Our research group recently published a study that proposed a new theoretical constraint on cloud formation. I'll give an overview of the main result and reflect on where this sits in the broader project of trying to understand and predict Earth's atmosphere.
What is a cloud? My working definition is "white object made of water in the sky". You might think scientists have a better definition than this, but I would disagree. There are certainly plenty of more precise definitions, but each one tends to be suitable for only a limited set of applications. Precision comes at the cost of generality. Some definitions are based on how the atmosphere interacts with light; others are based on how much condensed water (liquid/solid) is present. And no condensed water definition will draw the cloud boundaries in precisely the same place as an opacity definition.
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When faced with the question of how to define cloud, researchers will simply pick something that seems reasonable for their application and move on. On some level, the end result is nearly always based on intuition.
And when we look at the broader picture, it is easily forgotten how poorly defined clouds are. For example, one of the biggest foci for the climate science community is the question of how clouds will affect our future climate. It is currently the second most uncertain aspect of global warming, behind how much CO₂ humanity will emit.
The framing is in terms of the "cloud radiative effect", which is defined as the difference between how a cloudy and cloud-free atmosphere interacts with light. Implicit here is an assumption that it is possible to cleanly separate "cloudy" and "cloud-free".
So, we are in this situation: the cloud impact on climate is poorly understood and highly relevant to humanity's future, but the very definition of what a cloud is remains quite difficult to pin down. Worryingly, our situation has not been significantly improved in the last 40 years, so we need a new approach. As I see it, we have two options. The first is to search for a more physically realistic, i.e. not intuition-based, definition of cloud. The second is to rethink the very idea of the "cloud/not cloud" binary, perhaps in favor of some approach that admits of more nuance and gradation.
I am personally in favor of the second approach, and I'll discuss that much more in the future on this blog. But here I want to overview one reasonable attempt at the first option. This study was led by my graduate advisor Tim Garrett and was published about a month ago.
The basic premise is to shift perspective from the cloud itself to the cloud edge. Why? If we consider clouds as distinct entities, then for a cloud to grow, water (and, in fact, energy) must pass across the cloud boundary in order to accumulate inside the cloud. The cloud's ability to exchange water and energy with its environment is controlled by the geometry of the boundary: larger boundaries allow more water to pass, for example. This perspective was first articulated rigorously a few years before I joined the research group, and has informed all our work since.
The new study suggests that cloud edge is not only important as a surface that controls the passage of water and energy, but is also a physically unique location in an objective way. It provides, therefore, a new definition of cloud that is based on a physically meaningful property of the atmosphere, rather than human intuition.
Specifically, the new definition is based on buoyancy. For context, it is generally true that air inside clouds tends to rise while air outside of clouds tends to sink. The basic reason is that rising air creates clouds: as it rises, air cools, and water condenses out of sufficiently cold air. Sinking air, in contrast, warms, which causes water to evaporate and any cloud to dissipate.
Why would a bit of air rise? Because it is buoyant, i.e. less dense than its surroundings. So, beginning with the observation that in-cloud air is usually buoyant, and clear-sky air is usually negatively buoyant (so that it sinks), it follows that the boundary between cloudy and not cloudy might coincide with the boundary between positive and negative buoyancy. Tim often explains this by comparing cloud edge to the fulcrum of a seesaw: on one side, stuff goes up, on the other, stuff goes down. The cloud edge itself does not preferentially move up or down.
Cloud edge, when defined as neutrally buoyant, may also be more objective than more commonly-used definitions. The difference between up and down, or positive and negative, is qualitative as well as quantitative.
Here's a visual to explain the difference in perspective between "cloud" and "cloud-edge" thinking. The below is from a high-resolution cloud simulation. I color-coded the buoyancy using red to refer to positive buoyancy and blue to refer to negative buoyancy.
Previously, studies would focus on the red parts when thinking about clouds: the locations where clouds are. We focus on the white: the boundary which defines where clouds start. It's a subtle shift in perspective, but one that may prove useful.
It was already known that clouds tend to be positively buoyant, and it’s not much of a leap to realize that cloud edge is therefore neutrally buoyant. In fact, at an early stage we had discussions about whether this result was too trivial to publish. It felt kind of obvious, and in the paper, we call the cloud-edge-is-neutrally-buoyant result "somewhat trivial".
But we went ahead for one main reason. The "cloud-edge" vs. "cloud" perspective itself is much more novel than the actual mathematical derivation. And as I said above, the climate science community needs new approaches or perspectives if we are to make headway toward understanding the impact of clouds. So the hope is that looking at the problem from this new angle will stimulate some novel insight in the future.
Still, the "cloud-edge" perspective does conceptualize clouds as discrete entities. It has that in common with most other approaches. Personally, I am becoming increasingly convinced that we should dump that whole approach and consider clouds and clear-sky as a difference in degree rather than a difference in kind. I’ll return to that often in future blog posts. But I might be wrong, and if we must retain clouds as discrete entities, then it makes sense to search for a more objective definition.
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