Clouds, circulation and climate sensitivity

I’m a co-author on a paper that appeared last week in Nature Geoscience entitled “Clouds, Circulation and Climate Sensitivity”. This paper describes a “Grand Challenge”, one of several activities with that label, all begun in the last couple of years under the World Climate Research Programme (WCRP). For about another three weeks, the paper is freely available here. (After that, it’ll still be here, but with the full text behind the Nature paywall.) A Nature news piece on it is here.

Clouds over Addu Atoll, Maldives. From November 2011 during the DYNAMO field program. Photo by Adam Sobel.

The WCRP is an international organization with a long history of working to organize the efforts of climate scientists worldwide. The Grand Challenges are a new structure for doing that. There are six Grand Challenges; ours is, as you might guess, on Clouds, Circulation and Climate Sensitivity. The paper just published articulates what it is about. It tries to make the case for a particular set of research directions as ones that the climate science community would do well to focus on, and to emphasize the more basic point that there are big, fundamental problems left to solve in climate science.

The central scientific idea binding the piece together is that clouds and the large-scale circulation of the atmosphere are tightly coupled together and you can’t understand one without the other. This has been a widely accepted fact in tropical meteorology for decades; yet it has not been as well understood among those who study the roles of clouds in long-term climate change. My take is that this divide has occurred, historically, because clouds play different roles in the different areas.

For those studying tropical meteorology – trying to understand what makes the tropical atmosphere move, and how that motion is related to what we experience as weather – the clouds are important because of the precipitation they produce, and because the condensation of water vapor warms the atmosphere, inducing temperature and pressure contrasts which drive large-scale atmospheric motion (“circulation”). For those studying cloud effects on climate – where clouds are responsible for the largest uncertainties in our projections of anthropogenic climate change – what is important about clouds is how they interact with visible and infrared radiation. These roles of clouds are quite different, and historically studied separately. I see this paper as arguing that we need to bring them together.

The paper focuses on four questions¹:

1. What role does convection play in cloud feedbacks?

2. What controls the position, strength and variability of storm tracks?

3. What controls the position, strength and variability of the tropical rain belts?

4. What role does convective aggregation play in climate?

To understand properly what these questions mean, you’ll have to read the paper. But you can see in the first question what I’m talking about above. “Convection” is the word tropical meteorologists use to summarize the dynamical aspects of cloud, such as their updrafts, downdrafts, and interactions with temperature, pressure and wind. “Cloud feedbacks” is the word climate scientists use to describe how clouds modify the global warming caused by greenhouse gases. (We say the feedback is positive if they make the warming stronger, negative if they make it weaker.)

The second and third questions are similar to each other, but address different parts of the earth. “Storm tracks” is a term we use to describe the zones of greatest precipitation outside the tropics, while “tropical rain belts” refers, obviously, to the tropics. We often call the tropical rain belts “intertropical convergence zones” when they occur over the oceans, “monsoons” when they occur over land.

The last question may be the most opaque to a nonspecialist. “Convective aggregation” refers to a tendency of convective clouds (that is, clouds associated with strong circulations) to clump together. This happens in nature perhaps most strongly in tropical cyclones – a tropical cyclone can be described as a spinning clump of clouds – but there are other examples. Recent theoretical and modeling work has demonstrated that this clumping is something clouds tend to do if left to their own devices without anything else – land-sea contrasts, pole-equator temperature differences etc. – to disturb them, and it strongly alters the climate when it happens. So understanding how this tendency plays out in the real world, where all the complicating factors are present and the clouds are not left to their own devices, is important.

Why am I writing about this on the Extremes Initiative web page? What does this Grand Challenge have to do with extreme weather?

Nearly all the questions, actually, have something to do with extreme weather. The storm tracks are the zones of stormiest weather outside the tropics. Extreme winters like the one we just had in the northeast US result from long-term, persistent displacements of a storm track from its normal position. There’s a lot we don’t understand about these persistent anomalies, and the second question addresses that directly. The tropical rain belts, similarly, are where most extreme rainfall occurs, and major droughts or extreme rainy seasons occur when those belts are discplaced from their normal positions. The Columbia Initiative on Extreme Weather and Climate is co-sponsoring, with the WCRP Grand Challenge and others, a workshop on the third question (more or less) in September.

And convective aggregation is involved in the formation of tropical cyclones, as well as other extreme events.

Now there is also a separate Grand Challenge on Weather and Climate Extremes. That’s ok; the Grand Challenges are not meant to be mutually exclusive, but can overlap in their subject matter. We at Columbia are, in fact, talking to the people involved in the Extremes Grand Challenge about doing some joint activities in the not so far future. Stay tuned.

Footnote:

1. The structural resemblance to the Passover Haggadah was not intentional in the first discussions that led to this paper, but once the Jewish authors in the group explained it to the others, the group didn’t find it unwelcome. These first discussions did happen to occur just before the time of Passover, 2014, while the paper came out out just before Passover 2015; in both cases this is entirely coincidental.