Two-dimensional (2D) photochemical models are a widely used tool to study tracer species distribution in the middle-atmosphere. On the other hand, many features of the lower stratosphere are driven ...by troposphere-stratosphere exchanges at the tropical tropopause. Here 2D models suffer for the lack of appropriate mixing mechanisms in the troposphere, in particular cumulonimbus convection. It is developed here a first-order parameterization of this tropospheric mixing process, with the purpose of making these kind of models more meaningful for studies focusing on lower stratospheric chemistry. A validation is made with Rn-222 profiles from observations and three-dimensional calculations. Another validation is made using sulphur dioxide and sulphate. It is shown that inclusion of deep convection helps reconcile the calculated stratospheric aerosol load with SAGE-II observations: a factor of two increase of stratospheric sulphate mass is predicted with respect to a 'standard' 2D simulation without convection. The effects of the 'convectively' enhanced stratospheric aerosol surface area density on heterogeneous chemistry and ozone are also discussed: absence of tropospheric convection would produce a 3% overestimate of total ozone with respect to the realistic case including both convection and sulphate aerosol feedback on NOx and chlorine/bromine chemistry. The deep convection scheme adopted here is validated only for tracers of continental origin, but it could be easily extended also to oceanic source tracers. Zweidimensionale (2D) photochemische Modelle finden breite Verwendung, um die Spurenstoffverteilung in der mittleren Atmosphäre zu untersuchen. Andererseits werden viele Phänomene der unteren Stratosphäre durch Troposphäre-Stratosphäre-Austauschprozesse an der tropischen Tropopause angetrieben. Jedoch werden solche Prozesse, die zur troposphärischen Durchmischung führen, speziell Cumulonimbus-Konvektion, von 2D Modellen nicht angemessen berücksichtigt. Wir stellen eine einfache Parametrisierung dieser troposphärischen Mischungsprozesse vor, die entwickelt wurde, um eine bessere Eignung solcher Modelle für Untersuchungen der Chemie der unteren Stratosphäre zu erlangen. Die Modellergebnisse werden mit Rn-222 Profilen aus Messungen und dreidimensionalen Rechnungen validiert. Eine weitere Validierung wurde mit Schwefeldioxid und Sulfat durchgeführt. Es wird gezeigt, dass durch die Berücksichtigung der hochreichenden Konvektion eine bessere Übereinstimmung mit SAGE-II Beobachtungen erreicht wird: Die stratosphärische Sulphatmasse nimmt um den Faktor zwei zu im Vergleich zu einer Standard 2D Simulation ohne Konvektion. Die Auswirkung dieser durch Konvektion erhöhten stratosphärischen Sulphataerosoloberfläche auf die heterogene Chemie und auf Ozon wird ebenfalls diskutiert: Gesamtozon wird bei fehlender troposphärischer Konvektion im Vergleich zur realistischen Simulation um 3% überschätzt, die sowohl die Konvektion als auch die Wirkung des Sulphataerosols auf die NOx und Chlor-/Bromchemie beinhaltet.
The stratospheric age of air (AoA) is a useful measure of the overall capabilities of a general circulation model (GCM) to simulate stratospheric transport. Previous studies have reported a large ...spread in the simulation of AoA by GCMs and coupled chemistry-climate models (CCMs). Compared to observational estimates, simulated AoA is mostly too low. Here we attempt to untangle the processes that lead to the AoA differences between the models and between models and observations. AoA is influenced by both mean transport by the residual circulation and two-way mixing;we quantify the effects of these processes using data from the CCM inter-comparison projects CCMVal-2 (Chemistry-Climate Model Validation Activity 2) and CCMI-1 (Chemistry-Climate Model Initiative, phase 1). Transport along the residual circulation is measured by the residual circulation transit time (RCTT). We interpret the difference between AoA and RCTT as additional aging by mixing. Aging by mixing thus includes mixing on both the resolved and subgrid scale. We find that the spread in AoA between the models is primarily caused by differences in the effects of mixing and only to some extent by differences in residual circulation strength. These effects are quantified by the mixing efficiency, a measure of the relative increase in AoA by mixing. The mixing efficiency varies strongly between the models from 0.24 to 1.02. We show that the mixing efficiency is not only controlled by horizontal mixing, but by vertical mixing and vertical diffusion as well. Possible causes for the differences in the models' mixing efficiencies are discussed. Differences in subgrid-scale mixing (including differences in advection schemes and model resolutions) likely contribute to the differences in mixing efficiency. However, differences in the relative contribution of resolved versus parameterized wave forcing do not appear to be related to differences in mixing efficiency or AoA.
Geoengineering with stratospheric sulfate aerosols has been proposed as a means of temporarily cooling the planet, alleviating some of the side effects of anthropogenic CO2 emissions. However, one of ...the known side effects of stratospheric injections of sulfate aerosols is a decrease in stratospheric ozone. Here we show results from two general circulation models and two coupled chemistry climate models that have simulated stratospheric sulfate aerosol geoengineering as part of the Geoengineering Model Intercomparison Project (GeoMIP). Changes in photolysis rates and upwelling of ozone-poor air in the tropics reduce stratospheric ozone, suppression of the NOx cycle increases stratospheric ozone, and an increase in available surfaces for heterogeneous chemistry modulates reductions in ozone. On average, the models show a factor 20-40 increase of the sulfate aerosol surface area density (SAD) at 50 hPa in the tropics with respect to unperturbed background conditions and a factor 3-10 increase at mid-high latitudes. The net effect for a tropical injection rate of 5 Tg SO2 per year is a decrease in globally averaged ozone by 1.1-2.1 DU in the years 2040-2050 for three models which include heterogeneous chemistry on the sulfate aerosol surfaces. GISS-E2-R, a fully coupled general circulation model, performed simulations with no heterogeneous chemistry and a smaller aerosol size; it showed a decrease in ozone by 9.7 DU. After the year 2050, suppression of the NOx cycle becomes more important than destruction of ozone by ClOx, causing an increase in total stratospheric ozone. Contribution of ozone changes in this experiment to radiative forcing is 0.23 W m-2 in GISS-E2-R and less than 0.1 W m-2 in the other three models. Polar ozone depletion, due to enhanced formation of both sulfate aerosol SAD and polar stratospheric clouds, results in an average 5 percent increase in calculated surface UV-B.
PDF