Sensitivity of ozone and temperature to vertical resolution in a gcm with coupled stratospheric chemistry

Q. J . R. Meteorol. SOC. (1997), 123, pp. 1405-1431

Sensitivity of ozone and temperature to vertical resolution in a GCM with coupled stratospheric chemistry

By JOHN AUSTIN*, NEAL BUTCHART and RICHARD SWINBANK Meteorological Ofice, UK

(Received 26 January 1996; revised 2 September 1996)

SUMMARY

Results are presented from a general-circulation model with comprehensive stratospheric photochemistry and which includes the coupling between radiative heating and simulated ozone. Each model integration covers the 60-day period beginning 15 January during which episodes of polar stratospheric clouds (PSCs) are normally observed in the Arctic. Results from two versions of the model with different numbers of atmospheric levels near and above the tropopause are compared with observations. In the 19-level model the ozone transport is poorly simulated and, in particular, there is a significant increase in the tropospheric column. In contrast, in the 49-level model the simulated ozone distribution is in good general agreement with observations and reproduces well the steep vertical gradients in ozone mixing ratios in the tropical lower stratosphere, and the weak vertical gradients in the high-latitude middle stratosphere. This version also maintains a virtually constant tropospheric ozone column. Since in the 49-level model the ozone distribution is well simulated, including this ozone in the radiation calculation has only a moderate influence on the results. However, with the 19-level model, it substantially increases the global stratospheric temperature error. Increasing the number of levels improves the simulation of stratospheric temperatures but the zonal-mean temperatures in both versions of the model are generally lower than observed. Despite this, the modelled frequency of occurrences of PSCs is less than observed because of the underprediction of the zonally asymmetric component of the temperature distribution. The results suggest that if coupled chemistry4imate simulations are to proceed, it is important to have both a high upper boundary and good vertical resolution in the lower stratosphere to ensure a realistic meridional circulation and a more accurate representation of ozone transport across the tropopause.

KEYWORDS: General-circulation model Middle atmosphere Photochemistry Polar stratospheric clouds Vertical resolution

1. INTRODUCTION

As climate models have become more realistic, the need to take account of atmospheric chemistry has become increasingly apparent (e.g. Houghton et al. 1990, 1992 and references therein). Of the main radiative species, ozone is possibly the most complex as it is controlled by both dynamical and photochemical processes with a wide range of time-scales in different parts of the atmosphere. There are also many different chemical feedbacks on climate. For example, chlorofluorocarbons (CFCs) have both a greenhouse warming effect (Houghton et al. 1992) and deplete stratospheric ozone (Rowland and Molina 1975). Thus increasing CFCs may have a smaller impact on surface temperature than originally thought since the direct greenhouse warming may be partially offset by the cooling associated with lower ozone levels just above the tropopause (Lacis et al. 1990; Ramaswamy et al. 1992). The lower stratospheric region is also one of the most difficult to model accurately because of the presence of heterogeneous reactions on polar stratospheric clouds (PSCs) (Crutzen and Arnold 1986).

Because of their computational expense few climate models have been run with full chemistry. Rasch et al. (1995) have completed a 2-year integration of the National Cen- ter for Atmospheric Research Community Climate Model with gas phase chemistry but without coupling the ozone to the radiation scheme and without including heterogeneous chemistry. However, many climate models generate an unrealistically cold lower strato-

* Corresponding author: Meteorological Office, London Road, Bracknell, Berkshire RG12 2SZ, UK.

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sphere despite some noted improvements in the latest model versions (e.g. Hack et al. 1994). The remaining problems may be due, at least in part, to insufficient momentum deposition by vertically propagating gravity waves (Garcia and Boville 1994; Boville 1995). The performance of general-circulation models (GCMs) in the upper troposphere and lower stratosphere is also sensitive to the height of the upper boundary (Boville 1984; Boville and Cheng 1988). This in turn would affect the accuracy of both the transport and the photochemistry, possibly driving the model away from observations. Austin and Butchart (1995) have explored some of these problems using 40-day integrations of the UK Meteorological Office (UKMO) Unified Model (UM). These preliminary studies sug- gested that a bias in the lower-stratospheric temperatures could increase the amount of PSCs and produce excessive ozone depletion by heterogeneous processes. The cold bias would be further compounded by the feedbacks from the reduced solar heating, The effect of additional cooling might be to reduce the propagation of planetary waves into the polar vortex, leading to an atmospheric regime with a more stable polar vortex. Conse- quently there is the prospect that a fully coupled chemistry-climate model could generate for current conditions an ozone hole in the Arctic region, where an ozone hole does not presently occur. This contrasts with the results of Austin et al. (1992) which demonstrated the possibility of an Arctic ozone hole for doubled CO2 conditions but not for the current atmosphere. If such a spurious ozone hole were to occur, perturbations to the model lower- stratospheric climate might then follow similar patterns as those induced by the Antarctic ozone hole (Kiehl et al. 1988; Cariolle et al. 1990; Mahlmann et al. 1994).

