Cloud fields adopt many different patterns that can have a profound effect on the amount of sunlight reflected back to space, with important implications for the Earth’s climate. These cloud patterns can be observed in satellite images of the Earth and often exhibit distinct cell-like structures associated with organized convection at scales of tens of kilometres1, 2, 3. Recent evidence has shown that atmospheric aerosol particles—through their influence on precipitation formation—help to determine whether cloud fields take on closed (more reflective) or open (less reflective) cellular patterns4, 5. The physical mechanisms controlling the formation and evolution of these cells, however, are still poorly understood6, limiting our ability to simulate realistically the effects of clouds on global reflectance. Here we use satellite imagery and numerical models to show how precipitating clouds produce an open cellular cloud pattern that oscillates between different, weakly stable states. The oscillations are a result of precipitation causing downward motion and outflow from clouds that were previously positively buoyant. The evaporating precipitation drives air down to the Earth’s surface, where it diverges and collides with the outflows of neighbouring precipitating cells. These colliding outflows form surface convergence zones and new cloud formation. In turn, the newly formed clouds produce precipitation and new colliding outflow patterns that are displaced from the previous ones. As successive cycles of this kind unfold, convergence zones alternate with divergence zones and new cloud patterns emerge to replace old ones. The result is an oscillating, self-organized system with a characteristic cell size and precipitation frequency.
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- Supplementary Information (849K)
This file contains Supplementary Information comprising: Large Eddy Simulation; Animation of evolving open cellular structure; Animation of coupled oscillator; MSG SEVIRI satellite imagery; Lidar and radar data; Rayleigh-Bénard oscillations resulting from buoyancy reversals using a 2-D model, Legends for Supplementary Movies 1-3, Supplementary Figure S3 with legend and References.
- Supplementary Movie 1 - Supplementary Figure 1 (2.6M)
This movie contains an animation of open cellular structures in Fig. 2 (for full legend see Figure 1 in Supplementary Information file page 8).
- Supplementary Movie 2 - Supplementary Figure (17.3M)
This movie contains an animation of the coupled oscillator (for full legend see Figure 2 in Supplementary Information file page 8).
- Supplementary Movie 3 - Supplementary Figure 4 (6.4M)
This movie shows satellite imagery of oscillating open cells with animation of images at 30 min intervals (for full legend see Figure 4 in Supplementary Information file page 8).