Simone Dietmüller

Interactive ozone induces a negative feedback in CO2-driven climate change simulations

Interactively coupled climate chemistry models (CCMs) extend the number of feedback mechanisms in climate change simulations by including chemical feedback. In this study the radiative feedback from ozone changes on climate response and climate sensitivity is quantified for a series of simulations driven by CO2 increases on top of a present-day reference concentration level. Other possibly relevant feedback via atmospheric chemistry, e.g., via CH4 and N2O, is not fully quantified in the CCM setup as their concentrations are essentially fixed at the surface. In case of a CO2-doubling simulation, the ozone feedback reduces the climate sensitivity parameter by 3.4%, from 0.70 K/(W m−2) without interactive chemistry to 0.68 K/(W m−2). In case of a 4*CO2 simulation, the reduction of the climate sensitivity parameter increases to 8.4%. An analysis of feedback reveals that the negative feedback of stratospheric ozone and the associated negative feedback change in stratospheric water vapor are mainly responsible for this damping. The feedback from tropospheric ozone changes is positive but much smaller. The nonlinearity in the climate sensitivity damping with increased CO2 concentrations is shown to be due to nonlinear feedback of ozone and stratospheric water vapor.

Contrails, Natural Clouds, and Diurnal Temperature Range

Abstract The direct impact of aircraft condensation trails (contrails) on surface temperature in regions of high aircraft density has been a matter of recent debate in climate research. Based on data analysis for the 3-day aviation grounding period over the United States, following the terrorists’ attack of 11 September 2001, a strong effect of contrails reducing the surface diurnal temperature range (DTR) has been suggested. Simulations with the global climate model ECHAM4 (including a contrail parameterization) and long-term time series of observation-based data are used for an independent cross check with longer data records, which allow statistically more reliable conclusions. The climate model underestimates the overall magnitude of the DTR compared to 40-yr ECMWF Re-Analysis (ERA-40) data and station data, but it captures most features of the DTR global distribution and the correlation between DTR and either cloud amount or cloud forcing. The diurnal cycle of contrail radiative impact is also qualit...

Greenhouse Effect, Radiative Forcing and Climate Sensitivity

Temperature conditions and climate on Earth are controlled by the balance between absorbed solar radiation and outgoing terrestrial radiation. The greenhouse effect is a synonym for the trapping of infrared radiation by radiatively active atmospheric constituents. It generally causes a warming of the planet’s surface, compared to the case without atmosphere. Perturbing the radiation balance of the planet, e.g., by anthropogenic greenhouse gas emissions, induces climate change. Individual contributions to a total climate impact are usually quantified and ranked in terms of their respective radiative forcing. This method involves some limitations, because the effect of the external forcing is modified by radiative feedbacks. Here the current concept of radiative forcing and potential improvements are explained.

Can feedback analysis be used to uncover the physical origin of climate sensitivity and efficacy differences

Different strengths and types of radiative forcings cause variations in the climate sensitivities and efficacies. To relate these changes to their physical origin, this study tests whether a feedback analysis is a suitable approach. For this end, we apply the partial radiative perturbation method. Combining the forward and backward calculation turns out to be indispensable to ensure the additivity of feedbacks and to yield a closed forcing-feedback-balance at top of the atmosphere. For a set of CO2-forced simulations, the climate sensitivity changes with increasing forcing. The albedo, cloud and combined water vapour and lapse rate feedback are found to be responsible for the variations in the climate sensitivity. An O3-forced simulation (induced by enhanced NOx and CO surface emissions) causes a smaller efficacy than a CO2-forced simulation with a similar magnitude of forcing. We find that the Planck, albedo and most likely the cloud feedback are responsible for this effect. Reducing the radiative forcing impedes the statistical separability of feedbacks. We additionally discuss formal inconsistencies between the common ways of comparing climate sensitivities and feedbacks. Moreover, methodical recommendations for future work are given.

Global chemistry-climate modelling with EMAC

The Institute of Atmospheric Physics of the German Aerospace Center (DLR) uses the numerical model system ECHAM/MESSy Atmospheric Chemistry (EMAC). The model has a flexible modular structure and allows for coupled chemistry-climate simulations. Typical fields of application are related to questions regarding Earth’s climate, atmospheric chemical composition, and aerosol characteristics. In its current setup, the performance of EMAC on LRZ/ALTIX allows for multi-decadal simulations with climatologically significant results. The good performance demonstrates the multi-purpose capabilities of LRZ/ALTIX because EMAC involves various different numerical concepts and implementations of parallel decomposition. Our EMAC activities on LRZ/ALTIX are devoted to both model development and production simulations. The former comprise a new upper-boundary representation, a chemistry-transport mode, the inclusion of a mixed-layer ocean, and full-Lagrangian transport and dynamics. The latter tackle, for instance, questions related to the environmental impact of anthropogenic aerosol and gaseous substances.