Simone Tilmes - Investigation of chemical and dynamical processes using observations and chemistry-climate models
The chemical composition in the Earth's troposphere and stratosphere is controlled by the interaction of radiation, transport, chemistry and dynamics. The importance of individual processes depends on location and season. Various observations have been taken and models have been developed to understand processes and interactions and to estimate the impact of changing climate conditions. The current research of Simone Tilmes is to combine information from different observations (satellite, aircraft, in-situ) and chemistry-climate model results, to investigate atmospheric processes. Her topics are the polar region and the exchange region between stratosphere and troposphere at all latitudes, the UTLS (Upper Troposphere/Lower Stratosphere) region.
A very specific situation occurs each year in the winter polar stratosphere. Since the eighties, the Antarctic ozone hole develops as the result of enhanced halogens in the atmosphere. Activated halogen compounds under special polar conditions effectively destroy ozone. During the last decade, a lot of research was performed to understand the impact of chemical and dynamical processes on polar ozone, with increasing interest in the northern hemisphere as well. Simone enhanced and established a technique to quantify the chemical part of ozone depletion in the lower stratosphere. She derived chemical ozone loss for Arctic and Antarctica using different satellite observations and in-situ observations. The ILAS and ILAS-2 satellite observations were used to derive detailed information of the evolution of chemical ozone loss during the cause of the winter in both hemispheres. Further, she used these observations to derive chemical ozone loss in the early winter. Interesting dynamical behavior of a double transport barrier of the polar vortex was discussed (Figure 1).
Figure 1: Ozone distribution (in ppm) in the Polar vortex in Antarctica in April. The two black curves indicate the double transport barrier.
The impact of chemical ozone loss and changing climate conditions can be estimated from observations. Further, chemistry-climate models are used to understand, for example, the future impact of ozone depletion on the stratosphere and troposphere. However, it is important to be aware of shortcomings in the models. In the framework of the CCMVal (the Chemistry-Climate Model Validation Activity), Simone validated heterogeneous processes the NCAR Whole Atmosphere Chemistry Climate Model (WACCM3). Earlier analysis of observations and the development of diagnostics helped to localize shortcomings and to validate and improve the model. During her research, Simone addressed the question of how chemical ozone loss is changing with regard to temperature variability in both hemispheres.
Figure 2 shows the relationship between chemical ozone loss and her newly derived quantity, the potential for the activation of chlorine (PACl), which takes into account the effect of chlorine loading by using the Effective Equivalent Chlorine (EECl) index for the stratosphere.
PACl is an extension of another measure of potential ozone loss, the potential volume of Polar Stratospheric Cloud (PSC) formation. Both quantities depend strongly on temperature conditions in the polar vortex. PACl is suitable for comparing conditions in both hemispheres (Antarctica: squares, Arctic: triangles) between observations (open symbols) and WACCM3 results (colored symbols).
Simone's second research focus is the region encompassing the upper troposphere and the lower stratosphere, the UTLS. In this region, transport processes redistribute climate sensitive chemical species. The quantification of transport processes in this region is not straightforward. Mixing is a ubiquitous phenomenon in this region. Due to the small-scale nature of mixing processes, detailed observations from research aircrafts are important. A key step is to compile sporadic aircraft observations into a climatology, which can be readily used to guide the models. Currently, Simone is analyzing the seasonal and regional characteristics of transport processes near the tropopause based on available aircraft observations. Additionally, Simone uses two NCAR models to understand transport processes: MOZART3 (Model for Ozone and Related Tracers) and WACCM3. These models can be used to localize regions of possible mixing and therefore help planning upcoming future aircraft campaigns. On the other hand, these observations are used to validate chemistry climate models.
Figure 3 illustrates the distribution of ozone (in ppm) using MOZART3 for a specific transport event, the mixing across the subtropical jet. The tropopause is shown as solid circles.
ASP Spotlight September 2007
For more ASP spotlights click here http://www.asp.ucar.edu/spotlight/archive.jsp