Therefore, in modelling the chemistry-climate system, it is first necessary to establish the degree of complexity that is required in the stratospheric meteorology to avoid unrealistic perturbations to the chemistry. Such perturbations might take place over time-scales ranging from the lower-stratospheric radiative time-scale of a few months to much longer time-scales associated with climate drift. In this study two standard configurations of the UM with different numbers of vertical levels have been integrated with full stratospheric chemistry and using model-calculated ozone in the radiation scheme. Because of the computational expense the integrations have been restricted to the 60-day period following 15 January when the lower-stratospheric ozone is most vulnerable to chemical depletion before the final warming. However, for each model version, four integrations with different initial conditions were performed, allowing the results over four simulated winters to be compared with the observed behaviour in the four years 1992-95 in which high-quality stratospheric meteorological analyses have been produced by the UKMO. A possible dis- advantage of this approach is that the coupled model’s evolution may still be influenced by adjustment towards its own specific climatology, but the results suggest that this was nearly achieved by the end of the 60 days. Moreover, any adjustment is not expected to affect the particular results reported in this paper, nor the conclusions.

The two versions of the UM used in this study are a 19-level version similar to the atmosphere GCM used in climate studies by the Hadley Centre (e.g. Mitchell et al. 1995) and a 49-level version. This latter version was developed from a 42-level version of the UM used for stratospheric data assimilation (Swinbank and O’Neill 1994) by increasing the vertical resolution in the stratosphere. Extra levels were also added to avoid any potential problem associated with the placement of the 42-level model upper boundary near the polar-night jet maximum. The main aim is to study the effect of vertical domain and resolution on the simulation of stratospheric ozone and temperature for the two versions of the UM. Differences in simulated ozone, due to transport and chemistry changes, are investigated. The consequences for the thermal structure of the stratosphere are established due to the coupling between ozone and radiative heating within the model. Finally, the implications of the results for the simulation of PSCs are considered.

OZONE AND TEMPERATURE IN A GCM WITH COUPLED CHEMISTRY 1407

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2. MODEL DESCRIPTION AND INTEGRATIONS PERFORMED

(a) General model description The Meteorological Office Unified Climate and Forecast Model (UM) is a general-

circulation model with a flexible structure to allow a variety of different uses from the core- model computer code (Cullen 1993), although in practice its use is restricted to a small number of well-tested configurations. The UM is here used in two versions, with 19 and 49 levels, which are the standard configurations used in the UKMO for climate prediction and for stratospheric work respectively. Near the ground, sigma levels are employed going over to hybrid sigma/pressure levels and finally to constant pressure surfaces in the stratosphere. Figure 1 schematically illustrates the model levels. The 19-level model has a top level at 4.6 mb while the 49-level model has a top level at 0.1 mb. The first 12 levels are almost identical in both model versions and the levels are isobaric from levels 17 (19-level model) and 21 (49-level model) corresponding to 40 and 50 mb respectively. In the stratosphere of the 49-level model the levels become equally spaced in the logarithm of pressure, giving an approximate vertical resolution of 1.3 km. (See Table 1 for details of both models.)

The UM contains a full set of physical parametrizations and also has the facility to advect tracers (see section 2(b)). For this study an additional package has been developed to provide a comprehensive treatment of stratospheric photochemical processes and the UM radiation scheme modified to replace the usual climatological ozone data by model- calculated values (see section 2(c)). The 19-level model is as used in typical climate integrations except that the dynamical time step is reduced to 20 minutes for consistency with the 49-level model. Also, a proper Gregorian calendar is used as the photochemistry requires the correct solar zenith angles. Sixth-order diffusion is applied to the wind and temperature-related fields except that this is changed to normal (second-order) difhsion at the top level in the 19-level model. Compared with that used by Austin and Butchart (1995) there have been a number of minor changes to the 19-level model but the change


TABLE 1. MODEL DETAILS

19-level 49-level

Starting dates of runs Length of integrations Time step Horizontal resolution

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Sigma coordinates for the first Pressure coordinates starting at level

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to second-order diffusion at the top level is worth noting as it had a substantial effect on the model results in the lower stratosphere. The parameters used in this work are currently considered the best choice for the 19-level model for climate simulations.

Some aspects of the parametrizations have proved unsatisfactory in the upper stratosphere and lower mesosphere, since they were originally designed for tropospheric use. Following Morcrette et al. (1986), the treatment of gaseous effects in the long-wave radiation scheme has been modified in the 49-level model to give smoother and more accurate heating rates in the stratosphere. The effects of Doppler line broadening and of the atmosphere above the top of the model have also now been taken into account. The gravity- wave-drag parametrization is based on Palmer et al. (1986) but was found to introduce too much noise in the stratosphere in the 49-level model. Therefore in this version of the model it has been replaced with Rayleigh friction above 20 mb which relaxes the wind towards zero using a damping time-scale which decreases with height. This damping acts mainly above the the stratopause with a damping time-scale of about 1 day at the top of the model (0.1 mb).

