Coupled Chemistry Climate Model Research Articles

The global impact of supersaturation in a coupled chemistry-climate model

Abstract. Ice supersaturation is important for understanding condensation in the upper troposphere. Many general circulation models however do not permit supersaturation. In this study, a coupled chemistry climate model, the Whole Atmosphere Community Climate Model (WACCM), is modified to include supersaturation for the ice phase. Rather than a study of a detailed parameterization of supersaturation, the study is intended as a sensitivity experiment, to understand the potential impact of supersaturation, and of expected changes to stratospheric water vapor, on climate and chemistry. High clouds decrease and water vapor in the stratosphere increases at a similar rate to the prescribed supersaturation (20% supersaturation increases water vapor by nearly 20%). The stratospheric Brewer-Dobson circulation slows at high southern latitudes, consistent with slight changes in temperature likely induced by changes to cloud radiative forcing. The cloud changes also cause an increase in the seasonal cycle of near tropopause temperatures, increasing them in boreal summer over boreal winter. There are also impacts on chemistry, with small increases in ozone in the tropical lower stratosphere driven by enhanced production. The radiative impact of changing water vapor is dominated by the reduction in cloud forcing associated with fewer clouds (~+0.6 Wm−2) with a small component likely from the radiative effect (greenhouse trapping) of the extra water vapor (~+0.2 Wm−2), consistent with previous work. Representing supersaturation is thus important, and changes to supersaturation resulting from changes in aerosol loading for example, might have a modest impact on global radiative forcing, mostly through changes to clouds. There is no evidence of a strong impact of water vapor on tropical tropopause temperatures.

Evolution of Water Vapor Concentrations and Stratospheric Age of Air in Coupled Chemistry-Climate Model Simulations

Abstract Stratospheric water vapor concentrations and age of air are investigated in an ensemble of coupled chemistry-climate model simulations covering the period from 1960 to 2005. Observed greenhouse gas concentrations, halogen concentrations, aerosol amounts, and sea surface temperatures are all specified in the model as time-varying fields. The results are compared with two experiments (time-slice runs) with constant forcings for the years 1960 and 2000, in which the sea surface temperatures are set to the same climatological values, aerosol concentrations are fixed at background levels, while greenhouse gas and halogen concentrations are set to the values for the relevant years. The time-slice runs indicate an increase in stratospheric water vapor from 1960 to 2000 due primarily to methane oxidation. The age of air is found to be significantly less in the year 2000 run than the 1960 run. The transient runs from 1960 to 2005 indicate broadly similar results: an increase in water vapor and a decrease in age of air. However, the results do not change gradually. The age of air decreases significantly only after about 1975, corresponding to the period of ozone reduction. The age of air is related to tropical upwelling, which determines the transport of methane into the stratosphere. Oxidation of increased methane from enhanced tropical upwelling results in higher water vapor amounts. In the model simulations, the rate of increase of stratospheric water vapor during the period of enhanced upwelling is up to twice the long-term mean. The concentration of stratospheric water vapor also increases following volcanic eruptions during the simulations.

Assessment of temperature, trace species, and ozone in chemistry‐climate model simulations of the recent past

Simulations of the stratosphere from thirteen coupled chemistry‐climate models (CCMs) are evaluated to provide guidance for the interpretation of ozone predictions made by the same CCMs. The focus of the evaluation is on how well the fields and processes that are important for determining the ozone distribution are represented in the simulations of the recent past. The core period of the evaluation is from 1980 to 1999 but long‐term trends are compared for an extended period (1960–2004). Comparisons of polar high‐latitude temperatures show that most CCMs have only small biases in the Northern Hemisphere in winter and spring, but still have cold biases in the Southern Hemisphere spring below 10 hPa. Most CCMs display the correct stratospheric response of polar temperatures to wave forcing in the Northern, but not in the Southern Hemisphere. Global long‐term stratospheric temperature trends are in reasonable agreement with satellite and radiosonde observations. Comparisons of simulations of methane, mean age of air, and propagation of the annual cycle in water vapor show a wide spread in the results, indicating differences in transport. However, for around half the models there is reasonable agreement with observations. In these models the mean age of air and the water vapor tape recorder signal are generally better than reported in previous model intercomparisons. Comparisons of the water vapor and inorganic chlorine (Cly) fields also show a large intermodel spread. Differences in tropical water vapor mixing ratios in the lower stratosphere are primarily related to biases in the simulated tropical tropopause temperatures and not transport. The spread in Cly, which is largest in the polar lower stratosphere, appears to be primarily related to transport differences. In general the amplitude and phase of the annual cycle in total ozone is well simulated apart from the southern high latitudes. Most CCMs show reasonable agreement with observed total ozone trends and variability on a global scale, but a greater spread in the ozone trends in polar regions in spring, especially in the Arctic. In conclusion, despite the wide range of skills in representing different processes assessed here, there is sufficient agreement between the majority of the CCMs and the observations that some confidence can be placed in their predictions.

