Q. J . R. Meteorol. SOC. (1997), 123, pp. 785-800

Dynamical modelling of wintertime lidar observations in the Arctic : Ozone laminae and ozone depletion

By Y. J. ORSOLINI'*, G. HANSEN', U.-P. HOPPE3, G. L. M A " E Y 4 and K. H. FRICKE' Centre National de Recherches M&tkorologiques, France

2Norwegian Institute for Air Research, Norway 3Norwegian Defence Research Establishment, Norway

4Jet Propulsion Laboratoy, USA University of Bonn, Germany

(Received 26 January 1996; revised 8 July 1996)

SUMMARK A series of lower stratospheric ozone profiles taken by a lidar during February 1995 in northern Norway are

examined. The instrument location allowed continuous monitoring of rapidly evolving layered ozone structures, or laminae, near the polar vortex edge.

Observations under the vortex edge revealed laminae extending up to 20 km. Moreover, when the lidar was sampling vortex air, a thicker layer, characterized by unusually low mixing ratios, was also observed, thereby confirming satellite observations of Arctic ozone depletion during the winter 1994/95.

To unravel the respective role of dynamics and chemistry in generating these ozone-depleted layers requires better understanding of trace constituent lamination and filamentation through numerical modelling.

High-resolution modelled realistic transport of idealized tracers and satellite-derived ozone on many isen- tropes is therefore used to reconstruct fine-scale three-dimensional tracer fields. Tilted tracer or ozone sheets, peeled off near the vortex edge, lead to the formation of laminae in profiles. The model shows remarkable success in reproducing laminae intensification, thickening and lofting, analogous to the ones observed by the lidar over the course of a few hours.

KEYWORDS: Stratosphere Ozone Lidar Arctic polar vortex

1. INTRODUCTION

Fine-scale layering in lower stratospheric ozone profiles, or ozone laminae, has been suggested to be the signature of mixing processes in the vicinity of jet streams during the winter or spring (Dobson 1973; Reid and Vaughan 1991, hereafter RV91). Such ozone laminae are seen as layers enhanced in ozone, or alternatively depleted in ozone, and have typical thicknesses of about 1 km. They are most abundant in winter and spring near the polar vortex edge, and at a height of about 14 km (RV91). They have been identified in profiles derived from balloon-borne ozonesondes, and from ground-based and airborne ozone lidars. Other trace gases, and aerosol concentration, also exhibit laminated profiles highly correlated with ozone.

In the middle or upper stratosphere, vortex air mixes with extravortical air principally because of vortex erosion, that is, high-vorticity air is ejected into the mid-latitude surf zone in the form of filamentary extrusions. Entrainment into the vortex is not very common, and occurs during strong ridging events (Dahlberg and Bowman 1994; Plumb et al. 1994; Orsolini et al. 1995b). At lower heights, the circumpolar circulation does not create such a strong barrier to entrainment, and the vortex is more permeable to filamentary intrusions originating in mid latitudes (Chen 1994).

The aforementioned modelling studies showed that this process of erosion by fila- mentation induces an interlaced structure in trace constituents in the vortex edge region, and this is borne out by observations near the vortex edge in both hemispheres (Profitt et al. 1989; Murphy et al. 1989; Waugh et al. 1994a).

Orsolini (1995) also demonstrated in a model how such large-scale adiabatic motions give rise to tracer layering in the lower Arctic stratosphere. In qualitative agreement with high-latitude ozone soundings, laminae with vertical characteristic scales of about 1 km * Corresponding author: Mttto-France, CNRM, 42 avenue G. Coriolis, 31057 Toulouse Cedex, France.

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786 Y. J. ORSOLINI et al.

are generated. Such a layering is caused by the vertical tilt of tracer sheets being peeled off the vortex.

