All clouds great and small, instability made them all

The variability of cloud types in the deep convective region of the tropics is enormous, encompassing a range of processes commensurate in complexity with the amount of convective energy that is available, often sufficient to move air from the surface to the tropical tropopause. The fields relevant to understanding how clouds evolve are dynamic, radiative, microphysical and chemical, each sub-discipline having its own language while the physics of each is a function of the others. Adequately addressing the complexity can seem insurmountable, even as high tropical cloud behaviors are a primary source of uncertainty needing to be narrowed to improve predictions of future climate change using coarse-grid global models. There may be some hope: the atmospheric system does appear to follow some clear top-down constraints on cloud behaviors. One fascinating new result is that the albedos of the two hemispheres appears to be equal, even though the amount of land mass is very different, and even as the climate warms. This talk focuses on cloud size and amount. Ensembles of clouds viewed from space, whether in the tropics or elsewhere, appear to follow very simple power-law distributions for their sizes across a very wide range of scales, ranging in one regard from the millimeter to the planetary. Exponent values appear invariant to any dynamic, microphysical, or chemical considerations, and instead are more consistent with a theory that the total amount of cloud, most specifically the cloud perimeter, is governed uniquely by the thermodynamic stability, or the fact that the atmosphere is vertically “squashed”. The take-home message is that clouds at all levels compete for the finite energy that allowed them to form in the first place. Whatever the local, short-term details of cloud formation, one’s gain becomes another’s loss so that in the end the resulting distribution for large-scale distributions of cloud sizes remains stubbornly invariant.