Hemispheric ozone variability indices derived from satellite observations and comparison to a coupled chemistry-climate model

Abstract. Total column ozone is used to trace the dynamics of the lower and middle stratosphere which is governed by planetary waves. In order to analyse the planetary wave activity a Harmonic Analysis is applied to global multi-year total ozone observations from the Total Ozone Monitoring Spectrometer (TOMS). As diagnostic variables we introduce the hemispheric ozone variability indices one and two. They are defined as the hemispheric means of the amplitudes of the zonal waves number one and two, respectively, as traced by the total ozone field. The application of these indices as a simple diagnostic for the evaluation of coupled chemistry-climate models (CCMs) is demonstrated by comparing results of the CCM ECHAM4.L39(DLR)/CHEM (hereafter: E39/C) against satellite observations. It is quantified to what extent a multi-year model simulation of E39/C (representing "2000" climate conditions) is able to reproduce the zonal and hemispheric planetary wave activity derived from TOMS data (1996–2004, Version 8). Compared to the reference observations the hemispheric ozone variability indices one and two of E39/C are too high in the Northern Hemisphere and too low in the Southern Hemisphere. In the Northern Hemisphere, where the agreement is generally better, E39/C produces too strong a planetary wave one activity in winter and spring and too high an interannual variability. For the Southern Hemisphere we reveal that the indices from observations and model differ significantly during the ozone hole season. The indices are used to give reasons for the late formation of the Antarctic ozone hole, the insufficient vortex elongation and eventually the delayed final warming in E39/C. In general, the hemispheric ozone variability indices can be regarded as a simple and robust diagnostic to quantify model-observation differences concerning planetary wave activity. It allows a first-guess on how the dynamics is represented in a model simulation before applying costly and more specific diagnostics.

Quasi‐biennial oscillation and tracer distributions in a coupled chemistry‐climate model

We have used a fully coupled chemistry‐climate model (CCM), which generates its own wind and temperature quasi‐biennial oscillation (QBO), to study the effect of coupling on the QBO and to examine the QBO signals in stratospheric trace gases, particularly ozone. Radiative coupling of the interactive chemistry to the underlying general circulation model tends to prolong the QBO period and to increase the QBO amplitude in the equatorial zonal wind in the lower and middle stratosphere. The model ozone QBO agrees well with Stratospheric Aerosol and Gas Experiment II and Total Ozone Mapping Spectrometer satellite observations in terms of vertical and latitudinal structure. The model captures the ozone QBO phase change near 28 km over the equator and the column phase change near ±15° latitude. Diagnosis of the model chemical terms shows that variations in NOx are the main chemical driver of the O3 QBO around 35 km, i.e., above the O3 phase change.

