape) analyses to the coordinates of the point of interest. The MAT data are pseudo-synoptic analyses of the data in Fourier coefficient form consisting of the zonal mean and the first 6 zonal harmonics. Producing analyses in this form reduces the effects of random errors at the expense of degrading the temporal and spatial resolution by the application of a Kalman filter (Haggard etal. 1986; Remsberg et al. 1986). Therefore these analyses are not entirely appropriate for initializing the constituents of individual air particles. Instead, for this study it was decided to use the more local and unfiltered IPAT (Inverted Profile Archive Tape) data. These data are a set of vertical profiles of constituents along the sub-orbital track and are prone to random and systematic error (Gille and Russell 1984). The effects of the random errors were reduced by smoothing the co-located data along an air particle trajectory, that is, essentially in a Lagrangian sense. This was achieved in practice by fitting a linear regression line through the co-located data so that the ‘smoothed’ value at any trajectory coordinates was given by the regressed value. This implicitly assumes that the concentrations change linearly with time throughout the duration of the trajectory, as confirmed to a first approximation by a preliminary scan of the data. For simplicity the constituents 03, H 2 0 , and HN03 were initialized from regression lines for the forward trajectories already calculated (section 2) rather than the more natural choice of a combination of forward and backward trajectories from the space-time coordinates of interest. The approximately linear change with time would be expected from the long photochemical timescales of ten days or more (see, for example, Brasseur and Solomon 1986 or Austin et al. 1986) for the species H 2 0 , O3 and HN03 under the conditions prevailing at the high latitudes of the trajectories.

For NO2 there is a substantial diurnal variation and therefore linear regression using both day and night values is not appropriate. Analysis of LIMS data, however, suggested that, to within about lo%, total odd nitrogen (NO + NO2 + 2 x N205 + HN03) was


initially equal to about 16.5p.p.b.v. (parts per lo9 by volume) for all the particles. It was important to get the correct amount of odd nitrogen in the model, to guard against the possibility of calculating unrealistic production or destruction of O3 and HN03. The concentration of N205 was initialized to 2p.p.b.v. as inferred by Callis et al. (1986), and supported by modelling studies for a disturbed period in February 1979 (Austin et al. 1987). The species NO and NO2 strongly interact but their initial partitioning is not crucial because within a few minutes of the start of the model integrations these species come into photochemical equilibrium. For simplicity, therefore, NO was set to zero while NO2 was initialized to the remaining odd nitrogen of 12.5 p.p.b.v. minus the concentration of HN03. All the other radicals, including O(3P), O('D), OH and HOZ were initialized with zero concentrations. Clearly, this is not accurate for trajectories starting in the daytime but the model very quickly establishes appropriate mixing ratios without per- turbing other species so this does not lead to a significant error.

5 . RESULTS OF THE CHEMICAL MODEL INTEGRATIONS

In this section the chemical model results for 03, HN03 and NO2 are presented for typical examples of each of the categories of air particle motion described in section 2(b). To assess how these motions influence the photochemistry of an individual air particle, results are also presented for corresponding control integrations of the chemical model for a reference atmosphere in which the meridional dependence of the trajectory arising from either the distortion in shape and position of the vortex or the advection of the particle from the vortex was artificially removed. This was achieved by keeping the particle at its 'equivalent latitude' defined by its initial potential vorticity value. Full details of the concept of equivalent latitude can be found in Butchart and Remsberg (1986); here it suffices to say that it would be the natural latitude of the particle if the disturbance amplitudes were conservatively diminished to zero. In general this will not be the same as the initial latitude of the particle.