The explanation of lamina occurrences by height-dependent filamentation of the vor- tex is compatible with the large synoptic variability of such laminae, their preferred location near the vortex edge, and their seasonal distribution peaking in winter and spring. Much remains to be learned about this important topic of mixing near the vortex edge. The role of inertia-gravity waves in particular has not been assessed by models yet. Provided they have sufficiently large amplitudes, such waves may in principle induce additional vertical layering near the vortex edge.

Ozonesondes are not launched frequently enough to sample an evolving ozone lamina. In recent years high-quality lidar data have been acquired in the Arctic, which allow the near-continuous monitoring of lamina development over synoptic time-scales.

In this paper we describe the occurrence of such an ozone lamina in observations taken by a newly installed lidar system located in northern Norway, during mid-February 1995. A very high-resolution model, with horizontal resolution of about 60 km, is then used to study the fine-scale three-dimensional structure of tracer fields during that period.

Section 2 is a description of the lidar observations. Sections 3 and 4 are devoted to sim- ulations of the transport of an idealized tracer, and of satellite-derived ozone, respectively. Finally, conclusions are presented in section 5.

2. LIDAR OBSERVATIONS OF OZONE AND AEROSOL AT ANWYA IN FEBRUARY 1995

(a) Measurements Since December 1994 an ozone D W lidar system has been operational at the Arctic

Lidar Observatory for Middle Atmosphere Research (ALOMAR), near the Andgya Rocket Range in northern Norway (69.3"N, 16.O"E) (Hoppe et al. 1995). Owing to its high-latitude location, the instrument frequently samples air masses within the winter polar vortex. It also allows recovery of ozone profiles with high vertical resolution, and sufficient temporal coverage for studying ozone laminae. Several ozonesondes were launched to validate the measurements. In this paper we examine a set of measurements taken in mid-February 1995.

Figure 1 shows two lidar ozone profiles taken on 14 and 15 February 1995; ozone partial pressure is plotted as a function of altitude up to a height of 25 km. Profiles de- rived from ozonesondes launched from Andgya, 2 km away from the ALOMAR site, are also shown. Owing to the large dynamical range of the backscattered signal, the lidar sequentially takes measurements with matching configurations of neutral density filters and chopper cut altitudes in order to protect the photomultipliers from saturation. In the data set shown here, the backscatter profile was cut first at 9 km using an attenuation of 100, then at 12 km using an attenuation of 10, and then at 18 km without attenuation. The altitude ranges 9-13 km, 13-19 km and 19-40 km were evaluated separately, and finally put together to a full altitude profile. The pressure, total density and temperature data needed to calculate partial pressure and mixing ratio were taken from the ozonesonde sounding plus the CIRAt 86 reference atmosphere above the sonde-burst altitude.

The data have morevover been averaged over two-hour periods. The profile of 14 February is averaged between 1930 UTC and 2130 UTC, and its vertical resolution below 25 km is about 300 m. The profile of 15 February is averaged between 1830 UTC and

* Differential absorption lidar

Atmosphere COSPAR (Committee on Space Research (International Council of Scientific Unions)) International Reference

DYNAMICAL MODELLING OF LlDAR OBSERVATIONS 787

0 5 10 15 20 0 5 10 15 20

Figure 1. Two ozone profiles measured by the lidar at the ALOMAR at (a) 1930-2130 UTC 14 February and (b) 1830-2030 UTC 15 February 1995. The dashed line to the left is the measurement error. Tko ozonesonde profiles

are also plotted (dash-dotted lines). Prominent ozone laminae are seen below 20 km.

ozone partial pressure (rnPa)

2030 UTC, and its vertical resolution is about 200 m. On 14 February the ozonesonde was launched at 1826 UTC and reached an altitude of 14 km. On 15 February the launch time was 1827 UTC, and the sonde rose up to almost 25 km. The sondes drifted horizontally by about 20 km in a two-hour ascent period. Hence, ozonesonde and lidar measurements are not exactly coincident. Moreover, they sample slightly different air volumes.

