Dirty Clouds

In my previous post I gave a short summary of how clouds affect the radiation and heat budget of the Earth, and the difference between low and high clouds. In a world with a changing climate, we all want to know what the effects of CO2, aerosols, and associated global warming will be on clouds and their feedback on the system. 

In truth, clouds are difficult to incorporate in climate models. They are small, the physics behind them are complex and not always well understood, and clouds have strong regional effects, all of which makes them complicated to model. Even so, knowledge of clouds is greatly improving, and so models are becoming more certain (for those interested in climate model certainty, always a hot topic, please see NASA on climate uncertainties and the IPCC on the confidence of model predictions in the 4th Assessment Report). 

First, let's just do a quick recap on what aerosols are. Basically, aerosols are all sorts of types of suspended particulate matter. Some occur natural, such as dust or organic compounds. A famous example of the latter can be found in the Blue Mountains in New South Wales, Australia, where the eucalyptus trees release a sort of organic matter into the mountain air. Incoming bluish ultraviolet light is scattered by the particles, giving the Blue Mountains their etymological characteristic.

Blue Mountains, NSW, Australia

Source: National Geographic

However, humans are putting in a fair share of aerosols into the atmosphere as well. In a previous post I discussed the effect of soot deposition on snow in Antarctica. Other aerosols include sulphates from fossil fuels and other pollutants. In the picture below, Ramanathan and Feng (2009) have indicated the emissions of black carbon across the globe.

Emission of BC in tons per year across the globe. Note the emissions across the ocean along traffic routes of airplanes and ships. 

Source: Ramanathan & Feng 2009.

Aerosols alone can affect the incoming solar radiation by reflecting some of it back into space. In this sense, they increase the albedo. However, some of the radiation is absorbed, warming the atmosphere. As a net effect, the surface is cooled, and the atmosphere warmed (Ramanathan & Carmichael 2008).

In any case, like I said above, I want to investigate the effect of aerosols on clouds. Clouds are complicated, and studying the relation between clouds and aerosols is still not well understood. The IPCC rates the scientific understanding of the interactions generally as low to very low (also see the relevant section here). Even so, I want to discuss some of the basic processes.

Aerosols affecting cloud behaviour. 

Source: UK Met Office

Clouds form around small particles, called Cloud Condensation Nuclei (CCN). More aerosols mean more CCN, meaning more droplets that together form clouds. It also means that clouds become whiter, and thus more reflective. In this sense, aerosols can decrease cloud albedo and have a cooling effect (this is the first effect in the figure above).

Because there are more CCN, the droplets are smaller and it will take longer for clouds to form raindrops. This increases the lifetime of clouds as well as decreasing the number of rainfall events. This secondary effect, also known as the Albrecht effect, is not very well understood.

Lastly, there is the semi-direct effect. The aerosols in the clouds absorb radiation, and re-emit it, warming the clouds and reducing the upward flow of moisture - and the formation of clouds. This is perhaps the least-well understood effect.

I hope that this shows how complicated the relationship between clouds and aerosols is; there is no clear-cut correlation. Some effects increase cloud cover, whereas others reduce them. Clouds are difficult to study and model, which makes it tricky to incorporate them in climate projections.

Despite all this, scientists discover more and more about aerosols and clouds, which greatly improves the models for future climate projections. To finish off, I found this animated video of the cloud-aerosol effects I explained in this post.

Another interesting field related to clouds and aerosols has to do with geo-engineering, something I want to look at in my next blog post. For more information on research that is being done on clouds, I can recommend the Guardian article in the "Further Reading" section below, along with some other interesting websites. Happy browsing!

Further Reading

• "Creating clouds in the lab to better understand climate" via The Guardian. 
• "Climate science: The aerosol effect" - a commentary in Nature on progress in the cloud-aerosol research. 
• "What is global dimming?" - an explanation of air pollution and radiation at the Earth's surface, via The Guardian. 

References

• Ramanathan V. and G. Carmichael (2008), Global and regional climate changes due to black carbon, Nature Geoscience, 1, 221, doi:10.1038/ngeo156
• Ramanathan, V., and Y. Feng (2009), Air pollution, greenhouse gases and climate change: Global and regional perspectives, Atmospheric Environment, 43, 37, doi:10.1016/j.atmosenv.2008.09.063

Every Cloud Has a Silver Lining: Albedo & Clouds

One of the most important components of the weather system, clouds, actually has a very important effect on the world's albedo. A lot of research is being done on different types of clouds - darker clouds have a lower albedo than white ones - how clouds are formed, and what the effects of global warming will be.

Night clouds 

(Source: National Geographic)

Generally, clouds have a higher albedo than the surface underneath them, so they reflect more incoming sunlight than if they would be absent. In this sense, they have a cooling effect on the climate system. More clouds, more sunlight reflected, more cooling - or not?

The radiation coming from the sun is mostly in the same wavelength order as visual light, and is called shortwave radiation. The albedo of a given surface is the fraction of shortwave light that is being refracted. Nevertheless, not all of the light is reflected (as you'll remember from the first post). Some of the solar radiation is absorbed by the Earth, warming up the planet in the same way you get warmer from sitting in the sun. The Earth emits some of that energy in the form of longwave radiation, or heat.

In this image, you can see how clouds affect the distribution of shortwave and longwave radiation (Source: NASA)

Low and high clouds have different effects on the radiation budget. High clouds are usually thin and have low albedos. All things emit heat, even clouds, but because high clouds are quite cold, they emit less heat back into space. These two effects combined mean that high clouds have a net warming effect (also see NASA).

Low clouds do the opposite. They are thicker and have a higher albedo, and send more incoming shortwave radiation back to space before it can even reach the Earth's surface. They are closer to the Earth's surface, and their temperature is almost the same. This means that they radiate almost the same amount of longwave radiation as the planet. Even though they trap a great deal of longwave radiation as well, their net effect is cooling.

The image below is a compilation of cloud fraction cover as observed by NASA satellites. I recommend checking out the links below for further reading; these are also the links I used to do the research for this entry. Next entry I will look at the effect of pollution and aerosols on clouds and climate.

This animation shows the cloud fraction from 2005 to 2012

(Source: NASA Earth Observations)

Further Reading & References

• NASA's Earth Observatory - On clouds, cloud albedo and the effect on our climate.
• IPCC's Fourth Assessment Report - On the cloud-albedo feedback
• NOAA (National Oceanic and Atmospheric Administration) - On clouds