Understanding aerosol-cloud interactions in stratocumulus: a successful story about synergistic interactions in atmospheric science

Clouds can contain billions of water droplets and ice crystals. Each one of them reflect and absorb energy from the sun, giving to clouds a very important role in the climate of the planet. Clouds form when the wind blows up a parcel of air cooling it and causing water to condense over the surface of tiny particles called aerosols. Although the majority of aerosols are invisible to the naked eye, we can sometimes appreciate the largest ones in our daily life. For example, when wind blows spreading pollen, dust, or ashes from fireplaces.

With our activities, we can influence the amount and type of aerosols that can form clouds. Thinking that every aerosol particle behaves differently as every human being does, it is easy to understand why scientists spend so much time figuring out if clouds are changing and how this could be affecting the climate. In the recent study, researchers referred to the old saying “we can achieve more if we work together”. Uniting efforts, the Finnish Meteorological Institute and the University of Eastern Finland, studied the process of cloud formation from two different perspectives.

Cloud formation was modelled and followed by observations

First, the experimental team measured the number and size of droplets inside a cloud, while simultaneously also measured the number, size and composition of aerosol particles that generated them. To complete the cycle, they also measured how strong the vertical velocity of the wind was inside the cloud. In this way, they provided a very realistic “movie” of how clouds form. These observations were carried out at the top the Puijo tower, and the Savilahti and the Vehmasmäki stations in Kuopio, Finland, during autumn 2020.

After that, the modelling team benefited from UCLALES-SALSA, a novel computing tool created at the Finnish Meteorological Institute, that can solve all the mathematical equations that represent the formation of clouds. The tool gives also a unique and realistic description of how tiny aerosols, water droplets and ice crystals interact inside clouds.

The study was focused on two distinctively different cases. The first was a cloud case that occurred during the morning with the presence of sunlight and large amounts of aerosols due to pollution. The second was a cloud case that occurred during the night with lower aerosol levels, more representative of typical background conditions at the Puijo tower.

Intensity of vertical winds affects cloud droplets

Scientists discovered that the intensity of the vertical wind not only define how much water can be transformed to cloud droplets, but also can help to produce more numerous and smaller droplets when there are high aerosol levels. This, indeed, leads to brighter clouds that look whiter to our eyes and do not give us rain.

If on the contrary, aerosol levels are low and there is no sunlight, the intensity of the vertical wind can help to produce less but larger droplets than can transform into ice crystals if the temperature is below zero degrees.

The fruitful collaboration left a scientific lesson for future studies. The process of cloud formation can be estimated with the proper mathematical model if measurements give the aerosol properties and vertical velocities of the wind.

Further information:

Researcher Silvia Calderón, Finnish Meteorological Institute, silvia.calderon@fmi.fi.

Scientific article is available on Atmospheric Chemistry and Physics.

Reference: Calderón, S. M., Tonttila, J., Buchholz, A., Joutsensaari, J., Komppula, M., Leskinen, A., Hao, L., Moisseev, D., Pullinen, I., Tiitta, P., Xu, J., Virtanen, A., Kokkola, H., & Romakkaniemi, S. (2022). Aerosol-stratocumulus interactions: Towards a better process understanding using closures between observations and large eddy simulations. Atmospheric Chemistry and Physics Discussions, 2022, 1–39. https://doi.org/10.5194/acp-22-12417-2022

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