The measurement error in the lidar profiles is indicated on Fig. 1. The error due to aerosol backscatter is estimated to be in the range 5-10 per cent. The results from both techniques for 15 February nevertheless show that the ozonesonde profile nearly equals the time-averaged lidar profile during a portion of ascent.

The Rayleigh/Mie/Raman lidar of the ALOMAR facility also measured aerosol backscatter ratios at 532 nm wavelength during the night of February 15/16 1995. The vertical resolution was set to 150 m. The backscatter ratio is obtained from the ratio of the signals received in the Rayleigh/Mie and the Nitrogen/Raman channels, which is normal- ized above the aerosol layer. Details of the instrument are described by von Cossart et al. (1995).

(b) The prominent ozone laminae on 15 February Figure 2(a) shows the temporal development of laminae in the lidar profile throughout

the night of 15 February. Ozone mixing ratio is now plotted as a function of potential temperature, to facilitate comparisons with model results. The range is 375 K-500 K, that is about 14 to 20 km. Profiles have been reconstructed at standard meteorological times, every 3 hours, starting from 15 February (1800 UTC). The last profile, however, is at 0200 UTC owing to instrument shutdown. The potential temperature is derived from contemporaneous meteorological analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF). The 6-hourly temperatures at the horizontal resolution T106 have been used, at the closest grid point to Andaya. When contemporaneous temperatures are not available, at 2100 UTC for example, an arithmetic average between the two closest 6-hourly temperatures has been taken.

The major feature in this set of profiles is the development of an ozone-depleted lamina, near 450 K. The profile is near-uniform at 1800 UTC, but for a small dent at 450 K. At 2100 UTC one sees a drop in a layer centred near 435 K, and an increase below. At 0000 UTC considerable thickening of the depleted lamina is accompanied by its increase in magnitude. By 0200 UTC the lamina ceased to intensify but it further thickened.

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Figure 2. (a) A series of near 3-hourly ozone profiles (mixing ratios in tens of p.p.m.v.) measured in the night of 15/16 February 1995 by the ALOMAR ozone lidar. Height is indicated by potential temperature, 8. The profile derived from an ozonesonde is also shown at 1800 UTC (dotted line). The lamina near 450 K is evolving rapidly, rising slightly and thickening. (b) Similar series of 532 nm aerosol backscattering ratio profiles, as measured by

the ALOMAR Rayleigh lidar. Note the aerosol-rich layers, e.g. near 450 K.

Profiles of the aerosol backscattering ratio, R , at 532 nm are shown on Fig. 2(b) at standard meteorological times in the same layer. Potential temperature was again calculated from the ECMWF temperatures. The layered structure in R correlates with the layered structure in ozone mixing ratio, although the low-level laminae are more apparent in R than in ozone mixing ratio. At the time these measurements were taken, the volcanic aerosol layer which originated in the Mount Pinatubo eruption had nearly vanished, and winter- long observations above Andaya (Muller et al. 1995) showed that R was in the range from 1.1 to 1.2, except when polar stratospheric clouds (PSCs) were detected. For example, on February 16 (0000 UTC), a layer centred near 450 K is seen to have a high R , i.e. to be aerosol rich, and to correlate with the ozone-poor layer on Fig. 2(a). Just underneath and above are layers poor in aerosol, typical of vortex air. On February 15 (2100 UTC) note the correspondence between the three local maxima in R and the three local minima on the ozone mixing ratio profiles.

DYNAMICAL MODELLING OF LIDAR OBSERVATIONS 789

The two independent lidar observations reveal a pattern with multiple laminae. The good correlation in these two sets of measurements at high spatial and temporal resolution suggests a dynamical cause, and an extravortical origin for the aerosol-rich, ozone-poor layers. Such an anticorrelation of ozone and aerosol content in a lamina of tropical origin was observed by Pittock (1966) in the mid latitudes, after the eruption of Mount Agung in Bali. Later observations of highly correlated laminae in ozone, aerosol content or trace gases can be found in di Sarra et al. (1992) or Newman et al. (1996).

