Making model clouds more real

Foto: UiO
Inger Helene Karset, Department of Geoscience, University of Oslo. Photo: UiO

Clouds with myriads of tiny drops can last longer and grow bigger than clouds with fewer, larger drops. This is the theory, and in most climate models, this is what happens. An eruption from a volcano in 2014 shows that in the real world, the story is sometimes different.

In 2017, Inger Helene Karset participated in a study of how volcanic eruptions affect clouds. An eruption from the volcano Holuhraun in Iceland in 2014–2015 had sent huge amounts of ashes and other particles into the atmosphere.

Such particles – aerosols – are necessary ingredients in clouds, not by themselves, but because they act as small nuclei that water vapor can condensate on, forming tiny droplets. The clouds we see are by definition a collection of such small water drops or ice crystals. Not just volcanoes, but also pollen, sea salt and grains of sand can work as condensation nuclei in clouds. Aerosols can also form from pollution.

Whether the aerosols are natural or anthropogenic, they affect the climate on Earth. To be able to project our future climate, climate researchers need to know how aerosols influence clouds.

Aerosols cool the Earth

During eruptions such as the one in Iceland, volcanoes spew aerosols into the atmosphere. As the concentration of aerosols in the air increases, the number of cloud drops also increases. With more aerosols to condensate on, the water is distributed among a higher number of drops. This changes the clouds in two ways.

First, clouds with many, small drops reflect more of the radiation from the sun than clouds with fewer, larger drops. As a result, aerosols from a volcanic eruption will contribute to cooling the Earth.

Second, clouds feeded with areosols can grow larger and last longer. When the water is distributed on many, small droplets, fewer can grow big and heavy enough to fall down as raindrops. The water remains in the atmosphere, and clouds do to a lesser degree dissolve. Consequently, they block more radiation from the sun, also cooling the Earth.

In most climate models, this happens according to the theory. But, when Inger Helene Karset and her colleagues compared their model results with observations of the real world, they discovered that the theory was only partly supported.

Refining a theory

Satellite data after the Holuhraun eruption showed that clouds droplet did get smaller, just like the climate researchers had expected. But, there were neither more clouds, nor more water in the atmosphere. Could this mean that the effect of the aerosols from the eruption was too small to show? Can other factors have overshadowed it?

That is what Inger Helene Karset aims to find out now.

Water vapor condensating to form drops of water is not the only thing that occurs in clouds. There is a competing process: evaporation. Technically, condensation and evaporation occur at the same time. As soon as a cloud drop has formed, water will start to evaporate from the drop.

When the water is distributed over many, smaller drops, evaporation will be higher.

This is pure geometry. If a certain amount of water is to be distributed into two drops, the total surface area of the two drops will be larger than for a single drop. A larger surface area means a larger area for water to evaporate from. With more aerosols meaning that the water will be distributed into a higher number of drops, more drops will also dissolve. This may have stopped the clouds from growing as much as they would otherwise have done.

Making model clouds more real

Can too little evaporation in the climate models have prevented them from recreating what happened after the volcanic eruption in Iceland? In the climate models, the water content in the atmosphere increased with the aerosol concentration, and in some models more than in others.

In the real world there was no increase. Could the reason for the discrepancy be that the evaporation from the cloud drops in the models always is the same, not taking into account that drop sizes vary?

A very good thing with climate models is that reality can be tested over and over again. Inger Helene Karset has now refined the model she uses, so that small cloud drops evaporate faster than larger drops. The results are promising. So far, she has found a better correspondence between observations and her new model simulations than in the old simulations.