Meteorological implementation issues in chemistry and transport models

Abstract. Offline chemistry and transport models (CTMs) are versatile tools for studying composition and climate issues requiring multi-decadal simulations. They are computationally fast compared to coupled chemistry climate models, making them well-suited for integrating sensitivity experiments necessary for understanding model performance and interpreting results. The archived meteorological fields used by CTMs can be implemented with lower horizontal or vertical resolution than the original meteorological fields in order to shorten integration time, but the effects of these shortcuts on transport processes must be understood if the CTM is to have credibility. In this paper we present a series of sensitivity experiments on a CTM using the Lin and Rood advection scheme, each differing from another by a single feature of the wind field implementation. Transport effects arising from changes in resolution and model lid height are evaluated using process-oriented diagnostics that intercompare CH4, O3, and age tracer carried in the simulations. Some of the diagnostics used are derived from observations and are shown as a reality check for the model. Processes evaluated include tropical ascent, tropical-midlatitude exchange, poleward circulation in the upper stratosphere, and the development of the Antarctic vortex. We find that faithful representation of stratospheric transport in this CTM is possible with a full mesosphere, ~1 km resolution in the lower stratosphere, and relatively low vertical resolution (>4 km spacing) in the middle stratosphere and above, but lowering the lid from the upper to lower mesosphere leads to less realistic constituent distributions in the upper stratosphere. Ultimately, this affects the polar lower stratosphere, but the effects are greater for the Antarctic than the Arctic. The fidelity of lower stratospheric transport requires realistic tropical and high latitude mixing barriers which are produced at 2°×2.5°, but not lower resolution. At 2°×2.5° resolution, the CTM produces a vortex capable of isolating perturbed chemistry (e.g. high Cly and low NOy) required for simulating polar ozone loss.

Stratospheric water vapor trends in a coupled chemistry‐climate model

Stratospheric H2O trends are examined in a detailed, coupled chemistry‐climate model (CCM). The modeled H2O trend in the upper stratosphere (US) is mainly caused by CH4 oxidation while the trend in the lower stratosphere (LS) is largely related to changes in temperature and transport near the tropopause. Incomplete CH4 oxidation leads to a maximum upward H2O trend in the US of about 1.4× the imposed tropospheric CH4 trend. Cross‐tropopause exchange of water vapor gives rise to much larger trends (∼+50 ppbv/yr) in the LS. A trend of +0.44 K/decade in the 100 hPa temperature (T) contributes up to 70% (+35 ppbv/yr) of the model LS H2O trend while the remainder (around 30%) can be caused by changes in transport processes near the tropopause. In the LS the maximum modeled trend is close to observations although the globally averaged value is smaller than observed. Given an observed decrease in tropopause temperatures, our results suggest that the observed LS water vapor increases would require a significant change in transport.

Emissions from international shipping: 2. Impact of future technologies on scenarios until 2050

In this study the today's fleet‐average emission factors of the most important ship exhausts are used to calculate emission scenarios for the future. To develop plausible future technology scenarios, first upcoming regulations and compliance with future regulations through technological improvements are discussed. We present geographically resolved emission inventory scenarios until 2050, based on a mid‐term prognosis for 2020 and a long‐term prognosis for 2050. The scenarios are based on some very strict assumptions on future ship traffic demands and technological improvements. The four future ship traffic demand scenarios are mainly determined by the economic growth, which follows the IPCC SRES storylines. The resulting fuel consumption is projected through extrapolations of historical trends in economic growth, total seaborne trade and number of ships, as well as the average installed power per ship. For the future technology scenarios we assume a diesel‐only fleet in 2020 resulting in fuel consumption between 382 and 409 million metric tons (Mt). For 2050 one technology scenario assumes that 25% of the fuel consumed by a diesel‐only fleet can be saved by applying future alternative propulsion plants, resulting in a fuel consumption that varies between 402 and 543 Mt. The other scenario is a business‐as‐usual scenario for a diesel‐only fleet even in 2050 and gives an estimate between 536 and 725 Mt. Dependent on how rapid technology improvements for diesel engines are introduced, possible technology reduction factors are applied to the today's fleet‐average emission factors of all important species to estimate future ship emissions. Combining the four traffic demand scenarios with the four technology scenarios, our results suggest emissions between 8.8 and 25.0 Tg (NO2) in 2020, and between 3.1 to 38.8 Tg (NO2) in 2050. The development of forecast scenarios for CO2, NOx, SOx, CO, hydrocarbons, and particulate matter is driven by the requirements for global model studies of the effects of these emissions on the chemical composition of the atmosphere and on climate. The developed scenarios are suitable for use as input for chemical transport models (CTMs) and coupled chemistry‐climate models (CCMs).