(c ) Laminae and depletion on 14 February The profile on 14 February (2030 UTC) on Fig. 1 also shows several well-marked

laminae below 20 km, reaching large magnitudes at low levels. Further inspection reveals low ozone partial pressure in a thick layer above 15 km. For example, unusually low volume mixing ratios of 0.2-0.22 (in tens of p.p.m.v. (parts per million by volume)) are found near 465 K, or 18.5 km.

Ozone derived from the Microwave Limb Sounder (MLS) instrument aboard the Up- per Atmosphere Research Satellite (UARS) is available on 14 February (Manney et al. 1996; hereafter M96). The low spatial-resolution ozone distribution at 465 K is indeed indicative of low ozone mixing ratio, typically of 0.25-0.27 (in tens of p.p.m.v.) above the site, and lower values in a large ozone-depleted area to the north-east over northern Asia. The ozone-depleted region measured by the MLS actually extends as high as 520 K, or 22 km, at least. Hence the lidar measurements are compatible with the satellite measure- ments, and the thickness of the depleted layer suggests that it was not a laminar structure. Lidar ozone measurements in the Canadian Arctic also show that average mixing ratios decreased by 15 per cent from early January to mid-February 1995 below 500 K (Donovan etal. 1995).

3. AN IDEALIZED-TRACER SIMULATION

(a) A short description of the model The high-resolution Prather advection model described by Orsolini et al. (1995a) is

used to transport tracers passively on isentropes, over a time period of about two weeks. It is used here on a Gaussian grid T213, i.e. with a spacing of about half a degree. Six-hourly winds derived from the ECMWF analyses at the T106 resolution were used in order to capture synoptic events. There are about five pressure levels where the ECMWF winds are archived in the layer considered here. The simulations span 31 January to 16 February. The idealized tracer is the initial analysed potential vorticity, scaled to obtain a number between 0 and 1. It is hereafter considered as a proxy mixing ratio. Isentropic advections are carried out independently on a series of isentropes spaced every 5 K between 375 K and 500 K. The fine-scale three-dimensional tracer distribution is then deduced from the vertical superposition of the isentropic distributions, a method developed independently by Orsolini et al. (1995a) and Newman and Schoeberl(l995).

(b) The peeling of the polar vortex Figure 3 shows the tracer on the 450 K isentrope on 16 February (0000 UTC). The

ALOh4AR site is indicated by a star. As in many other studies, there is ubiquitous fila- mentation of the tracer field. Particularly obvious on this map is the erosion of the vortex by filamentation at its edge. As a filament is drawn out, pairs of rich-poor streamers form, nearly parallel to the distribution edge. Over northern Europe and the northern Atlantic,

790 Y. J. ORSOLINI et al.

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Figure 3. ECMWF model tracer distribution at 450 K on 16 February 1995 (0000 UTC). The star indicates the location of A n d ~ y a (69.3"N, 16.O"E). near the vortex edge.

Figure 4. Distribution of potential vorticity tracer in a potential-temperatureilatitude cross-section near the vortex edge, at the longitude of Anddya (1h"E) on 16 February 1995 (0000 UTC). The latitude range is only 8'.

two such pairs of streamers are present at that time near the 70"N latitude circle. A large blob, rich in tracer, is also drawn out over eastern Asia, and cyclonically wrapping up.

The contouring mainly emphasises vortex-edge filamentation, but strong mixing is present in the mid latitudes. Well-marked tracer-poor filaments are seen to meander over Europe, with a south-westerly origin.

