Modelling study to quantify the impact of ice nucleation active bacteria on weather and climate
Maher Sahyoun 1,2 , Niels Woetmann Nielsen 1 , Jens Havskov Sørensen 1 , Kai Finster 2,4 , Ulrich
Gosewinkel Karlson 3 , Tina Šantl-Temkiv 4 , and Ulrik Smith Korsholm 1

1 Department of Research and Development, Danish Meteorological Institute, Copenhagen, Denmark
2 Department of Bioscience, Aarhus University, Aarhus, Denmark
3 Department of Environmental Science, Aarhus University, Roskilde, Denmark
4 Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark

Abstract: Scientific interest in bacteria and their impact on ice nucleation in mixed phase clouds became more focused since they were found to be efficient in nucleating ice at temperatures close to –2 °C, in addition to being abundant in the atmosphere, including: cloud water, precipitation, and air. In modeling studies, the potential impact of bacteria on ice nucleation and precipitation formation is still uncertain and this was due to their low concentration compared to other types of ice nuclei, i.e. dust. In this study, we investigate the sensitivity of modeled cloud ice and other meteorological parameters to the changes in the fraction of cloud droplets containing bacterial ice nucleation particles (INP), and thereby the potential atmospheric impact of bacteria in nucleating ice in mixed phase clouds. For this purpose, a module that calculates the probability of ice nucleation as a function of ice nucleation rate and fractions of cloud droplets containing different types of ice nuclei was built and implemented in a numerical weather prediction model. In the case study, it was found that changing the fraction of cloud droplets containing bacteria causes a perturbation in the forecast leading to some differences in several meteorological variables including: cloud ice, solar radiation and precipitation. In particular, the cloud ice was sensitive to the increase in the fraction of cloud droplet containing bacterial INP. The fraction of bacteria needed to have a certain amount of cloud ice was determined. Finally, we found that the impact of bacteria on cloud ice and other meteorological variables appears to be present and more efficient in local areas especially during convective events.

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