Solar cycle modulation of the Arctic Oscillation in a chemistry‐climate model

The structural modulation of the Arctic Oscillation (AO) induced by a realistic enhancement of the ultraviolet (UV) radiation associated with the 11‐year solar cycle is investigated with a coupled Chemistry‐Climate Model. It has been found that the main features of the modulation are an eastward shift of the surface Atlantic center of action into Europe and a vertical extension into the stratosphere in solar maximum conditions. These model results are in good agreement with observational results in the North Atlantic Oscillation (NAO) centers of action. Confirmation of the modulation effect of solar activity on the NAO spatial structure by a Chemistry‐Climate Model simulation gives support to the occurrence of a real solar effect on the tropospheric circulation.

Long-term changes and variability in a transient simulation with a chemistry-climate model employing realistic forcing

Abstract. A transient simulation with the interactively coupled chemistry-climate model (CCM) E39/C has been carried out which covers the 40-year period between 1960 and 1999. Forcing of natural and anthropogenic origin is prescribed where the characteristics are sufficiently well known and the typical timescales are slow compared to synoptic timescale so that the simulated atmospheric chemistry and climate evolve under a "slowly" varying external forcing. Based on observations, sea surface temperature (SST) and ice cover are prescribed. The increase of greenhouse gas and chlorofluorocarbon concentrations, as well as nitrogen oxide emissions are taken into account. The 11-year solar cycle is considered in the calculation of heating rates and photolysis of chemical species. The three major volcanic eruptions during that time (Agung, 1963; El Chichon, 1982; Pinatubo, 1991) are considered. The quasi-biennial oscillation (QBO) is forced by linear relaxation, also known as nudging, of the equatorial zonal wind in the lower stratosphere towards observed zonal wind profiles. Beyond a reasonable reproduction of mean parameters and long-term variability characteristics there are many apparent features of episodic similarities between simulation and observation: In the years 1986 and 1988 the Antarctic ozone holes are smaller than in the other years of that decade. In mid-latitudes of the Southern Hemisphere ozone anomalies resemble the corresponding observations, especially in 1985, 1989, 1991/1992, and 1996. In the Northern Hemisphere, the episode between the late 1980s and the first half of the 1990s is dynamically quiet, in particular, no stratospheric warming is found between 1988 and 1993. As observed, volcanic eruptions strongly influence dynamics and chemistry, though only for few years. Obviously, planetary wave activity is strongly driven by the prescribed SST and modulated by the QBO. Preliminary evidence of realistic cause and effect relationships strongly suggests that detailed process-oriented studies will be a worthwhile endeavour.

A Strategy for Process-Oriented Validation of Coupled Chemistry–Climate Models

Accurate and reliable predictions and an understanding of future changes in the stratosphere are major aspects of the subject of climate change. Simulating the interaction between chemistry and climate is of particular importance, because continued increases in greenhouse gases and a slow decrease in halogen loading are expected. These both influence the abundance of stratospheric ozone. In recent years a number of coupled chemistry–climate models (CCMs) with different levels of complexity have been developed. They produce a wide range of results concerning the timing and extent of ozone-layer recovery. Interest in reducing this range has created a need to address how the main dynamical, chemical, and physical processes that determine the long-term behavior of ozone are represented in the models and to validate these model processes through comparisons with observations and other models. A set of core validation processes structured around four major topics (transport, dynamics, radiation, and stratospheric chemistry and microphysics) has been developed. Each process is associated with one or more model diagnostics and with relevant datasets that can be used for validation. This approach provides a coherent framework for validating CCMs and can be used as a basis for future assessments. Similar efforts may benefit other modeling communities with a focus on earth science research as their models increase in complexity.