(a) Particles which peeled from the uortex, after completing one circuit The results of integrating the chemical model along trajectory 5 (see Fig. 4) are

illustrated in Fig. 13 by the solid line. The broken line is for the control trajectory in which the particle remained at its equivalent latitude of 55.6"N. In the control integration there is a steady decrease in O3 due to destruction by the odd nitrogen catalytic cycle (e.g. see Brasseur and Solomon 1986). For the true trajectory, O3 remained constant for the first 1-4 days while the particle was inside the polar night. During this time the NO2 concentration fell steadily owing to the production of Nz05. Consequently, when the particle emerged from the polar night, the odd nitrogen O3 destruction cycle was much less effective and O3 increased for a short period. However, subsequent fast photolysis of NzO5 released NO2 in sufficient quantities to destroy O3 up to about day 4 at approximately the same rate as for the control particle. The actual air particle then experienced a second period of extended darkness, again reducing the amount of NOz and slowing the O3 destruction rate. In the final few days the particle was transported to low latitudes where O3 production occurred. The eccentricity of the vortex played an important role in transporting the particle into the polar night, which reduced the effect of the NO catalyst in destroying 03, relative to the control. On the other hand, the eventual displacement of the particle to low latitudes for part of the time increased the production rate of 03. The net effect was a slight decrease of O3 (3%), compared with a decrease of 11% in the control.


0 2 4 8 8 1 0 Time (days)

I I I 0 2 4 6 8 10

Time (days1

01 I 0 2 4 6 8 1 0

Time ldaya)

Figure 13. Results of the photochemical model integrated along trajectory number 5 (see Table 1 and Fig. 4). The solid line indicates the results obtained for the true trajectory and the broken line denotes the results obtained for the control integration (see text). (a) 0, volume mixing ratio (p.p.m.); (b) HNO, volume mixing ratio (p.p.b.); (c) NOz volume mixing

ratio (p.p.b.).

The model concentration of HN03 is determined by the balance between the production and destruction reactions

O H + N O z + M + H N 0 3 + M R1

H N 0 3 + h v - ,OH+NOz FU HN03 + OH-, NO3 + H 2 0 R3

where M is any molecule. Since the rate for reaction R3 is generally small, HN03 decreased or increased slowly when NO2 was low, but when NOz was high, HN03 increased. Further, HN03 in the model was constant in the polar night, although HN03 was observed to increase in the polar night at lower levels in late December 1978 by up to 0.1 p.p.b.v. per day, possibly owing to heterogeneous reactions (Austin et al. 1986) which are absent from the current model. These results contrast with the behaviour of 03, which decreased when NOz was high. Thus the concentrations of HN03 and O3 are anticorrelated in the model when integrated along trajectory 5 , as shown in Fig. 13, and the effect of the disturbed flow was to increase HN03 at a slower rate than in the control.

(b ) Particles which were transported around the Aleutian high The results of integrating the chemical model along trajectory 10 (Fig. 5 ) are

illustrated in Fig. 14 together with the results for the corresponding particle fixed at an equivalent latitude of 47.1"N. As with the previous trajectory, ozone decreased steadily in the integration when the particle remained at its equivalent latitude. However, at


5

0" 6 5

I 4 I I

0 2 4 0 8 1 0 Time (days1

10 1-

01 , I I I 0 2 4 8 8 10

Time Idaya)

Figure 14. As Fig. 13 but for trajectory number 10.

1 I 1 I 0 2 4 6 8 10

Time (days)

I L I I 0 2 4 6 8 10

Time (days)

Y , I I

0 2 4 6 8 10 Time (dayr)

Figure IS. As Fig. 13 but for trajectory number 13.

these slightly more southerly latitudes the O3 production term was larger and hence the net O3 change was less. The true trajectory partly traversed higher latitudes than 47", which led to a smaller O3 production term. For the short time the particle spent in high latitudes, the NO2 concentration fell, as along trajectory 5 (section 5(a)), although the net O3 change was not significantly affected. After an initial fall, possibly due to incorrect initialization, the calculated concentration of HN03 remained approximately constant, independent of whether the particle followed the true trajectory or stayed at its equivalent latitude. From these results it may be concluded that the transport of particles around the Aleutian high had only a small impact on the final O3 and HN03 concentrations.