(b) The UM tracer advection scheme The calculation of transport in GCMs (e.g. Thuburn 1993) is commonly followed by

a flux correction to limit the inherent numerical diffusion. Tracer advection in the UM uses the flux redistribution scheme of Roe (1985) which is positive definite and mass conserving. The scheme is also 'total variation diminishing' thereby avoiding spurious oscillations which might otherwise occur near very sharp gradients. Tracer values remain bounded by their initial values and the scheme uses the superbee limiter. For linear advection this was shown by Morton and Sweby (1987) to be the most successful of a range of limiters in common use, with only a relatively minor tendency to remove sharp peaks from the waves. For the three-dimensional advection problem, operator splitting is adopted and the flux distribution is applied separately in each coordinate direction, allowing the scheme to be used near the poles. The advection scheme assumes zero vertical tracer flux at the top and bottom of the model. Further information may be obtained from Cullen (1992).

(c) The stratospheric photochemistry scheme The photochemical model is adapted from that used by Butchart and Austin (1996) in

a stratosphere only model, Thirteen photochemically active tracers are included (HN03, NzOS, H202, HCl, HOC1, C10N02, H2C0, 0,, BrON02, HN03(s) + HN03, H20(s), NO,, H20(s) + H20), together with a further two passive tracers to determine the transport of the long-lived species CH4, total inorganic C1 and total inorganic Br, required in the


photochemical calculation. In the above, the species are assumed to be in the gas phase except for those indicated by (s) which are held in the solid phase in PSCs. 0, is the total odd oxygen (03 + O(3P) + O(’D)) and NO, is the total odd nitrogen (HN03 + HN03(s) + 2N205 + C10N02 + BrONOz + NO + NO2 + NO3). A simple PSC scheme is included which condenses gas phase HN03 and H 2 0 into frozen nitric acid trihydrate and ice clouds according to the ambient temperature. Two particle sizes are assumed (see Butchart and Austin 1996) together with appropriate sedimentation rates. Because of sedimentation local conservation is not satisfied, and hence both variables HN03(s) and HzO(s) are required to be transported as well as the total nitrogen, NO,, and the total water, HzO(s) + H20. In addition to these transported species the following species are derived from the reservoirs by photostationary state and conservation assumptions: 03, O(3P), O(’D), NO, NO2, NO3, OH, H02, C1, C10, C1202, C12, Br, BrO, HBr and HOBr. Seventeen photolytic reactions are included and their rates are determined from the altitude, ozone column and solar zenith angle using a pre-computed look-up table. The ozone column is calculated assuming the ozone mixing ratio is constant above the top of the model. Five heterogeneous reactions are included in the model together with 42 gas-phase reactions which cover the most significant processes occurring in the stratosphere, within the limits of our current understanding. Photochemical data are taken from DeMore et al. (1994).

Below 400 mb all the tracers are treated as passive in the absence of a suitably developed tropospheric chemistry scheme and similarly in the absence of a mesospheric chemistry scheme, all the tracers are treated as passive above 1 mb. This restricts the length of integrations that can realistically be performed but is unlikely to affect the model results in the lower stratosphere during the 60-day periods considered in this study. The two passive tracers are chosen to represent the initial ‘equivalent latitude’ (Butchart and Remsberg 1986) and the initial potential temperature of the air parcels. The photochemistry is coupled to the dynamics, by using the model computed ozone in the radiation scheme, and is computed by process splitting using a 5-minute time step, so that four chemical time steps are performed every dynamical time step.

( d ) Initial conditions The dynamical initial conditions were taken from the UKMO stratospheric data assim-

ilation archive (Swinbank and O’Neill 1994) which was originally produced as correlative data for the Upper Atmosphere Research Satellite (UARS) project. The chemical initial conditions were determined using the method of Lary et al. (1995b). At each longitude and latitude point the equivalent latitude was first computed from the dynamical fields as a function of potential temperature and used to derive species values from the two- dimensional model results kindly supplied by D. Lary in equivalent latitude and potential temperature coordinates. A simple smoothing operator was then applied to the results to remove some of the sharpest features. However, the chemical balancing step described by Lary et al. (1995b) was considered unnecessary for the seasonal length integrations of this study which were not intended as hindcasts for the individual winters. In using this approach, rather than observations specific to the simulated winters, a level of consistency in the chemistry between each integration was ensured.

Initializations were first established as described above for the 49-level model configuration and later interpolated to the 19-level model grid. Thus in some of the diagnostics presented later in the paper, minor differences may exist between the initial conditions depending on the interpolating algorithm.


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Figure 2. Meridional cross-section of tracer-2 at the end of the integrations, rescaled to indicate the approximate mean ascent over the 60-day period. (a) Average for the four 49-level model simulations, and (b) average of the

four 19-level model simulations. Positive values are shaded.