Comment on the paper: On the design of practicable numerical experiments to investigate stratospheric temperature change, by S. Hare et al. (2005)

AbstractIf stratospheric temperature trends are to be understood, coupled chemistry climate models will need to be run. Simulations with fixed ozone trends might provide a misleading indication of future temperature trends. Copyright © 2005 Royal Meteorological Society

Simulation of stratospheric water vapor trends: impact on stratospheric ozone chemistry

Abstract. A transient model simulation of the 40-year time period 1960 to 1999 with the coupled climate-chemistry model (CCM) ECHAM4.L39(DLR)/CHEM shows a stratospheric water vapor increase over the last two decades of 0.7 ppmv and, additionally, a short-term increase after major volcanic eruptions. Furthermore, a long-term decrease in global total ozone as well as a short-term ozone decline in the tropics after volcanic eruptions are modeled. In order to understand the resulting effects of the water vapor changes on lower stratospheric ozone chemistry, different perturbation simulations were performed with the CCM ECHAM4.L39(DLR)/CHEM feeding the water vapor perturbations only to the chemistry part. Two different long-term perturbations of lower stratospheric water vapor, +1 ppmv and +5 ppmv, and a short-term perturbation of +2 ppmv with an e-folding time of two months were applied. An additional stratospheric water vapor amount of 1 ppmv results in a 5–10% OH increase in the tropical lower stratosphere between 100 and 30 hPa. As a direct consequence of the OH increase the ozone destruction by the HOx cycle becomes 6.4% more effective. Coupling processes between the HOx-family and the NOx/ClOx-family also affect the ozone destruction by other catalytic reaction cycles. The NOx cycle becomes 1.6% less effective, whereas the effectiveness of the ClOx cycle is again slightly enhanced. A long-term water vapor increase does not only affect gas-phase chemistry, but also heterogeneous ozone chemistry in polar regions. The model results indicate an enhanced heterogeneous ozone depletion during antarctic spring due to a longer PSC existence period. In contrast, PSC formation in the northern hemisphere polar vortex and therefore heterogeneous ozone depletion during arctic spring are not affected by the water vapor increase, because of the less PSC activity. Finally, this study shows that 10% of the global total ozone decline in the transient model run can be explained by the modeled water vapor increase, but the simulated tropical ozone decrease after volcanic eruptions is caused dynamically rather than chemically.

A new coupled chemistry–climate model for the stratosphere: The importance of coupling for future O<SUB>3</SUB>-climate predictions

We have created a new interactive model for coupled chemistry–climate studies of the stratosphere. The model combines the detailed stratospheric chemistry modules developed and tested in the SLIMCAT/ TOMCAT off-line chemical transport models (CTM) with a version of the Met Office Unified Model (UM). The resulting chemistry–climate model (CCM), called UMCHEM, has a detailed description of stratospheric gas-phase and heterogeneous chemistry. The chemical fields of O3, N2O, CH4 and H2O are used interactively in the radiative heating calculation. We present results from a series of 10-year ‘time-slice’ experiments for 2000 and 2050 conditions. The UMCHEM model performs well in reproducing basic features of the stratosphere. The distribution of long-lived tracers and ‘age of air’ compare well with observations. For O3, the model tends to underestimate the stratospheric column at high latitudes by ∼ 20 DU. This is due to an underestimate of poleward transport in the mid-low stratosphere. The UMCHEM reproduces well the seasonal cycle in monthly mean column O3 at mid-high latitudes, though the variability is slightly smaller than observations and smaller than in the SLIMCAT CTM. Other comparisons with the CTM, which has an identical chemistry scheme, show differences resulting from the models' meteorologies. For example, while the CTM reproduces the observed NOy versus N2O correlation, the UMCHEM overestimates the slope by about a factor of 2. Including full chemistry in the UM causes important differences in the model's meteorology. As the zonal mean ozone climatology used in the UM is larger than that calculated in UMCHEM, with a maximum difference of 4 ppmv in the upper stratosphere, the UMCHEM temperature is about 4 K lower in the Antarctic lower stratosphere and 1–6 K higher in the upper stratosphere. The age of air is less in the basic UM by about 1–3 months in the lower stratosphere but slightly greater in the upper stratosphere. Coupling of the more realistic stratospheric chemistry water vapour warms the model stratosphere by ∼1–2 K. This water vapour coupling also results in a decrease in ozone with a maximum difference of about 250 ppbv in the tropical and southern high-latitude upper stratosphere, while in the tropical lower stratosphere ozone concentrations are increased by up to 30 ppbv. For 2050 conditions, the model produces a column O3 5% higher than present-day values in the tropics, about 15% higher in the Arctic winter/spring and up to 90% higher in the much smaller Antarctic O3 hole. This large O3 increase more than offsets the effect of cooling in the Antarctic late spring induced by greenhouse gases. Copyright © 2005 Royal Meteorological Society