(c ) Particles which remained on the edge of the vortex The trajectories are illustrated in Fig. 6 and the chemical model results for a typical

example of this category, trajectory 13, are shown in Fig. 15. For the first seven days the particle has the same behaviour as the particles which peeled from the vortex (section 5(a)) so the results for O3 and HN03 have many features in common. The eccentricity of the vortex caused the particle to experience extended periods of darkness, which led to increased NO2 and reduced O3 destruction. The time spent in low latitudes also resulted in enhanced production of O3 relative to the results of the control integration. Hence for two reasons the meridional excursions of the particle resulted in a smaller net O3 destruction. The model HN03 production was also less, for similar reasons to those given in section 5(a). However, the HN03 increase in the model is 31% along trajectory 13 compared with only 18% along trajectory 5 and, despite a large difference in initial values, both calculations give a final concentration of 4-9p.p.b.v. The difference in initial values reflects the importance of photochemistry in establishing a gradient in HN03 concentration around the initial potential vorticity contour (see Fig. 11).


0 2 4 8 8 10 Time ldays) Time (days)

z L -2 __,-_/--,-' ,_-- --

_-I

0 2 4 8 8 10 0 2 4 6 8 1 0 Time (days) Time (days)

(C) (C)

g - P 10 10 - -

B 2

$ 5 $ 5

0 0 o 2 4 6 8 10 0 2 4 8 8 10


Time IdaW Time (days)


( d ) Particles which were transported to the subtropics The trajectories are illustrated in Fig. 7 and the chemical model results for a typical

trajectory, number 19, are shown in Fig. 16. The particle entered the polar night almost immediately which, overall, reduced O3 destruction (see previous subsection). Then after partially completing a circuit of the vortex, the particle was transported to the subtropics where the O3 production term was substantially increased. The net effect was an increase in the model O3 concentration of 9%, whereas if the flow had been undisturbed and the particle had remained at its equivalent latitude the O3 concentration would have decreased. The model O3 concentration for the true trajectory was dominated, in general, by the photolytic production term (0, + hv) rather than by the destruction cycle involving NO2. On the other hand, for this particle, the change in the HN03 concentration was still dominated by the production reaction with NO2, (Rl), and hence HN03 increased outside the polar night. Therefore, as NO2 did not dominate the behaviour of both HN03 and 03, the two species concentrations are not anticorrelated, in contrast to the results of sections 5(a) and (c) .

(e ) Particles which were transported to middle latitudes The trajectories are illustrated in Fig. 8 and the photochemical model results for a

typical trajectory, number 26, are shown in Fig. 17. The trajectory is quite similar in appearance to that considered in section 5(d) but the differences in detail had a major effect on the model chemistry. This was particularly noticeable for 03, which steadily decreased, compared with an increase for trajectory 19 (section 5(d)) . This was partly a result of the higher initial O3 concentration for trajectory 26 but also the particle's O3 production was less owing to its generally more northward trajectory than those described in section 5(d) . Very similar results were obtained in the calculation at the equivalent latitude and, in particular, the HN03 increased steadily in both model integrations.


However, the initial HN03 concentration was considerably lower than in the calculations of sections 5(a)-(d) because of the low-latitude initial position. Nitric acid more than doubled in the course of the integration but the final value of 4-6p.p.b.v. is within 10% of the final concentrations for all the other categories of particle motions except for the particles which were transported around the Aleutian high.

6. CHEMICAL MODEL COMPARISONS WITH OBSERVATIONS

The main advantage of trajectory models is that the results can be compared locally with individual observations of atmospheric constituents, as shown by Austin et al. (1987). In this section, the usefulness of the technique is explored further by comparing model results presented in section 5 with the corresponding LIMS observations.