3. SUMMARY OF PASSIVE TRACER RESULTS

Figure 2 shows the meridional cross-section of the difference in the second passive tracer in the model between the beginning and end of the simulations, averaged for the four winters. The values shown have been suitably rescaled so as to indicate the approximate zonal-mean ascent in kilometres over the 60-day period. Below 100 mb both model results show a similar pattern with ascent in the tropical regions and descent over the winter and, to a lesser extent, the summer pole. This general behaviour was continued in the stratosphere of the 49-level model (Fig. 2(b)) and is consistent with that expected from the Brewer- Dobson circulation (Brewer 1949; Dobson 1956) between solstice and equinox as well as observations of passive tracers such as N20 and C& (Jones and Pyle 1984) and other theoretical meridional distributions of passive tracers (e.g. KO et al. 1985). Quantitatively, the 49-level model results in the tropics are in good agreement with ascent rates estimated by Holton et al. (1995) and those which may be deduced from observations using total hydrogen as a passive tracer (Mote et al. 1996). In contrast, the 19-level model results (Fig. 2(a)) became increasingly unrealistic above 100 mb with an accumulation of relatively low


tracer mixing ratio near the top levels of the model at all latitudes. With only four levels above 100 mb in the 19-level model, this behaviour was probably largely due to poor vertical resolution, rather than the position of the upper boundary itself, since similar effects did not occur near the upper boundary (not shown) of the 49-level model. Figure 2 also shows significant quantitative differences between the two model versions below 100 mb, particularly towards the winter pole, where the maximum descent in the 19-level model is more than 50% greater than in the 49-level model. For this particular feature it is not possible, on the basis of the results obtained here, to distinguish between transport errors associated with an inaccurate meridional circulation and those associated with the coarse vertical resolution near the tropopause.

4. CHEMICAL CHARACTERISTICS

Figures 3 and 4 summarize the effects of model domain on the concentrations of the reservoir species. The high-latitude peak in HN03 was significantly higher in the 49- level model, but was about 15% lower than observed by the Cryogen Limb Array Etalon Spectrometer ( C W S ) in this month (Roche et al. 1994), probably because the initial conditions used two-dimensional model total odd nitrogen, rather than observed amounts, and the model does not take into account the heterogeneous reactions on aerosol that were present during 1992 and 1993 from the eruption of Mt. Pinatubo (Randel et al. 1995). Concentrations of HN03 in both model versions were also higher than those for January in the three-dimensional model results of Rasch et al. (1995), possibly because of the absence of heterogeneous reactions on PSCs in their model. Nitric acid values were generally low in the tropics but the very cold modelled tropical tropopause resulted in significant denitrification. The spurious peak in the tropical troposphere resulted from the absence of tropospheric chemistry and the condensation of the HN03 in clouds. Concentrations of N205 were also higher in the 49-level model than in the 19-level model, although absolute values were still considerably lower than Rasch et al. (1995), again probably because of the absence of heterogeneous reactions in their model. Nonetheless, the N205 results in the lower stratosphere were broadly consistent with Atmospheric Trace Molecule Spec- troscopy (ATMOS) measurements (Toon et al. 1986) and showed the expected reduction above 10 mb. Chlorine nitrate values in both model versions were broadly consistent with CLAES data (Roche et al. 1994) but the high-latitude peak between 10 and 20 mb is about 0.5-0.75 parts per billion ( lo9) by volume (p.p.b.v.) higher in the 49-level model. For HCl, marked differences occurred, particularly in the tropics, where, in the 19-level model, the 0.25 p.p.b.v. contour extended above 5 mb but only just rose above 20 mb in the 49-level model. In comparison with Halogen Occultation Experiment ( W O E ) data (Park and Russell 1994) the model values were generally too low, particularly in the tropics, but were more realistic with the 49-level model. The poor partitioning of model chlorine has become apparent from other model studies in recent years (e.g. Eckman et al. 1995; Lary et at. 1995a; Chance et al. 1996). They suggest that improvement in model performance can be achieved by providing additional HC1 production from the reaction OH + ClO + HC1 + O2 although there is currently no definite laboratory evidence for this channel (DeMore etal. 1994).

Only limited measurements exist for H202, HOCl, H2CO and BrON02. H2C0 and BrON02 were qualitatively similar in the two model versions but with significantly higher values in the 49-level model. The impact of the upper boundary was noticeable in affecting the chemistry of H202 and HOC1. Finally, H202 and H2C0 would normally have much lower concentrations in the troposphere because of their solubility in clouds, a process that is ignored in these calculations.

1412 J. AUSTIN et uZ.

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Figure 3. Meridional distributions of the main model-transported species on 16 March averaged for the four winters from the 19-level model. The contour intervals are (in p.p.b.v.) (a) HN03: 2.0, @) N205: 0.05, (c) H202: 0.05, (d) HCI: 0.25, (e) HOC1: 0.03, ( f ) ClON02: 0.5, (g) H2CO: 0.05 and (h) BrON02: 0.002. The density of

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Figure 5. Meridional cross-sections of 0 3 during the 19-level (left-hand panels) and 49-level (right-hand panels) model integrations at 20-day intervals averaged for the four winters. The contour interval is 1 p.p.m.v. and the density of shading increases towards the higher values. (a) and (b) Initial conditions (15 January); (c) and (d) 4

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Figure 6. Evolution of total ozone averaged globally for the 19-level and 49-level models in comparison with Total Ozone Mapping Spectrometer (TOMS) data for the period 1988-91.

