Diagnostics of the Tropical Tropopause Layer from in-situ observations and CCM data
1ISAC-Institute for Atmospheric Sciences and Climate, National Research Council, Italy
2CMCC-Centro Euro-Mediterraneo per i Cambiamenti Climatici, Italy
3ENEA-Ente Nuove Tecnologie Energia e Ambiente, Rome, Italy
4INGV-Istituto Nazionale di Geofisica e Vulcanologia, Italy
5FZJ, Forschungzentrum Julich, GMBH, Germany
6INOA-CNR, Istituto Nazionale di Ottica Applicata, Italy
7J.W. Goethe University, Frankfurt, Germany
*now at: Department of Physics, University of Wuppertal, Germany
Abstract. A suite of diagnostics is applied to in-situ aircraft measurements and one Chemistry-Climate Model (CCM) data to characterize the vertical structure of the Tropical Tropopause Layer (TTL). The diagnostics are based on the vertical tracers profiles, relative vertical tracers gradients, and tracer-tracer relationships in the tropical Upper Troposphere/Lower Stratosphere (UT/LS), using tropopause coordinates.
Observations come from the four tropical campaigns performed from 1998 to 2006 with the research aircraft Geophysica and have been directly compared to the output of the ECHAM5/MESSy CCM. The model vertical resolution in the TTL allows for appropriate comparison with high-resolution aircraft observations and the diagnostics used highlight common TTL features between the model and the observational data.
The analysis of the vertical profiles of water vapour, ozone, and nitrous oxide, in both the observations and the model, shows that concentration mixing ratios exhibit a strong gradient change across the tropical tropopause, due to the role of this latter as a transport barrier and that transition between the tropospheric and stratospheric regimes occurs within a finite layer. The use of relative vertical ozone gradients, in addition to the vertical profiles, helps to highlight the region where this transition occurs and allows to give an estimate of its thickness. The analysis of the CO-O3 and H2O-O3 scatter plots and of the Probability Distribution Function (PDF) of the H2O-O3 pair completes this picture as it allows to better distinguish tropospheric and stratospheric regimes that can be identified, first, by their differing chemical composition.
The joint analysis and comparison of observed and modelled data allows us to evaluate the capability of the model in reproducing the observed vertical structure of the TTL and its variability, and also to assess whether observations from particular regions on a monthly timescale can be representative of the fine scale mean structure of the Tropical Tropopause Layer.