(a) Data analysis LIMS data centred within 5 great circle degrees of the particle positions of the quasi-

isentropic trajectories described in section 2 were determined as in Austin et al. (1987). This produced along each ten-day trajectory typically 30 observations of each LIMS species, sufficient to enable the results to be analysed statistically. However, because of random and systematic errors in the observations only the broad features of the data can be considered. In particular, the species concentrations along each trajectory were linearly regressed against time. The mean rates of change of the species concentrations along the trajectories were then determined both from the model and from the regression lines through the data. As noted in section 4, to a first approximation the concentrations change linearly with time, although the regression lines for different sections of some trajectories would imply different rates of change. The non-uniformity in the rate of change of HN03, for example, may arise from the changing photochemical timescale due to the variation in latitude (see Austin et al. 1986). In this work, however, the statistic of interest is the net change in concentration over the ten days and this may be equivalently expressed as a rate of change averaged for the complete ten days of the trajectories. Regression statistics (e.g. Bendat and Piersol 1971) have been used to compute 95% confidence intervals for the rates of change and include random and some systematic errors (see Austin et al. (1987) for a discussion of LIMS errors in the context of trajectory modelling). To reduce the effect of LIMS data errors and random trajectory errors, mean rates of change have been determined for each category. In computing the average, the rate of change for each trajectory in the category has been weighted by its inverse variance, so that the confidence levels can be determined (Bendat and Piersol 1971). The results, together with 95% confidence intervals, are presented in Table 2. In some categories, particularly for the particles which peeled from the vortex, a number of observations appear as near coincidences for several trajectories. Since these observations were not independent, allowance has been made for them in the analysis of each trajectory category. The discrepancies (in standard deviations) between model and data are also included in Table 2 and those discrepancies which are significant at the 95% confidence level are printed in bold. Mean rates of change are given both in absolute amounts and in per cent per day. The values in per cent per day were calculated by dividing the mean rate of change by the initial concentration.

(b) Results Figure 18 shows the model results (solid lines) and LIMS observations (solid circles)

for the particles which peeled from the vortex (see section 5(a)). The pecked lines are the linear regression lines through the data, which are prone to some systematic and


TAFILE 2. COMPARISON BETWEEN PHOTOCHEMICAL MODEL RESULTS AM) LIMS DATA FOR QUASI-ISENTROPIC TRAJECTORIES

O3 Rate of change

Category (ppm/day) (%/day) (ppm/day) (%/day) (standard dev.)

Data Model Discrepancy

0.055 * 0.055 0.9 -0.010 -0.2 -2.4 -1.7 0.016 f 0446 0.3 -0.023 -0.4

-0.016 2 0.043 -0.7 -0426 -0-4 0.9 0.197 f 0.049 3.2 0.053 0.8 -5-9

-0448 2 0.042 -0.6 -0.088 -1.1 -1.9

(a) (b) ( 4 ( 4 (el

HN03 Rate of change

Category (ppb/day) (%/day) (ppb/day) (%/day) (standard dev.)


(4 0.126 2 0-057 3-0 0.066 1.6 -2.1 0-133 f 0.070 2.6 0.001 0.0 -3.8 0.141 2 0.061 3.4 0.054 1.3 -2.9

(b) ( 4

-0.131 f 0.047 -3.1 0.053 1.3 7.9 0.263 2 0.034 9.2 0.196 6.8 -3.9

( 4 (el

H 2 0 Rate of change

Category (ppm/day) (%/day) (PPm/daY) (standard dev.)


(a) -0-008 f 0.036 -0.2 O*OOO (b) ( 4 ( 4 (el

0.015 f 0.030 0.4 0.OOO 0.052 2 0.046 1.2 0-OOO 0.012 2 0.042 0.3 0-OOO 0.050 f 0.031 1.2 0.OOO

0.4 -1.0 -2.3 -0.6 -3.2

Discrepancies significant at the 95% confidence level shown in bold.

Trajectories 143

(a) l2C

0 2 4 8 8 1 0 Time (days)

1 I I I 0 2 4 8 8 10

Time (day.)

Figure 18. Photochemical model results (solid lines) for the air particles which peeled from the vortex (trajectories 1-6, see Table l), together with co- located LIMS observations (solid circles). The pecked lines are linear regression lines through the data. (a) 0 3 volume mixing ratio (p.p.m.); (b) HNO, vol-

ume mixing ratio (p.p.b.).


Trajectories 7-11

(a) l2I Trajectories 12-16

..

