Mystery of the converging clouds that cool

The convective aggregation phenomena

In 1993, Isaac Held, a wavy-haired, soft spoken, Princeton professor, a pioneer of precipitation and global warming research, was doing some simulations of the atmosphere over the ocean, when he and his colleagues found something quite mysterious. [1] Beginning with clouds in random places, they ran their simulation, leading to clouds forming, disappearing and forming again. The simulation would, for some reason, always end with a clump of clouds all in the same place. The phenomena was named convective aggregation.

This is a sequence from one such simulation (see a video of this effect here), from the work of Allison Wing, of a 1000km by 1000km area over the ocean [2-4]. The grey spots are clouds, the red area indicates the amount of water vapor in the air, and the blue area indicates very little water vapor. The clouds initially form and un-form in different parts of the square until they begin to aggregate together over a period of 15-100 days.

In the above picture we can see that initially there are convection loops that bring up water vapor to create clouds. The loops are not synchronized with each other. Each convection pattern is an example of the maxim “Energy flows, matter cycles”. As water vapor ascends it carries latent heat, which it then releases when it forms a cloud. That water can then rain back down. Its similar to a small water cycle, only over the ocean. Each of these convection loops is an example of what Ilya Prigigone calls a dissipative structures - structures that self-organize as they dissipate energy. What’s curious in this scenario is that the different dissipative structures will begin to attract each other, synchronizing, and ending up as one larger dissipative structure.

One of the archetypal examples of a dissipative structure is Rayleigh-Bernard convection, i.e. when a whole array of convective loops that appears when, say, you heat up a pan of oil. In that pan, the convective loops seem to rearrange and self-organize. Is something similar happening here in how the clouds self-organize? The jury is still out.

These clouds congregations are believed to help cool the planet because they manage to radiate more long wave radiation into space than if they were separate clouds patterns. When convective aggregation is observed over oceans, it is usually accompanied by less high cloud cover. Reducing high cloud cover lessens the amount of water vapor that is trapped as a greenhouse gas. Researchers David Coppin and Sandrine Bony write that their “experiments show that the triggering of convective self-aggregation results in a strong and abrupt drop in global mean surface temperature.” [5] The convective aggregation water cycle is in essence, self-organizing to release heat.

Researchers also conjecture these convective aggregations can turn into hurricanes or into the Madden-Julian Oscillation (MJO) . Hurricanes pass heat from regions closer to the equator to regions closer to the poles, and help cool the warmer parts of the planet. The convective aggregation thus leads to larger processes that cool the planet even more. The Madden-Julian Oscillation is a disturbance of clouds, rain, and wind that moves eastward nearer the equator, every 30-60 days, influencing the probability of floods, heat waves and tornadoes wherever it travels. Thus some floods or heat-waves that we see trace back to the convective aggregation. The cause of the MJO is still also somewhat mysterious to researchers.

The exact cause of these convective aggregations is still being debated. Many think it has to do with the way the water cycle carries heat upwards, and how that heat is then radiated into space. There is a general feeling that areas with more clouds create more clouds with a number of theories why that should be, those being - a) The drier areas radiate more long wave radiation which causes the air there to sink lower, and the humidity in that area then migrates to the humid area, as there are convective updrafts there. “Convection arranges itself to balance radiation” says Allison Wing. Wetter areas become wetter, while drier areas become drier. The wetter areas have higher clouds, which are assumed to trap more of the water greenhouse gas, and so warms area up. But the drier areas cool more, so overall there is cooling over whole area b) As clouds form they can swirl in more air to make more clouds. If that air has more humidity from previous clouds being there then it will make more clouds. Thus areas that have been previously cloudy are more likely to become cloudy again c) Cold pools of air form under clouds that then spread, creating more clouds. d) Convection triggers other convections via gravity waves.

[a) High clouds are wetter area. Low clouds are dryer area. Lower clouds radiate more heat. Higher clouds act as greenhouse traps for water vapor, and so heat up the local area. Diagram from Allison Wing talk]

Thomas Kuhn, the philosopher of science who made the idea of paradigm shifts famous, wrote that an anomaly is an unexpected discovery one's paradigm cannot explain, which includes discovery of problems the paradigm cannot solve. Solving the mystery of convective aggregation and the Madden-Julian Oscillation phenomena will shed light on the formation of extreme weather events on earth, and illuminate how the earth self-organizes to cool itself.

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References

[1] Held, Isaac M., Richard S. Hemler, and V. Ramaswamy. "Radiative-convective equilibrium with explicit two-dimensional moist convection." Journal of Atmospheric Sciences 50, no. 23 (1993): 3909-3927

[2] Holloway, Christopher E., Allison A. Wing, Sandrine Bony, Caroline Muller, Hirohiko Masunaga, Tristan S. L’Ecuyer, David D. Turner, and Paquita Zuidema. "Observing convective aggregation." Surveys in Geophysics 38 (2017): 1199-1236

[3] Wing, Allison A., Kerry Emanuel, Christopher E. Holloway, and Caroline Muller. "Convective self-aggregation in numerical simulations: A review." Shallow clouds, water vapor, circulation, and climate sensitivity (2018): 1-25

[4] Wing, Allison A. "Self-aggregation of deep convection and its implications for climate." Current climate change reports 5 (2019): 1-11

[5]Coppin, D., & Bony, S. (2018). On the interplay between convective aggregation, surface temperature gradients, and climate sensitivity. Journal of Advances in Modeling Earth Systems, 10, 3123–3138. https://doi.org/10.1029/2018MS001406