LAUSR

Abstract Forecasts of atmospheric composition in the Ex-UTLS (extratropical upper troposphere – lower stratosphere) are highly sensitive to uncertainties in the source regions of air masses. This study investigates the reliability of calculated source regions and their effects on trace gas distributions in this region. For this purpose, an ensemble-based probabilistic retroplume approach is developed to track source regions of air masses. This approach provides the ability to account for both, (i) the contribution of different source regions to an air mass by adjoint diffusion and (ii) the uncertainty of theses source regions with respect to forecast dynamics. Probabilistic retroplume calculations are applied to an air mass in the vicinity of a weak saddle-point in the Ex-UTLS in order to investigate its effect on source regions. The results indicate high sensitivity of source regions to uncertainties in atmospheric transport revealed by perturbed meteorological forecasts. Additionally considering the sources of surrounding air masses leads to even larger spread of source regions. Spreading over large parts of Europe and altitudes between 8 and 14 km within two days, probable source regions comprise completely opposite transport-directions. This large uncertainty of source regions show significant effect on atmospheric composition in the vicinity of the saddle-point on a short timescale. Evaluating the effect of meteorological uncertainties on local distributions of trace gases, ozone concentrations range from 50 to 350 ppbv at the same location. Probabilistic retroplume calculations suggest a clear connection between these concentrations and related source regions.