I I I I I 0 2 4 6 8 1 0 10 oak+++-+- Time (days) Time (days)

0 2 4 6 8 1 0 Time Idaysl Time (days)

Figure 19. As Fig. 18 but for the air particles which were transported around the Aleutian high (tra-

jectories 7-11, see Table 1).

Figure 20. As Fig. 18 but for the air particles which remained on the vortex edge (trajectories 12-16, see

Table 1).

random errors, as suggested by the scatter of the data in the figure. On average the LIMS HN03 and O3 concentrations increased faster than in the model results but the discrepancies between model results and data are only just significant. However, the co- located observations show a strong latitudinal dependence and these fluctuations were not reproduced in the model. The discrepancies between model results and data for O3 and HN03 are smaller than the differences between the model results for the true trajectory and the control trajectory for the undisturbed atmosphere.

The model results and LIMS observations for the particles which were transported around the Aleutian high are presented in Fig. 19 (see section 5(b)). On average, the observed concentrations of HN03 and O3 increased at a faster rate than the model results although this was not quite statistically significant for 03. Although not shown here, the observed variation of NO2 (largely dominated by diurnal processes) was quite well reproduced in the model, in agreement with Solomon et al. (1986). The discrepancies between model results and data for O3 and HN03 are much larger than the differences between the model results for the true trajectory and the control.

The model results and LIMS observations for the particles which remained on the edge of the vortex are presented in Fig. 20 (see section 5(c)). Overall, the model O3 concentration decreased at a slower rate than in the observations but the discrepancy is not statistically significant. The discrepancies between model results and data for both HN03 and H20 are in the opposite sense to that for 03. For these particles the observed rates of change of the constituents O3 and HN03 are much closer to the model results calculated with the equivalent latitude rather than the true latitude.

The model results and LIMS observations for the particles which were transported to the subtropics are presented in Fig. 21 (see section 5(d)). The observed O3 concentration increased substantially, whereas the model maintained approximately constant O3 levels. In contrast the HN03 concentration decreased in the observations but increased in the model. Both O3 and HN03 discrepancies are highly statistically significant. The discrepancies between model and data for O3 and HN03 are larger than the differences between the model results for the true trajectory and the control.

The model results and LIMS observations for the particles which were transported to middle latitudes are presented in Fig. 22 (see section 5(e)). Both data and model


Trajectories 22-30 Trajectories 17-21

" 4 t 0 2 4 6 8 1 0

Time ldaysl 0 2 4 6 8 10

Time ldaysl

I I I I I I 4 I I 0 2 4 6 8 1 0 0 2 4 6 8 1 0

Time (days) Time (days)

Figure 21. As Fig. 18 but for the air particles which were transported to the subtropics (trajectories 17-

21, see Table 1).

Figure 22. AS Fig. 18 but for the air particles which were transported to middle latitudes (trajectories 22-

30, see Table 1).

concentrations of O3 decreased, although the change was greater in the model than in the data. Nitric acid concentrations increased in the model but not as fast as in the observations. This discrepancy is significant at the 95% confidence level. The H20 data also show a significant net increase above that of the model. The discrepancies between model and data for O3 and HN03 are much larger than the differences between the model results for the true trajectory and the control.

7. DISCUSSION AND CONCLUSIONS

In this paper a photochemical trajectory model (Austin et al. 1987) has been used to extend the analysis of the erosion of the stratospheric circumpolar vortex observed in late January 1979 (Butchart and Remsberg 1986) to include some quantitative estimates of the photochemical changes in constituents. An important prerequisite, however, was the demonstration in section 2(a) that a limited number of trajectories could be used to capture the correct signal of vortex erosion. By considering the behaviour of isentropic trajectories calculated for a large number (126) of particles initially densely packed within a small segment of the vortex edge, it was found that only four distinct categories of motion could be identified. Each category was composed of several trajectories with little spread in behaviour compared with the differences between the categories, and was therefore thought to be a useful indicator of actual air particle motion. The small spread also implied that any trajectory chosen from this segment would be broadly representative of one of the categories. It was assumed that similar arguments applied to each segment of the vortex edge and would also apply to the quasi-isentropic trajectories. There was then good justification for believing that the relatively few (30) trajectories used in the remainder of the paper were each representative of one of the broader categories of motion occurring around the vortex rim. Furthermore, the evidence suggested that only a limited number of these categories were likely to occur and therefore the results of integrating a photochemical model along the representative trajectories could be interpreted in a meaningful manner.