The three-dimensional structure of this peeling process can be seen on Fig. 4, which shows a meridional cross-section at the longitude of Andoya (16"E) on 16 February (0000 UTC). The latitudinal range is narrow on that figure (go ) , but the model's high resolution

DYNAMICAL MODELLING OF LIDAR OBSERVATIONS 79 1

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Figure 5. Tracer meridional profiles o n four isentropes at the longitude of 16"E on 16 February 1995 (0000 UTC). Note the varying vertical scale. The thick (thin) line is for the T213 (T106) simulation.

allows a detailed view of the vortex edge region. Several interleaved sheets of tracers are intersected, which slope equatorward with height. Two major tracer-poor sheets are indicated by dotted lines, and can be traced at 450 K on Fig. 3 as well. The scales of such features, several hundred kilometres in latitude, roughly 1 km in height, are compatible with airborne observations of ozone and aerosol sheets (Reid etal. 1993; Plumb etal. 1994; Orsolini 1995). Figure 5 provides another view of these meridionally sloping sheets: tracer mixing ratio is plotted on several isentropes over a broad range of latitudes, again at 16"E on 16 February (0000 UTC). The thin lines correspond to a simulation at twice-degraded resolution (T106). Superposed on the general tendency for tracer mixing ratio to increase poleward, one sees the signature of enhanced (depleted) filaments in the vortex edge region near 70 N. One sees clearly the locations of these maxima (minima) being displaced in latitude at varying heights, indicating meridionally sloping peels. Note that at 450 K there is much improvement in the edge region by doubling the resolution, one of the two enhanced filaments of Fig. 3 being hardly noticeable at the coarser resolution. Locally, the relative change in mixing ratio between the two simulations is of the order of 50 per cent.

( c ) Laminae on 15 February In the vortex edge region, the tilt of such tracer sheets, or peels, leads to strong

lamination of vertical profiles. The laminated structure varies strongly over time-scales of hours, as narrow filaments pass over a grid point. Figure 6 shows 3-hourly model profiles at two grid points close to Andoya throughout the night of February 15/16. The vortex edge at 450 K can be seen to straddle A n d ~ y a on Fig. 3; hence, two points at the longitude of Andoya but separated by a degree in latitude were used for the comparison. It turned out that the profiles at 68.3'" mimic the observed profiles better than those at 69.3"N, which may mean that the model has a slight phase shift relative to the observed pattern.

792 Y. J. ORSOLINI et al.

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Figure 6. A series of 3-hourly tracer profiles at two locations near Andoya ((69.3"N, 15.9"E) - thin dashed lines, and (68.3"N, 15.9"E) - thick lines) throughout the night of 15/16 February 1995. Height is indicated by potential

temperature, 0

For example, on 16 February (0000 UTC), one sees, at 68.3"N, the major enhanced/depleted laminae near 450 K. For example, the enhanced lamina, peaking at 445 K, is caused by the eroded tracer sheet which comprises the filament seen at 450 K just south of Andsya on Fig. 3 (see also Fig. 4). The tracer sheet slopes equatorward with height, and passes over Andsya slightly below 450 K.

The progressive thickening of the depleted lamina seems to be well captured by the model. Interestingly, both in the model and in the ozone lidar observations, there is a lofting of the laminae pair with time, both the minimum and maximum ascending. This lofting, seen clearly on Fig. 6, is caused, in the model, by meridional motions of the sloping peels, mostly aligned east-west (see Fig. 4): in this particular case, northward motions of the stacked peels induce upward displacement of tracer maxima and minima, and in fact of the laminae pair, over a few hours. The lofting rate is about 5 K per hour, both in the lidar observations and in the model. The aerosol concentration also reveals such a lofting with time (see Fig. 2(b)). There is fast intensification of the laminae between 1800 UTC and 0000 UTC in the lidar observations. In the model, it is occurring more slowly and it has not ended at 0300 UTC.

Finally, Fig. 5 indicates that, during the filamentation, tracer-poor air masses from low latitudes are drawn in contact with polar air (see also Fig. 4) (Waugh et al. 1994b).

Although the run at the highest resolution shows more resolved filaments at high latitudes, Fig. 5 shows a lack of low values which would betray the recent entraiment of mid-latitude air within the vortex. This is in agreement with the climatological study of Dahlberg and Bowman (1994), and the numerical simulation of Pierce and Fairlie (1993), which both showed mixing within the wintertime vortex but lack of entrainment, except during strong ridging.