Extrapolating future Arctic ozone losses

Abstract. Future increases in the concentration of greenhouse gases and water vapour may cool the stratosphere further and increase the amount of polar stratospheric clouds (PSCs). Future Arctic PSC areas have been extrapolated from the highly significant trends 1958-2001. Using a tight correlation between PSC area and the total vortex ozone depletion and taking the decreasing amounts of ozone depleting substances into account we make empirical estimates of future ozone. The result is that Arctic ozone losses increase until 2010-2015 and decrease only slightly afterwards. However, for such a long extrapolation into the future caution is necessary. Tentatively taking the modelled decrease in the ozone trend in the future into account results in almost constant ozone depletions until 2020 and slight decreases afterwards. This approach is a complementary method of prediction to that based on the complex coupled chemistry-climate models (CCMs).

The impact of horizontal transport on the chemical composition in the tropopause region: lightning NO x and streamers

The chemical composition of the region of the extra-tropical tropopause is influenced by chemistry and transport processes of either troposphere and stratosphere. On the basis of simulations with the coupled chemistry–climate model ECHAM4.L39(DLR)/CHEM two specific chemistry–transport interactions are investigated aiming to determine their importance for the ozone budget in the tropopause region. First, the transport and chemistry of nitrogen oxides from tropical lightning are analyzed. Second, the impact of low ozone air masses transported from the tropics into higher latitudes by wave breaking events (so-called streamers) is investigated. The first mechanism leads to an increase of ozone in the order of 5–10%. This additional ozone is not only formed in the tropical upper troposphere and then transported into the lower-most stratosphere (LMS), but it is also produced in the LMS, where the production is small but the residence time is long. The second mechanism is only important in the area 100–30 hPa, where ozone is not solely chemically controlled. In the lowest stratosphere at around 30°N and 30°S ozone decreases by up to 80%. In the tropopause region ozone decreases by 30% in summer and 50% in winter. The analysis clearly shows that the interactions of transport and chemistry in the tropopause region are highly complex and a simulation of that area requests a reasonable representation of the troposphere and the stratosphere.

Impact of dynamically induced ozone mini-hole events on PSC formation and chemical ozone destruction

The impact of ozone mini-holes over the extra-tropics of the northern hemisphere on the heterogeneous ozone chemistry is investigated, based on simulations with the coupled climate-chemistry model ECHAM4.L39(DLR)/CHEM. Ozone mini-holes are synoptic-scale regions of strongly reduced total ozone, directly associated with upper troposphere high pressure systems. The simulated mini-hole events are validated with a mini-hole climatology based on daily ozone measurements with the total ozone mapping spectrometer (TOMS) instrument on the satellite Nimbus-7 between 1979 and 1993. Furthermore, the impact of mini-holes on the stratospheric heterogeneous ozone chemistry is investigated indirectly. For this purpose, polar stratospheric cloud formation inside mini-holes is suppressed during the model simulation. Heterogeneous processes inside mini-holes amount to one third of the heterogeneous ozone destruction in general over northern mid- and high-latitudes during winter (January–April). This ozone perturbation subsides and recovers during summer with an e-folding time of two months.

Coupled chemistry–climate model simulations for the period 1980 to 2020: Ozone depletion and the start of ozone recovery

