Research on Atmospheric Water Vapour Sources and Transport
The atmospheric branch of Earth’s water cycle couples the major water reservoirs, namely oceans, ice shields, rivers, and the land surface via evaporation and precipitation. Water vapour is a crucial ingredient for many weather-related phenomena, such as rain, snow, and clouds. As a greenhouse gas, it is also important for the radiative budget of the Earth. However, many physical processes related to water vapour are associated with large uncertainties. This research aims at improving the understanding of weather and climate processes by studying the evaporation and transport processes of atmospheric water vapour.
Lagrangian diagnostic for water vapour origin
We have developed a method which allows to trace water vapour in the atmosphere backward in time to identify the location from which it has evaporated. The method is a diagnostic based on backward trajectories, and provides quantitative information on the various contributions from moisture sources to the water vapour in traced air parcels. This provides, for example, a physical link between anomalous sea surface temperatures and extreme precipitation events.
We applied the method to identify the inter-annual variability of moisture sources for water transport to Greenland. Our results based on the ERA-40 reanalyis data show that the North Atlantic and the Nordic Seas are Greenlands main moisture sources during winter. However, moisture source locations and strengths can change strongly from winter to winter, in correspondence with the North Atlantic Oscillation index. In a further study, we investigated the implications for stable isotopes in Greenland winter precipitation. The method was also applied to study the moisture source seasonality of the European Alps.
Figure 1: Sketch of the Lagrangian water vapour transport diagnostic.
Water vapour tracer simulations in a regional model
An alternative approach to tracing water transport in the atmosphere is explored by means of a regional numerical weather prediction model (CHRM). In this apporach, we follow water vapour tracers that only evaporate from restricted areas of the surface to follow the distribution of the water vapour during specific meteorological events. The advantage of this method is the detailed consideration of physical processes which are not resolved at the model grid scale, such as convective precipitation.
Our analyis currently focuses on the examination of the Elbe flood, which during which large parts of Central Europe suffered in August 2002. Figure 2 shows how sub-tropical moisture was advected into the domain and contributed to precipitation in the model domain. Details about the method and this particular case study are available in Sodemann et al., 2009.
Figure 2: Tracer from a Sub-tropical water vapour source region is advected into the domain during the Elbe flood (12 August 2002).
- Sodemann, H., Wernli, H. and Schwierz, C., 2009: Sources of water vapour contributing to the Elbe ﬂood in August 2002 – A tagging study in a mesoscale model, Quart. J. Royal Meteorol. Soc., 135, 205-223, doi:10.1002/qj.374. [Abstract and PDF]
- Sodemann, H., Masson-Delmotte, V., Schwierz, C., Vinther, B. M. and Wernli, H., 2008: Inter-annual variability of Greenland winter precipitation sources. Part II: Effects of North Atlantic Oscillation variability on stable isotopes in precipitation, J. Geophys. Res., 113, D12111, doi:10.1029/2007JD009416. [Abstract, PDF, 1.6MB]
- Sodemann, H., Schwierz, C., and Wernli, H., 2008: Inter-annual variability of Greenland winter precipitation sources. Lagrangian moisture diagnostic and North Atlantic Oscillation influence, J. Geophys. Res., 113, D03107, doi:10.1029/2007JD008503. [Abstract, PDF, 6.4MB]
- Stohl, A., Forster, C. and Sodemann, H., 2007: Remote sources of water vapor forming precipitation on the Norwegian west coast at 60°N – a tale of hurricanes and an atmospheric river, J. Geophys. Res., 113, D05102, doi:10.1029/2007JD009006. [Abstract, PDF, 7MB]
- Sodemann, H. and Zubler, A., 2007: Herkunft des Niederschlagswassers im Alpenraum. DACH Meteorologentagung 2007, Hamburg. [Extended Abstract, PDF, 656KB]