( d ) Remarks on filamentation at low levels Ozone laminae are frequently observed as low as 12 km (RV91) (see also Fig. 1).

Intense filamentary mixing occurs in the model at the lowest levels, although horizontal variations in mixing ratio are masked by the stratification in previous figures. The filaments

DYNAMICAL MODELLING OF LIDAR OBSERVATIONS 793

Figure 7. Geopotential height (in km) at 150 mb on 16 February 1995 (0000 UTC), derived from the ECMWF analyses. Cyclonic and anticyclonic centres are seen over eastern Asia, the northern Pacific, and the eastern

Mediteranean.

at these levels are strongly drawn into the mid latitudes, especially in two geographical regions, over Europe and the northern Pacific. Figures 7 and 8 show the geopotential height field at 150 mb on 16 February (0000 UTC), and the 400 K tracer map on the same date. It can be seen that strong cyclonic, or anticyclonic, centres are instrumental in creating the interwoven filamentary extrusions and gyres of Fig. 8. Note on Fig. 7 the tripole of cyclones/anticyclones over the northern Pacific and eastern Asia, and the cyclone over the eastern Mediterranean. These geopotential disturbances are evanescent with height, and only the cut-off low over eastern Asia is still prominent at 50 mb. Hence, low-level exchange processes and lamination are likely to be stronger in these preferred longitude ranges, owing to the climatological features of the upper tropospheric flow, such as blocking ridges or cut-off lows (Price and Vaughan 1992; O’Neill et al. 1994; Norton and Chipperfield 1995).

Fine-scale intrusions into the high latitudes occur at these low levels, due to a weak- ened vortex barrier. Depleted laminae can be seen at the northernmost latitudes, as the result of such intrusions (see Fig. 8, over northern Greenland).

Appenzeller and Davies (1992) found that downward intrusions of stratospheric air into the troposphere were apparent as fine streamers in Meteosat water vapour channel im- ages. These correlated nicely with the coarser potential-vorticity analyses of the ECMWF (T213). These satellite images displayed remarkable fine-scale details (200 km) in the formation and evolution of filaments, such as fragmentation and break-up into vortices, and roll-up of filament tips. Our model resolution is still too coarse to model fragmentation of filaments, but nevertheless, at 380 K, there is evidence for initial roll-up of streamers in S-shaped features (not shown for brevity).

794 Y. .I. OKSOLINI et ul.

4. S I M U I ~ A T I O N WITH MLS OZONE

( a ) lnitializution of the ozone field A more direct comparison with the lidar ozone observations can be carried out if

the model is initialized with a global, but approximated, low-resolution ozone field, such as one derived from 205 GHZ MLS observations. MLS observations were performed infrequently in February 1995, and they had degraded vertical resolution. However, MLS observations are available on 1 February. These data, taken over a 24-hour period centred near 12 uric' o n I February 1995, are griddcd at 4' latitude by 5 longitude as described by M96, and interpolated on the 420 K, 465 K and 520 K isentropes. Ozone on each isentrope used in the model between 410 K and 500 K was determined by a linear fit between the three isentropic values at each grid point. The initial ozone distribution hence increases linearly with potential temperature, and contains no laminar structures. The MLS isentropic distributions were computed in M96 using temperatures from the U K Meteorological Office assimilations, but the discrepancies with the ECMWF temperatures were checked so as not to exceed 2 K. The initial ozone field in the thin stratospheric layer considered here (375-500 K) includes mainly data from the MLS retrievals for 100, 46 and 22 mb. The precisions of high latitude MLS data at these levels were estimated by Froidevaux et al. (1996) to be approximately 50, 10 and 3 per cent. They furthermore compared about 30 ozonesonde profiles taken at Gardermoen (60 'N, 1 1 ',E), Norway, throughout 1993-but mostly during winter-with MLS profiles; they found a systematic bias of 0.3 p.p.m.v. in that height range, with MLS being larger.

