Coupled Chemistry Climate Model Research Articles

A Three-Dimensional Coupled Chemistry–Climate Model Simulation of Past Stratospheric Trends

Abstract A coupled chemistry–climate model is integrated for the period March 1979–January 2000 with sea surface temperatures and sea ice amounts specified from observations. Greenhouse gas concentrations and halogen loading are also taken from observations. The model contains a detailed stratospheric chemistry package that responds to and in turn influences the model temperature fields. The simulated temperature and ozone trends over the period 1980 to 2000 are broadly in agreement with observations. However, the Arctic ozone depletion is smaller than observed, and the impact of the Antarctic ozone hole lasts too long into summer. The coupling between the model ozone and temperature trends is demonstrated to occur in a similar way to that inferred from observations and could be important in generating the observed interannual temperature variability. It is concluded that further improvement in climate models is necessary before future trends in stratospheric ozone and temperature can be predicted with co...

Origin and variability of upper tropospheric nitrogen oxides and ozone at northern mid-latitudes

Year-long measurements of NO x and ozone performed during the NOXAR project are compared to results from the ECHAM4.L39(DLR)/CHEM (E39/C) and GISS coupled chemistry–climate models. The measurements were taken on flights between Europe and the eastern United States and between Europe and the Far East in the latitude range 40–65°N. Our comparison concentrates on the upper troposphere and reveals strong longitudinal variations in seasonal mean NO x of more than 200 pptv, which both models are able to reproduce qualitatively. Vertical profiles show maximum NO x values 2–3 km below the tropopause (“E-shape”) with a strong seasonal cycle. E39/C simulates a maximum located at the tropopause and with a reasonable seasonal cycle. The GISS model reproduces the seasonal cycle but not the profile's shape due to its coarser vertical resolution. A comparison of NO x frequency distributions reveals that both models are capable of reproducing the observed variability, except that E39/C shows no very high NO x mixing ratios. Both models show that lightning and surface NO x emissions contribute the most to the seasonal cycle of NO x at tropopause altitudes. The impact of lightning in the upper troposphere does not vary strongly with altitude, whereas the impact of surface emissions decreases with altitude. Among all sources, lightning contributes the most to the variability of NO x in the upper troposphere in northern mid-latitudes during summer.

Results of an interactively coupled atmospheric chemistry – general circulation model: Comparison with observations

Abstract. The coupled climate-chemistry model ECHAM4.L39(DLR)/CHEM is presented which enables a simultaneous treatment of meteorology and atmospheric chemistry and their feedbacks. This is the first model which interactively combines a general circulation model with a chemical model, employing most of the important reactions and species necessary to describe the stratospheric and upper tropospheric ozone chemistry, and which is computationally fast enough to allow long-term integrations with currently available computer resources. This is possible as the model time-step used for the chemistry can be chosen as large as the integration time-step for the dynamics. Vertically the atmosphere is discretized by 39 levels from the surface up to the top layer which is centred at 10 hPa, with a relatively high vertical resolution of approximately 700 m near the extra-tropical tropopause. We present the results of a control simulation representing recent conditions (1990) and compare it to available observations. The focus is on investigations of stratospheric dynamics and chemistry relevant to describe the stratospheric ozone layer. ECHAM4.L39(DLR)/CHEM reproduces main features of stratospheric dynamics in the arctic vortex region, including stratospheric warming events. This constitutes a major improvement compared to earlier model versions. However, apparent shortcomings in Antarctic circulation and temperatures persist. The seasonal and interannual variability of the ozone layer is simulated in accordance with observations. Activation and deactivation of chlorine in the polar stratospheric vortices and their inter-hemispheric differences are reproduced. Considering methane oxidation as part of the dynamic-chemistry feedback results in an improved representation of the spatial distribution of stratospheric water vapour concentrations. The current model constitutes a powerful tool to investigate, for instance, the combined direct and indirect effects of anthropogenic trace gas emissions.Key words. Atmospheric composition and structure (middle atmosphere – composition and chemistry) – Meteorology and atmospheric dynamics (general circulation; middle atmosphere dynamics)