The quasi-isentropic trajectory analysis in section 2 showed that, on the 27 January 1979, the region at the edge of the vortex on the 850 K isentropic surface was composed


of air particles which could be divided into five categories according to their subsequent transport over the next ten days. One category represented the anticipated continued transport of air around the edge of the vortex. Two other categories involved the removal of air from the vortex by the action of the expanded Aleutian high. In the first of these the removal occurred almost immediately after the 27 January and the particles then followed trajectories around the Aleutian high. In the second the particles completed a circuit of the vortex before being removed. Similar erosion of the material vortex has been inferred from IPV maps for this period (McIntyre and Palmer 1983, 1984, 1985; Dunkerton and Delisi 1986) and, based on other potential vorticity studies (e.g. Clough et al. 1985; Juckes and O’Neill 1988; Fairlie and O’Neill 1988; O’Neill and Pope 1987), probably occurs frequently in the winter stratosphere where a persistent, but fluctuating, Aleutian high continually removes small pieces of the vortex. The final two categories found with the trajectory analysis had not been fully identified using the potential vorticity maps and indicated that the vortex was also being eroded in the eastern hemisphere. In the first of these, air particles removed on the 31 January ended up in the subtropics while in the second, removal occurred on the 30 January with the air particles ending up in mid-latitudes.

It is worth emphasizing that four of the five categories of motion identified in section 2 represented the removal of air from the vortex into the surf zone, while in the fifth the air remained at the edge of the vortex. No evidence was found which would suggest that air from the edge would be advected further into the vortex. This would support the conclusions obtained from some very-high-resolution one-layer model experiments (Juckes and McIntyre 1987) that the erosion of the vortex is very much a one-sided process. Indeed the picture that is now beginning to emerge is of a polar vortex that fluctuates in shape and position while remaining materially isolated from its surroundings. Of course it is possible that the restricted analysis involving only 30 trajectories may have failed to identify air particles penetrating the vortex though this seems unlikely from existing evidence. Certainly it would be desirable in future research to extend the trajectory analysis of section 2 to include air particles initially densely packed around the entire rim of the vortex. Results from such an analysis could also form the basis for estimating near-instantaneous erosion rates at the preferred locations of erosion and the total erosion rate checked against more conventional calculations. Work along similar lines has already been described by Lyjak (1987), Lyjak and Smith (1987) and Plumb (1987).

A possible source of doubt over the conclusions of section 2 is the inability of the satellite observations to resolve fine-scale structure similar to that found in numerical model simulations (e.g. Juckes and McIntyre 1987; Mahlman and Umscheid 1987). It is not known how sensitive the trajectories are to these small scales and a proper test could only be pursued with the aid of a comprehensive general circulation model (e.g. Mahlman and Umscheid 1987), which is quite beyond the scope of this work. However, there is an encouraging property of potential vorticity dynamics which suggests that resolution of the wind field may not be crucial for calculating trajectories. Essentially, it appears from observational and theoretical data (Juckes and McIntyre 1987; McIntyre and Palmer 1984) that in the situation of interest in this study Q will act in the large part like apussive tracer as scales become smaller. It is then likely that its evolution and the associated transport will be determined primarily by the large-scale wind field even when this leads to small scales in the Q field (Hoskins et al. 1985). Therefore, in the highly disturbed situation analysed in this paper, a satisfactory degree of confidence can still be placed in satellite-derived trajectories.

