SPP 1167: “Quantitative Precipitation Forecast”

Forecasting precipitation is one of the most challenging tasks of weather forecast models. Especially heavy events are difficult to predict correctly for the present models. Though some improvements were made during the last years especially regarding the qualitative prediction of precipitation, the quantitative forecast is still not satisfying. The Priority Program 1167 “Quantitative Precipitation Forecast” combines various groups from universities and research institutions to meet the challenge with a joint and coordinated effort.

Project contribution: “Spectral Microphysics in Weather Forecast Models with Special Emphasis on Cloud Droplet Nucleation” (duration: April 2004 – March 2008)

In our approach, we combine a Spectral Bin Microphysics model [1,2] with the mesoscale Lokalmodell (now called COSMO model, Consortium for small-scale modelling), which is the regional part of the forecast system of the German Weather Service (DWD). Aerosol particles in the atmosphere have a huge impact on the formation of droplets and thus on the amount of precipitation. Such particles serve as nuclei for cloud droplets. It depends on on the size and number of available particles as well as on the chemical composition how many cloud droplets will evolve and to which size they grow through condensation. Those droplets grow further through coalescence which finally leads to the formation of rain. Also, droplet freezing can take place at temperatures below 0°C being dependant, e.g., on the availability of ice nuclei, which are typically insoluble particles. The presence of the ice phase promotes the formation of larger particles and, therefore, precipitation. Thus it is a promising approach to have a closer look on the interaction of aerosols with clouds which may eventually lead to

  • improving the quantitative precipitation forecast in operational models through offering new approaches to the parameterization of cloud microphysics and
  • investigating in detail the influence of aerosol particles (their number and size as well as their chemical composition) on rain formation, which e.g. makes it possible to analyse the effect of air pollution on the weather.

[1] Diehl et al. (2006), J. Geophys. Res. 111. D07202, doi:10.1029/2005JD005884.
[2] Simmel and Wurzler (2006), Atmos. Res. 61, 135-148.