5. SIMULATED OZONE DISTRIBUTIONS

Figure 5 shows the zonal-mean distribution of ozone for the 19-level (left-hand panels) and 49-level (right-hand panels) models at 20-day intervals, averaged for the four winters. In the initial conditions and throughout the 194evel model integrations, the equatorial peak in ozone in the upper stratosphere was not properly resolved, and the effects of the inadequate vertical resolution at the upper levels are clearly apparent. Also, the assumption of constant mixing ratio above the top level of the model leads to a higher ozone column above 5 mb than observed, leading to a reduced ultra-violet (UV) flux into the middle stratosphere. With 49 levels the initial ozone distribution was more properly resolved in accordance with climatology (e.g. Russell 1986) and the model simulated the upper- stratospheric ozone quite well over this period. In the lower stratosphere the relatively strong vertical gradients in ozone were generally maintained by the 49-level model but weakened during the 19-level model integrations, particularly in the tropics. For the 19- level model results the 1 part per million by volume (p.p.m.v.) contour descended slightly with time, while in the 49-level results the 1 p.p.m.v. contour remained close to 100 mb at the poles. Tropospheric ozone remained low throughout all the integrations but there were significant differences in behaviour between the two model versions which are not visible in Fig. 5 and are analysed in greater detail below.

Globally averaged ozone column abundances are shown in Fig. 6. Initial readjustments between the model dynamics and photochemistry caused a decrease at first, particularly in the 19-level model. Thereafter the column abundance increased steadily in the 19-level model but remained almost constant in the 49-level version. For comparison, the mean for the years 1988-91 of globally averaged data from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) (Heath et al. 1975; Stolarski 1993) is also shown in Fig. 6. Only the most recent years of data have been used as these are more representative of the high chlorine situations simulated by the model. The years 1992 and 1993 have, however, been excluded as large ozone depletion was observed in the northern hemisphere, probably


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OZONE AND TEMPERATURE IN A GCM WITH COUPLED CHEMISTRY

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due to the effects of aerosol from the eruption of Mt. Pinatubo in 1991 (Randel et al. 1995). Where no TOMS data were available in the polar night, the column amount was assumed to be the same as that at the most northerly latitude where measurements were possible. With this proviso, the data indicate a slight rise in globally averaged ozone, due to a combination of transport and chemistry effects in the northern hemisphere winter, and confirm the gradual increase in the 49-level model.

Increases in the column ozone in the 19-level model occurred at all latitudes (see Fig. 7(a)) but relative to the 49-level model (Fig. 7(b)) the greatest increases were in the subtropics (see Fig. 7(c)). In comparison with TOMS data averaged for 1988-91 (see Fig. 8) the 49-level model reproduced better the strong gradients in the zonal-mean total ozone near 30"N and also performed much better in the tropics. However, the low tropical total ozone extended over a greater latitudinal range and was more symmetric about the equator than observed (cf. Figs. 7(b) and 8). The observed springtime increase in zonal-mean total ozone in polar latitudes was apparently better simulated by the 19-level model, but further analysis shows that this was only the result of a spurious build-up of tropospheric and lower-stratospheric ozone.

Figure 9 shows for each model version the profile of the zonal-mean ozone partial pressure at the end of the simulations at different latitudes and averaged for the four winters, together with the initial ozone profiles. In the 19-level model the peak partial pressure has descended and increased in magnitude in high latitudes. Tropospheric ozone increased substantially, by over a factor of two near 300 mb in the tropics and subtropics. At 30"N the ozone partial pressure was unrealistic and almost uniform between about 800 and 30 mb. Comparison with the corresponding January and March monthly mean profiles obtained from the Microwave Limb Sounder ( M U ) channel 205 on UARS (see Fig. 9) confirms that the increase in ozone in the troposphere and lower-stratosphere in the 19-level model was unrealistic.

During the northern winter months global average chemical production of ozone within the troposphere has been calculated to be comparable with the dry deposition rate (0.30 DU day-' and 0.43 DU day-' respectively (Strand and Hov 1994)). The balance of 0.13 DU day-' is made up by the stratospheric influx. Thus in the current study in which both tropospheric production and surface deposition are neglected but stratospheric

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influx is included, the average ozone would be expected to increase by only about 8 DU during the 60 days. This is comparable with the discrepancy between the TOMS data and the 49-level model but somewhat smaller than the 19-level model increase. Further, the unrealistic behaviour of the 19-level model extended well into the lower stratosphere (see Fig. 9) where the chemistry was properly modelled. Consequently, accurate simulation of ozone over long time-scales will require accurate transport as well as the inclusion of tropospheric and surface chemical processes.