AbstractTwo simulations of a coupled chemistry–climate model are completed for the period 1980 to 2020, covering the recent past during which extensive satellite ozone and temperature data exist, and covering the near future when ozone levels are expected to begin to recover. In the first simulation, Rayleigh friction is used to decelerate the polar night jet. In the second simulation, a parametrized spectral gravity‐wave forcing scheme is included. This has the effect of considerably reducing the model temperature bias in the polar regions and weakening the polar night jet. In the simulations the concentrations of chlorine, bromine and the well‐mixed greenhouse gas concentrations are specified in accordance with past observations and future projected values. The calculated trends in temperature and ozone in the two runs are similar, indicating that model internal variability does not have a significant impact and suggesting that the trends arise largely from changes in external parameters. Typically, after about the year 2000, the trend in the modelled annually averaged ozone changed from a decrease to a small increase. The change was found to be statistically significant in the upper stratosphere and in the lower stratosphere over Antarctica, which are the regions most affected by halogen chemistry. Globally averaged temperature results suggest that the best place to look for future atmospheric change is in the upper stratosphere. Decadally averaged statistics are used to estimate the timing of the start of recovery of total ozone. The simulations indicate no significant further ozone loss from the current atmosphere with minima typically occurring in the years from 2000 to 2005, except in the spring Arctic where ozone values continued to decrease slowly until the end of the integrations. One major problem with the detection of the start of ozone recovery, is that the concentrations of halogens are expected to reduce only slowly from their peak value. Hence, no substantial recovery is simulated before the year 2020. The difficulty in detecting the start of ozone recovery suggests the need to continue the model simulations until the second half of this century which would also help to establish the timing of complete ozone recovery. © Crown copyright, 2003. Royal Meteorological Society

Impact of ozone mini-holes on the heterogeneous destruction of stratospheric ozone.

A comprehensive study of ozone mini-holes over the mid-latitudes of both hemispheres is presented, based on model simulations with the coupled climate-chemistry model ECHAM4.L39(DLR)/CHEM representing atmospheric conditions in 1960, 1980, 1990 and 2015. Ozone mini-holes are synoptic-scale regions of strongly reduced total ozone, directly associated with tropospheric weather systems. Mini-holes are supposed to have chemical and dynamical impacts on ozone levels. Since ozone levels over northern mid-latitudes show a negative trend of approximately -4%/decade and since it exists a negative correlation between total column ozone and erythemally active solar UV-radiation reaching the surface it is important to understand and assess the processes leading to the observed ozone decline. The simulated mini-hole events are validated with a mini-hole climatology based on daily ozone measurements with the TOMS (total ozone mapping spectrometer) instrument on the satellite Nimbus-7 between 1979 and 1993. Furthermore, possible trends in the event frequency and intensity over the simulation period are assessed. In the northern hemisphere the number of mini-hole events in early winter decreases between 1960 and 1990 and increases towards 2015. In the southern hemisphere a positive trend in mini-hole event frequency is detected between 1960 and 2015 in spring associated with the increasing Antarctic Ozone Hole. Finally, the impact of mini-holes on the stratospheric heterogeneous ozone chemistry is investigated. For this purpose, a computer-based detection routine for mini-holes was developed for the use in ECHAM4.L39(DLR)/CHEM. This method prevents polar stratospheric cloud formation and therefore heterogeneous ozone depletion inside mini-holes. Heterogeneous processes inside mini-holes amount to one third of heterogeneous ozone destruction in general over northern mid- and high-latitudes during winter (January-April) in the simulation.

Impact of aircraft NOx emissions. Part 2: Effects of lowering the flight altitude

Aircraft emissions of NO x amount to a small proportion of total emissions of NO x from man-made and natural sources. However, NO x from subsonic aircraft are directly emitted in the upper troposphere and lower stratosphere and have a relatively strong effect on the production of ozone (O 3 ), a greenhouse gas, in that region. Furthermore air traffic is expected to increase significantly in the next decades. Possibilities for reducing the environmental impacts of air traffic by operational, technological and economic measures are currently under discussion. One potential option is to reduce flight altitudes. As a first step we investigate the effect of lowering cruise altitudes by 1 km on the chemical composition of the atmosphere for a subsonic fleet in 2015. Other parameters are deliberately kept constant, e.g., fuel consumption and NO x emissions. The simulations with the coupled climate-chemistry model E39/C clearly show that this fleet leads to a significantly smaller ozone increase compared to a fleet with a standard cruise altitude. In the northern hemisphere aircraft cause an ozone increase of 12.5 Tg for a 2015 simulation. The reduction in the ozone increase of 1.5 Tg is greater than the estimated additional ozone increase of 0.3 Tg due to decreased aerodynamic efficiency and therefore higher fuel consumption and NO x emissions. This value (1.2 Tg) is in the order of 10% relative to the simulated aircraft induced ozone increase of 12.5 Tg.