By Allison A. Wing
If you take a look at nearly any satellite image of clouds in the tropics (for example, the GOES west geostationary satellite image from a few days ago), you’ll notice that the clouds tend to be organized into clusters. Organized cloud clusters can be many different shapes and sizes, from groups of thunderstorms like squall lines and mesoscale convective complexes, to spiral-shaped tropical cyclones, to planetary scale phenomena such as the Madden-Julian Oscillation. A large fraction of weather in the tropics (clouds and rain) is associated with these organized clusters, so it is important that we understand how they form.
I’ve spent most of the last five years working on one specific type of cloud organization called “self-aggregation.” Self-aggregation is the tendency of tropical clouds to spontaneously clump together, solely due to interactions between the clouds and the surrounding environment. It occurs in numerical model simulations of an imaginary patch of the tropical atmosphere, in which the model starts off with the same temperature and moisture everywhere over a uniform, fixed temperature ocean surface. To see an example of one of these simulations, watch the movie below. The white shading indicates clouds and the colors indicate rainfall. Make sure to watch the whole thing; the most exciting part is about 1 minute into the movie.
Quickly after the simulation begins, clouds and rainfall start to form. In the beginning of the simulation, the clouds occur randomly throughout the domain; they look a bit like bubbles in a pot of boiling water. Sometimes it stays like this forever, but in this case, in some areas of the domain the air starts to get drier and in other areas the air starts to get moister. As this happens, the clouds and rainfall are increasingly confined to the moistest regions, and help them become even moister. Eventually, all the clouds and rainfall are confined to a single cluster. This dramatic evolution is what we call “self-aggregation.” Sometimes we end up with a circular cluster, as in the movie above, but other times the clouds organize into elongated bands, which is the topic of a new study I recently completed in collaboration with Timothy Cronin, a postdoc at Harvard University, titled “Self-aggregation of convection in long channel geometry.” To learn more about the results from that study and more about what causes self-aggregation, check out an extended version of this post on the Extreme Weather Blog over at the Columbia Initiative on Extreme Weather and Climate.
We think that self-aggregation might be important for the growth of the Madden-Julian Oscillation, and that it could also play a role in tropical cyclone formation (a problem I am currently working on). But the impact of self-aggregation might be much broader than its role in these specific (but very important!) phenomena. Clouds and humidity play a big role in determining how easily the atmosphere warms in response to a forcing like increased greenhouse gas concentrations. The transition from random to clustered clouds that occurs with self-aggregation is not just a reorganization; the domain overall is drier when the clouds are aggregated. So, if self-aggregation really happens in nature, it could affect climate sensitivity. This is one of the central questions of the WCRP Grand Challenge on Clouds, Circulation and Climate Sensitivity, and is being actively studied by researchers across the world.
Allison A. Wing is an NSF postdoctoral research fellow at Lamont-Doherty Earth Observatory. Her research on organization of tropical convection and tropical cyclones is a component of the Columbia Initiative on Extreme Weather and Climate.