In quantlfying the photochemical changes of air particles it is important to verify the


photochemical model as far as possible. Direct comparison with LIMS data (Austin ef al. 1987) has gone some way to achieving this, although in the upper stratosphere such comparisons revealed the well-known model underpredictions of O3 (WMO 1986) when the recommended reaction rates were used (DeMore et al. 1985). However, in this paper the general effects of the transport on the photochemistry on timescales of about ten days is of primary consideration, rather than the details of the chemistry itself, and so the reaction rates adopted are probably not of major importance except possibly in the direct comparisons with observations (section 6).

The effects the transport had on the photochemistry of air particles originally distributed around the edge of the circumpolar vortex on the 27 January 1979 were quantified by comparing the results of integrating the photochemical model along representative ten-day trajectories with the corresponding results for trajectories calculated for an axially symmetric reference stratosphere. In this manner it was found that the distortion and displacement of the vortex from the pole during the minor warming was likely to decrease substantially the net destruction of O3 and the net production of HN03 at the edge of the material vortex. Even for particles which finally peeled away from the edge of the vortex after one circuit the photochemical decrease in O3 was found to be reduced from 11% to 3% and the increase in HN03 reduced from 20% to 12% in the ten-day period considered. On the other hand those particles which were eroded from the vortex earlier in the ten-day period experienced smaller perturbations in their photochemical evolution except when transported as far as the subtropics. In the latter case the results of the integrations showed that O3 increased by about 9% in the ten days as opposed to decreasing by 74% in the corresponding control integration. The effect on HN03 was less marked and the transport to the subtropics reduced the increase in concentration from 40% to 23%.

The use of LIMS observations in the initialization of the photochemical model also allowed the model results to be compared with co-located observations. However, unlike the above results where it was only important for the trajectories to be qualitatively realistic, the direct comparison of the model results with observations requires the additional consideration of trajectory accuracy. Although Austin (1986) suggested that trajectories may have large absolute errors of order loo0 km, these will not be too critical for many of the comparisons made in this study because of the weak horizontal gradients of species, particularly in the surf zone. Nevertheless, it was still decided to limit the emphasis to comparing only the mean rates of change of constituents for a complete ten- day model integration to the corresponding rates of change obtained from a linear regression fit to the co-located data. The initialization procedure then ensured that the concentrations of species in the model at the start of the integrations were the same as those given by the regression lines, thus removing another possible source of error. Moreover, the existence of categories of trajectories displaying similar behaviour allowed the effects of random errors to be further reduced by averaging the results for each category.

A statistical analysis of each category of motion revealed a number of significant discrepancies between the modelled and observed rates of change of species concentrations. This was especially true for trajectories ending in the subtropics. For this category the model showed O3 increasing by only 8% in ten days compared with the observed increase of 32%. Similarly HN03 decreased substantially in the observations (31% in 10 days) in contrast to an increase of 13% in the model. In general it does not appear that all the discrepancies between model and data can be directly attributable to the same reaction rate errors. For example the reaction rate R1 (see section 5 ) for the formation of HN03 has an estimated error of about 20% (DeMore et al. 1985) and


possibly could be the source of the HN03 discrepancy for those particles which were transported to middle latitudes. However, none of the other discrepancies can be due to this error, unless of course the reaction rate error is substantially underestimated.

The most likely explanation for the discrepancies, which is broadly consistent for all the categories and species, is the rapid transport of particles into regions where the concentrations of their constituents are no longer representative of the ‘averaged’ values observed by the LIMS instrument, with its spatial resolution of about 500km. This interpretation is consistent with the expected behaviour in wave-breaking situations in which air is eroded from the circumpolar vortex and mixed into the surrounding region while the vortex itself remains relatively unmixed. For those particles eroded from the vortex the discrepancies between the model and the observations were generally larger than the changes in photochemical evolution induced by the erosion itself (cf. sections 5 and 6). This suggests that on the ten-day timescale, perturbations to photochemistry during the erosion process are likely to be less significant than mixing in determining the observed distributions of species within the surf zone. It should be cautioned, however, that this does not imply that photochemistry will be unimportant on the longer timescales.