6. THE IMPACT OF SIMULATED OZONE ON MODEL TEMPERATURES

In this section the radiative impact of the improved ozone simulation on the meteorology is assessed in comparison with the direct affect of increasing the number of stratospheric levels. In order to do this all the integrations were repeated without the computationally expensive chemistry and transport and with the radiative heating calculated using the monthly varying zonally symmetric ozone climatology currently used by other UM users (e.g Swinbank and O’Neill 1994; Mitchell et al. 1995). Mean temperatures from the four 19-level and four 49-level model integrations with, and without, chemistry are compared with the mean of four years of observations obtained from the UKMO’s UARS correlative data assimilation archive (Swinbank and O’Neill 1994).

Figure 10 shows that globally both versions of the model produced a systematic cooling of the lower and middle stratosphere irrespective of whether or not fully coupled stratospheric photochemistry was included. By the end of the integrations the deviation from observations at 10 mb was, on average, between 5 and 8 K, except for the 19-level model simulations with coupled photochemistry when the deviation was over 12 K. Simi- larly, at 46 mb the cold bias was greatest for the 19-level model integrations with simulated ozone. At 100 mb there was little difference between the model versions, though by the end of the integrations the model temperatures differed from the observations by 4-6 K. With climatological ozone, changing from 19 to 49 levels improved the global simulation of temperatures at 10 and, particularly, 46 mb but had a slightly detrimental effect at 100 mb. Including photochemically determined ozone had only a moderate influence on the 49-level model results whereas with the 19-level model it substantially increased the global temperature error in the lower and middle stratosphere.

The impact of the poorly simulated ozone distribution in the 19-level model is more clearly visible in Fig. l l(a) which shows the mean globally averaged vertical temperature profiles at the end of each set of four integrations on 16 March compared with the 4-year mean observed profile for that date. All the integrations in this study used the same climatological sea surface temperatures, and the mean simulated lower-tropospheric temperatures from all the model configurations agreed quite well with observations and are not shown in Fig. 11. The cold model stratosphere shown in Fig. l l(a) is also symptomatic of most GCMs. However, what is particularly noticeable in the figure is the significant increase in the cold bias throughout the lower and middle stratosphere for the 19-level model with simulated ozone. In contrast, there was little difference between the 19- and 49-level model results using climatological ozone and 49-level model with simulated ozone. Moreover, Figs. ll(b)-ll(d) show that these conclusions also apply when the tropical and extratrop- ical regions are considered separately.

7. POLAR STRATOSPHERIC CLOUDS

The simulated thermal structure of the stratosphere in chemistry4imate models is significant because of the temperature dependence of the chemical processes. In particular


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Figure 10. Evolution of the global mean temperature (K) at (a) 10 mb, (b) 46.42 mb and (c) 100 mb. The solid lines are the average temperatures from the four 49-level model integrations with the thick line denoting results using simulated ozone and the thin line the results using climatological ozone; similarly, the dashed lines refer to results from the 19-level model. The dotted line is the average of 4-years (1992-95) of observations obtained from

the UKMO's stratospheric data assimilation.


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Figure 11 . Vertical temperature profiles for the eud (16 March) of the 60-day integrations. (a) Global average, (b) southern hemisphere average (30"S-90"S), (c) tropical average (30"N-3OoS), and (d) northern hemisphere average

(3OoN-90"N). The line styles are as in Fig. 10.

an important component to the ozone budget in winter in the lower stratosphere is the heterogeneous chemistry associated with PSCs (e.g. Crutzen and Arnold 1986; Turco et al. 1989; Drdla and Turco 1991). PSCs are highly temperature dependent and modelling heterogeneous processes demands a realistic simulation of the temperature behaviour of the lower stratosphere.

The global and area-averaged systematic cooling of the UM stratosphere with coupled photochemistry (see section 6) was also evident in the zonal-mean temperatures north of 20"N at 46 mb (Fig. 12). In this region the cooling resulted from the failure of both 19- level and 49-level models to maintain the correct pole-to-equator temperature gradients and in particular the observed mid-latitude band of higher temperabres near SOON. In polar latitudes, however, except for the first six days, neither model simulated temperatures below 206 K, in contrast with the observations. For both models, simulated temperatures in the sub-tropics (and also the tropics-results not shown but see Fig. 1l(c)) were much lower than observed.

Despite the above overall cold biases, neither model version with fully coupled chemistry was able to reproduce the observed frequency of occurrence of temperatures locally


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Figure 12. Evolution in the northern hemisphere of the zonal-mean temperatures at 46.42 mb. (a) Mean for the years 1992-95 from the UKh4O’s stratospheric data assimilation, (b) mean of the four 49-level model simulations with coupled photochemistry, and (c) mean of the four 19-level model simulations with coupled photochemistry.