Thus, due to the coarse vertical resolution of the current MLS retrievals, the large uncertainties in MLS ozone at the lowest levels, and the crude vertical interpolation, the initial ozone field has to be understood as a first guess, giving the general morphology of the large-scale ozone field.

The MLS observations in early February (M96) indicate that strong Arctic ozone depletion had occurred then, following a long period of cold stratospheric temperatures, favourable for PSC formation. PSCs were detected above Andoya (Muller et a/. 1995) in all observations made between 11 and 21 January 1995.

(6) ReJults of the ozune simulation The region of low ozone in the interior of the vortex observed by the MLS on 1

February, and advected by the model, is seen o n Figs. ',(a) and (b), on 14 February (2100 UTC) and I6 February (0000 u IC) respectively, both at the 450 K level. Ozone-rich filaments edged away from the vortex on these figures are seen to be strongly correlated with the tracer filaments of Fig. 3. At high latitudes, however, such a correlation is not seen, since ozone in the interior of the vortex was chemically depleted by the initialization data and, therefore, no longer correlated with potential vorticity. Figure 9(a) indicates that, on 14 February, the lidar sampled air advected from the region where ozone depletion had occurred in January; this transport of depleted air leads to observed ozone-poor filaments (additional chemical depletion may also have occurred between 1 and 14 February (M96)). The high-resolution simulations suggest that the low ozone levels observed by the ALOMAR lidar above 15 km on 14 February are not caused by extravortical intrusions. These observations confirm the occurrence of ozone depletion in the Arctic during the winter 1994/95.

On 15 February i t was sampling the ozone distribution along the vortex edge. A sequence of model profiles throughout night-timc on 15 February, near Andoya, shows remarkable similarity with the lidar profiles (Fig. 10). One can see the rapid evolution from a near-uniform profile at 1800 u rc to a profile characterized by a strong ozone-depleted lamina near 450 K at 2100 U I C . Contrary to what is seen in the lidar observations, the

DYNAMICAL MODELLING OF LIDAR OBSERVATIONS 795

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Figure 8. ECMWF model tracer distribution at 400 K on 16 February 1995 (0000 UTC), poleward of 25.". The horizontal resolution of the figure is degraded by a factor two, compared with Fig. 3.

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Figure 9. Transported MLS ozone at 450 K on (a) 14 February 19Y5 (2100 UTC) , and (b) 16 February (0000 UTC). The uni t s are tens of p.p.m.v.

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Figure 10. A series of 3-hourly profiles of transported MLS ozone at two locations near Andoya (69.3"N, 16.0"E, thin 1ine)and (68.3"N, 16.0"E, thick line) throughout the night of 15/16 February 1995.The unitsare tensof p.p.m.v.

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Figure 11. The 3-hourly ozone mixing ratio at 450 K above And~ya (68.3"N, 16.O"E) from 14 February (1200 UTC) to 16 February 1995 (0600 UTC), from th'e MLS transport simulation (full line). The lidar observations are

also shown (diamonds). The deep minimum indicates the passage of a depleted lamina above the site.

lamina does not further intensify after 2100 UTC, and it is not as thick. The lofting rate is, however, well reproduced.

The 3-hourly evolution of mixing ratio at 450 K above Andoya between 1200 UTC 14 February and 0600 UTC 16 February is shown on Fig. 11, along with the lidar observa- tions, including the 3-hourly observations taken during the night of February 15/16. The minimum in model ozone is reached on 16 February (0000 UTC), in excellent agreement with the observations. This provides strong evidence for the passage above Andoya of an ozone-poor lamina. Variations in mixing ratio are larger in the lidar measurements,

DYNAMICAL MODELLING OF LIDAR OBSERVATIONS 797

and these are slightly lower than the transported MLS ozone mixing ratios. This may be caused by an overall off-set of satellite measurements, and the initial vertical smoothing of the satellite-derived ozone field. Weak variations in ozone mixing ratios can be seen throughout 14 and 15 February, when Andaya was under ozone-depleted inner-vortex air masses. The decrease associated with the lamina, and with the presence of extravortex air, culminating on February 16 (000 UTC), is however more dramatic and abrupt.