Although only one particular minor warming has been studied in this paper the results provide general insight into the coupling between dynamical and radiative/ photochemical processes in the wintertime middle stratosphere with important rami- fications for photochemical modelling. Whilst the results support the picture of a materially isolated circumpolar vortex they suggest that in general the distributions of chemical species within that vortex are likely to be strongly influenced by the perturbations in photochemistry induced by the reversible fluctuations in the shape and position of the vortex. The main cause of these perturbations appears to be the sharp transition in photochemistry which occurs on entering the polar night. In the January 1979 minor warming, mid-latitude air at the edge of the vortex was brought into the polar night by the disturbed flow. This then had two effects on the O3 destruction rate, firstly in reducing the odd nitrogen species which catalytically destroy 03, and secondly in reducing the sunlight available to drive the catalytic cycle. Low values of NO2 also reduced the production rate of HNO,. Obviously the time of year, air particle positions and the amplitude of the fluctuations of the vortex about the pole will all affect the precise details of how the presence of the polar night influences the photochemical evolution within the vortex and will therefore introduce an additional degree of complexity for a proposed new generation of two-dimensional chemical models based on the material vortex (McIntyre 1987).

Further, comparisons of the model results with observations suggest that there are important limitations in the use of chemical trajectory modelling. Although Austin et al. (1987) recognized the potential importance of inhomogeneities on spatial scales smaller than that observed, their impact was not fully appreciated. Unfortunately the technique is, in principle, of most value when considering particles which experience large displacements in latitude, and these situations are often associated with vortex erosion and the generation of small scales. Trajectory modelling is still of major value when applied to regions such as deep inside the circumpolar vortex where small-scale processes do not appear to be significant (Juckes and McIntyre 1987). One possible way of avoiding these difficulties is to develop a two-dimensional latitude-longitude chemical model on an isentropic surface with wind fields either supplied from a full three-dimensional dynamical model or derived from satellite observations. Since air particle motions are approximately isentropic, such a model could for periods of ten days or so include the effect of large- scale dynamics as well as the complete chemistry, just as in trajectory modelling. In addition, by solving the equations in an Eulerian framework, a diffusion term can be


incorporated to parametrize the important subgrid-scale processes. Such a model is currently under development (Austin and Holton, ‘A model of stratospheric chemistry and transport on an isentropic surface’, in preparation).

ACKNOWLEDGMENTS

We would like to thank Peter Haynes, Peter Jonas, Rod Jones, Michael McIntyre, Alan O’Neill, Vicky Pope, Ellis Remsberg and Adrian Tuck for their many helpful suggestions on earlier drafts of this paper. We would also like to thank Betty Kingston for producing the potential vorticity plots.

Austin, J.

Austin, J. and Tuck, A. F.

Austin, J., Garcia, R. R., Russell, J. M. 111, Solomon, S. and Tuck, A. F.

Austin, J., Pallister, R. C., Pyle, J. A., Tuck, A. F. and Zavody, A. M.

Bendat, J. S. and Piersol, A. G. Brasseur, G. and Solomon, S. Butchart, N.

Butchart, N. and Remsberg, E. E.

Callis, L. B., Natarajan, M., Boughner, R. E., Russell, J. M. I11 and Lambeth, J. D.

O’Neill, A.

Molina, M. J., Watson, R. T., Golden, D. M., Hampson, R. F., Kurylo, M. J., Howard, C. J. and Ravishankara, A. R.

Dunkerton, T. J. and Delisi, D. P.

Fairlie, T. D. A. and O’Neill, A.

Clough, S. A., Grahame, N. S. and

DeMore, W. B., Margitan, J. J.,

Gille, J. C. and Russell, J. M. 111

Haggard, K. V., Remsberg, E. E., Grose, W. L., Russell, J. M. 111, Marshall, B. T. and Lingenfelser, G.

Hoskins, B. J., McIntyre, M. E. and Robertson, A. W.

Jones, R. L. and Pyle, J. A.

1986

1985

1986

1987

1971 1986 1987

1986

1986

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1985

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Documents

A study of air particle motions during a stratospheric warming and their influence on photochemistry