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Recent satellite surveys show that these striking 200-300-km-wide radial cloud structures are more prevalent than previously supposed.
Clouds are the most familiar aspect of our atmospheric environment. Schoolchildren around the world learn the names of the different cloud types, names that were devised just over 200 yr ago by the amateur meteorologist, Luke Howard. Clouds take their names-cirrus, cumulus, and stratus-from Latin terms that describe the way clouds appear to someone standing on the ground. Cirrus clouds resemble curls of hair, cumulus clouds look like piled-up heaps, and stratus clouds are spread out like blankets. While individual clouds may take innumerable forms, Howard's system works because the shape of clouds is not completely random, but is instead governed by some organizational principle.
Because clouds also play a significant role in determining the radiation budget of the planet and serve as an important link in the hydrological cycle, they are studied using a variety of ground-based and satellite techniques. Ground-based observers often use Howard's cloud classification system, but clouds look very different when viewed from space. This is because satellites are able to see, much more easily than a single observer on the ground, the range of scales at which clouds occur within the atmosphere.
The scale of satellite observations falls within the mesoscale, defined by Orlanski (1975), to range from a few to a few thousand kilometers. Cloud organization at the mesoscale comes about because clouds act as tracers of vertical motion within the atmosphere, which is primarily due to convection. Depending on the strength of the vertical motion, convection can be deep or shallow. Deep convection extends through the depth of the troposphere and includes mesoscale convective systems and hurricanes. Shallow convection is much weaker and typically occurs within the lowest few kilometers of the atmosphere. While deep convective systems have been extensively studied due to their impact on human life and property, shallow convective systems are also of interest because they reveal complex and poorly understood processes that affect cloud appearance and behavior in regions that play a significant role in the global climate system, particularly the tropical oceans (e.g., Blaskovic et al. 1991; Klein and Hartmann 1993).
This paper describes actinoform clouds, a class of shallow convective cloud systems that show...