5. SUMMARY A N D DISCUSSION

Several major laminae form in tracer profiles by the peeling and shearing of several interleaved layers of tracer. It is captured in this model because of the high horizontal and vertical resolution. Multiple laminae, e.g. an enhanced layer paired with a depleted layer underneath or above, appear as the result of the interleaving of air masses of different ozone content, and of different latitudinal origins. The statistical study of Reid and Vaughan (1991) shows that the number of major laminae per profile is between 1 and 2 in the height range of 10 to 20 km. Major laminae means that the thickness is between 200 m and 2.5 km, and the magnitude in partial pressure greater than a threshold of 20 nbar. The model results are compatible with these findings, and two tracer peels are seen on Fig. 4.

Strong filamentation and vertical shearing of filaments occur atop of lower-level cyclones or anticyclones, hence preferably in regions of upper-troposphere climatological disturbances.

Such model laminae are strongly variable on synoptic time-scales and over a scale of one hundred kilometres. In agreement with lidar observations, a large laminae pair is seen in the model on 16 February (0000 UTC) near Andaya at the 450 K level. The model lamina reaches its largest magnitude at the same time as the one in the observations.

Model laminae move up or down over a few hours, by the meridional motions of tracer sheets, slanting with latitude. This property has not been discussed before, to the authors' knowledge, but is borne out by the lidar observations.

The depleted laminae in the modelled profiles obtained in a simulation using potential vorticity as a proxy tracer exhibit a deepening and thickening more consistent with the observations than those obtained in a simulation with satellite-derived ozone. This could be due to the fact that the former tracer field is initially in balance with the flow, while the latter is derived from asynoptic measurements; hence the latter has initial isopleths which are not exactly aligned with the polar vortex.

There are obvious limitations to this modelling approach. First, the simulation is based on an hypothesis of isentropic motions. Diabatic motions will influence tracer dis- tributions on a time-scale of about two weeks. Because of its finite resolution and its underlying large-scale dynamics, the model necessarily captures only a fraction of the real filamentation process, likely to be characterized by a tracer cascade down to ever-thinning scales. Nevertheless, laminae similar to those observed can form in the model tracer fields, through differential advection by vertically varying winds.

On 15 February the laminae taken at 1" south of Andaya display more similarity with the laminae seen in the lidar observations. Other models have also a moderate degree of success in predicting the occurrence and location of filaments (Waugh et al. 1994a), even in the northern hemisphere extratropics where the quality of the lower-stratosphere analyses are the best. This predictability capability of our model has improved since Orsolini (1995) owing to increased resolution, and to careful time matching with the lidar observations.

The remarkable degree to which the model is able to reproduce some properties of the laminae, such as intensification, thickening and lofting, demonstrates the value of global high-resolution off-line transport models in interpreting ground-based measurements.

798 Y. J . OKSOLINI et ul,

ACKNOWLEDGEMENTS

This work was done while the first author was a Visiting Scientist at the Centre National de Recherches Miteorologiques (CNRM), and was supported by the European Union (EU) Environment and Climate Programme, under the HODAS project. P. Simon of CNRM is gratefully acknowledged for providing the transport code. The support of the ozone lidar by the Norwegian Industrial and Regional Development Fund, by the Norwegian Space Centre, and by the Norwegian Department of Defence is gratefully ac- knowledged. The excellent support of the AndGya Rocket Range in running the ALOMAR facility and launching ozonesondcs is gratefully acknowledged. The Rayleigh/Mie/Raman lidar was supported by the EU Environment and Climate Programme, the Bundertninis- terium fu r Forschnung und Technologic (Germany), the Science and Engineering Research Council (UK), and the Centre National de Recherche Scientifique (France). Work at the Jet Propulsion Laboratory was done under contract with the National Aeronautics and Space Administration (US A).

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