Small Parts, Major Effect

Today, weather forecasts are based on well-developed computer models, yet which still do not take into account many important processes within the atmosphere – for instance, floating particles such as dust, pollen or chemical compounds. Such so-called aerosols not only reduce visibility but influence also the temperature distribution across various altrimetric levels; they can react with one another and as condensation nuclei can cause the formation of clouds and precipitation. Yet in the established models used by weather services, aerosols are included, if at all, only generally because they absorb part of solar radiation. “We now observe the various aerosol particles in much greater depth and model an important part of processes, in particular also the formation of clouds and their interaction with radiation”, explains Dr. Bernhard Vogel from the KIT’s Institute for Meteorology and Climate Research. He and his colleagues have been working for about five years on COSMO-ART, a module with aperture sizes of a few kilometres only, which can be connected to the weather forecasting model COSMO used by the Deutscher Wetterdienst DWD (German Weather Service). The abbreviation ART stands for “Aerosol and Reactive Trace Gases”. By now, COSMO is being used not only by the DWD but also by many other weather services and research institutions in Europe, Canada, Russia and the United Arab Emirates, who are also interested in the ART module.

For aerosols are everywhere, they diffuse the light and change transport processes between air layers. Natural aerosols are volcanic ash, sea salt, sand particles from the Earth’s deserts, plant pollen, evaporations from forests or cattle herds. To these are added anthropogenic aerosols such as soot, nitrogen oxides and fine dust from traffic, households and industry. Some particles are only a few nanometres thick, other floating particles measure some hundredths of a millimetre in diameter. “We calculate the concentration of different kinds of aerosols from a set of initial data by way of dispersion calculation”, says Vogel. Thus, information regarding land development, agriculture, forestation, flow of traffic as well as natural and anthropogenic emissions is entered into the model. The modelling itself is based on the knowledge about relevant physical processes and chemical reactions of the different types of aerosols.

Hitherto, the atmosphere researchers tested their model on past periods in time in order to test how realistic the results are. Of particular interest is the question how the concentration of aerosols affects the formation of clouds and thus also the amount of precipitation. For instance, the model shows how high concentrations of aerosols can delay rain as regards time and location: For high concentrations of aerosols cause countless cloud condensation nuclei amongst which the given amount of moisture is spread out so that the droplets remain floating. Conversely, if there are only few aerosol particles, less but larger drops form, which then rain down.

COSMO-ART can also calculate how sand particles spread across Europe after a sandstorm in the Sahara or which paths the ash takes after a volcanic eruption. By now, the calculated sequences increasingly match the satellite pictures, so that they could be used for prognosis very soon. The pollen forecast for allergic people is almost ready for application, whereas currently it is still painstakingly compiled by hand nearly without computer support. Presently and in cooperation with the KIT, the Deutscher Wetterdienst tests COSMO-ART with a module for birch pollen into which were incorporated locations of birch tree forests; a module for the dangerous ragweed pollencould follow once the locations of the plants are known more accurately. “The United Arab Emirates now also approached us, intending to use an ART module for dust prognosis for aviation”, Vogel states.

Likewise, the usual climate models on regional level are to be incorporated into the ART modules. This is because the interaction between aerosols and cloud formation is badly portrayed in these, with different studies hitherto arriving at contradictory results as to how high concentrations of aerosols can affect climate and precipitation. “In two to three years, we want to create a regional climate model including aerosols from out of COSMO-ART”, says Vogel. “This is an integral contribution on part of the KIT towards the Helmholtz Climate Initiative REKLIM”, explains Institute Director Professor Dr. Christoph Kottmeier. And then one could do a computer test on how stricter critical limits for fine dust, better filters or the cutting down of entire forests affect the regional weather and the long-term climate development.