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Figure 13. Daily minimum temperatures (K) north of 61.25"N at 46.42 mb. (a) 49-level model including simulated ozone, and (b) 19-level model including simulated ozone. The dotted, solid, dashed and chain-dashed line styles are for the integrations initialized on the 15 January 1992,1993,1994 and 1995 respectively. Also indicated is the daily range of observed minimum temperatures for these four years as obtained from the UKMO's stratospheric data assimilation. The dashed line at 195.3 K represents a typical temperature threshold below which polar stratospheric

cloud formation can be expected to occur.

low enough for PSC formation. For instance, Fig. 13 shows for the four integrations of each model version the minimum temperature north of 61.25"N at 46 mb in comparison with the range of minimum temperatures obtained from the UKMO stratospheric data assimilation. The thick dashed line at 195.3 K is the PSC formation threshold calculated by the photochemical model for this pressure with typical ambient levels of nitric acid and water vapour. Minimum temperatures in three of the four 19-level model simulations were


well above those observed for any of the four winters from 1992 to 1995, and after the first few days PSC formation would not have occurred at this level in these integrations. Some general improvement was obtained with 49-levels, and three of the integrations now produced temperatures low enough for PSC formation up until early February. Minimum temperatures in two of these integrations continued within the observed range up until March. Observed minimum temperatures nearly always occur poleward of 61.25"N, however, because of the absence of the mid-latitude band of higher temperatures in the model (see Fig. 12), the lowest simulated temperatures sometimes occurred further equatorward. However, in terms of PSC formation, the only significant low-temperature occurrences equatorward of 61.25"N were in March of the 1993 simulation.

Given that these integrations were generally longer than the probable predictability time-scales, an interesting feature of the results shown in Fig. 13 is the broad similarity in the behaviour of the two model versions in three out of the four years shown. In both integrations initialized with the 1993 observations, temperatures low enough for PSC formation remained until mid-February whereas in the pairs of integrations initialized with 1992 or 1995 data the minimum temperatures rose rapidly above the range of observed values. In contrast, in the 1994 simulations the 49-level model minimum temperatures remained within the observed range until March, and the 19-level model minimum temperature again rose above the range of observations. It should also be noted that all these integrations used the same climatological sea surface temperatures.

The poor performance of the model in simulating temperature sufficiently low for PSC formation resulted from an unrealistic decay in amplitude of the planetary-wave disturbances in the lower stratosphere. This is illustrated by Fig. 14 which shows for 46 mb the standard deviation of temperature around a latitude circle. Here, the standard deviations were first calculated for each integration or year of observations and then averaged over the four integrations of a particular version of the model, or the four years of observations. The maximum variability around a latitude circle was observed to occur near 70"N throughout the 60 days and, apart from short periods at the end of February and again at the beginning of March, the average daily standard deviations at this latitude were more than 8 K (Fig. 14(a)). At 70"N the standard deviations were more than 12 K in midJanuary when the integrations were initialized, but with both model versions this dropped to under 8 K by the end of January (Figs. 14(b) and 14(c)). Thereafter the 49-level model performed better with the 46 mb temperature field having retained, on average, a greater degree of zonal asymmetry than the 19-level model, even though it was considerably less asymmetric than observed. By wave-mean flow interaction theory the loss of planetary-wave propagation in the simulations is also consistent with the cold bias referred to above, at least in the northern hemisphere extratropics.

Although on average the simulations failed to maintain the observed degree of zonal asymmetry in the 46 mb temperature fields an analysis of the results from the individual integrations shows that the model can, nevertheless, produce a spontaneous amplification of the planetary waves. For example, Fig. 15 is similar to Fig. 14 but shows only the observations for 1993 and the results from the integrations initialized from data for that year. Of the four years 1992-95 the flow in the lower stratosphere in 1993 was the least disturbed in January but there was a significant growth in the planetary-wave disturbances in February and again at the beginning of March (Fig. 15(a)). Pulses in planetary-wave disturbance amplitudes occurred in February and March in both versions of the model (Figs. 15(b) and 15(c)) and whilst these were stronger in the 49-level model they were still much weaker than observed. Interestingly, the temperature field in both models became much more zonally asymmetric in late January, in contrast to the observed behaviour. Again the disturbance growth was greatest with the 49-level model. The most likely explanation for differences


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Figure 14. The daily values of the standard deviation of temperature around a latitude circle for the northern hemisphere at 46.42 mb; (a) for years 1992-95 from the UKMO’s stratospheric data assimilation, (b) for the four 49-level model integrations with simulated ozone, and (c) for the four 19-level model integrations with simulated

ozone. The contour interval is 2 K and the shading denotes a standard deviation greater than 8 K.


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Figure 15. As Fig. 14 but for the single year 1993.


in behaviour between the two model versions and the observations is that the tropospheric source of planetary waves was independent of the different model stratospheres and was broadly realistic. Both models then underestimated the upward flux of planetary waves into the lower stratosphere but, as with the ozone transport (see section 5), improvements were gained by changing from 19 to 49 vertical levels.

8. DISCUSSION AND CONCLUSIONS

This study has examined aspects of modelling the interaction between chemistry and dynamics important for the future development of more comprehensive climate-simulation models. A detailed stratospheric photochemical model was added to a state-of-the-art atmosphere GCM that was also modified to allow the solar heating to be calculated from the model-simulated ozone. Two versions of the GCM were used. The first was similar to that used by climate-change experts (e.g. Mitchell et al. 1995) and had a top boundary near 5 mb with poor vertical resolution in the stratosphere. In the second the top boundary was placed at 0.1 mb and the vertical resolution in the stratosphere considerably improved. Ensembles of four 60-day integrations starting from different initial conditions for 15 January allowed climatological details of the chemistry and transport in the northern winter to be considered without the need for a computationally expensive multi-year integration.

After 60 days both versions of the model produced realistic stratospheric distributions of the main reservoir species, which corresponded qualitatively with available satellite observations but differed in detail. There was also reasonable agreement with results from other GCMs incorporating stratospheric chemistry. On the other hand, the HCl results were still in poor agreement with observations, possibly because of the absence of an additional source term (e.g. Eckman et al. 1995), though poor transport of total chlorine is likely to be a significant contributor. In some cases the simulated chemical distributions were rather sensitive to the number of stratospheric levels. The results indicated that by raising the upper boundary the photochemistry in the model became more active and the ozone distribution was much improved in the middle stratosphere. These aspects could have arisen from two factors, which could not be separated within the current model formulations. Firstly, ozone columns above 5 mb were higher in the 19-level model than in the 49-level model, leading to a reduction in the UV flux into the middle stratosphere. Secondly, the excessive vertical transport near the top of the 19-level model could have transported relatively low amounts of radicals such as NO,.

There were also considerable inadequacies of the transport in the version of the model used by the climate experts due to the poor representation of the stratosphere. This resulted in a significant build-up in the concentration of ozone in the upper troposphere and lower stratosphere and a concomitant unrealistic increase in the global-mean ozone column abundance. In contrast, when compared with observations, the increased number of stratospheric levels resulted in a realistic simulation of the ozone distribution. In particular, in the lower stratosphere, the steep vertical gradients in the tropical ozone concentration were well simulated. On the other hand, the transport of a passive tracer with roughly uniform vertical gradients was qualitatively similar in both model versions below about 100 mb, suggesting that the poor simulation of ozone in the 19-level model resulted from the inability of the model to resolve the steep vertical gradients near the tropopause. One way of reducing errors in the transport is to use a separate ‘off-line’ chemistry-transport model (e.g. Chipperfield et al. 1993), in which more accurate vertical velocities, for instance, are derived from an independent diabatic heating calculation. Such a model would still need to be run with sufficient vertical resolution to allow the vertical flux of constituents to be


accurately calculated. The weakness of this approach, however, is that it does not allow a proper radiative coupling between the simulated ozone and dynamics.

Both versions of the model used in this study suffered, in general, from unrealistically cold stratospheres, which is symptomatic of most GCMs (e.g. Hack et al. 1994). On the other hand, over the Arctic the simulated temperatures in the lower stratosphere were too high. Additionally, the planetary-scale disturbances were underpredicted as the integrations proceeded. Consequently neither version of the model was able to reproduce the observed frequency of occurrences of temperatures locally low enough for PSC formation, and the lowest simulated temperatures tended to occur further equatorwards than observed. A brief analysis of the simulated temperatures in the northern winters in a 10-year benchmark integration of the UM for the Atmospheric Model Intercomparison Project (AMIP) (Gates 1992) revealed no period when PSC formation could occur using the standard climate configuration of the model.

The mid-latitude cold bias in the model in the northern hemisphere stratosphere is consistent with the reduced planetary-wave amplitudes. Hence, the situation was improved somewhat by the increased number of stratospheric levels which allowed better upward propagation of the planetary waves to the higher levels. Lower down in the troposphere both versions of the model were able to reproduce a spontaneous amplification in wave amplitudes as would occur in the atmosphere, for example, before a stratospheric warming. For the standard climate-prediction model the poor simulation of ozone generally exac- erbated the problem of the cold stratosphere through the radiative terms. This confirms the need to treat chemistry-climate models as complete systems, rather than concentrating on improving the dynamics or chemistry separately. Even over a 60-day period this study has demonstrated that including the radiative coupling between the model’s chemistry and dynamics can have a serious impact on the model’s meteorological performance. Longer integrations will require an improved representation of the tropospheric sources and sinks of the chemical constituents. Nevertheless, such integrations would be of little scientific value without the more accurate representation of transport that was achieved by increasing the number of stratospheric levels and raising the upper boundary. Finally, work in progress suggests that, in this version of the model, the stratospheric climatology can be improved further by modifications to physical parametrizations.

ACKNOWLEDGEMENTS

Terry Davies and Mark Mawson provided helpful comments on the use of the transport scheme. The two-dimensional chemical-model results used in the chemical initialization were kindly supplied by David Lary. The TOMS ozone data used in the model comparisons were obtained from the British Atmospheric Data Centre at the Rutherford Appleton Laboratory. Tony Slingo and William Ingram provided helpful advice on the interpretation of the model heating and cooling rates. We also acknowledge useful comments from the referees. This work was supported by the UK DOE Stratospheric Change contract PECD7/12/146.

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Documents

Sensitivity of ozone and temperature to vertical resolution in a gcm with coupled stratospheric chemistry