Earth’s balmy, relatively stable temperature relies on a complex balancing act. Much of the Sun’s heat is lost to space through radiation emitted by Earth, a process called radiative cooling. Simultaneously, however, the atmosphere is warmed when water vapor condenses into droplets, releasing energy, and currents of air transfer heat from Earth’s surface into the atmosphere.
For decades, scientists attempting to simulate Earth’s climate have known that the global atmosphere as a whole maintains an idealized state called radiative-convective equilibrium (RCE), in which energy lost through radiation is balanced by heat released through the condensation of water vapor and the direct transfer of heat from the surface. When researchers observe Earth’s atmosphere at smaller scales, however, it is often out of equilibrium, raising concerns about whether RCE exists at local levels.
A new study by Jakob et al. identifies the scale at which RCE breaks down in the tropical atmosphere—around 1 million square kilometers. The team used several data sets to test whether RCE is present at different scales, including satellite observations of radiative cooling and convection, precipitation records, and images of clouds collected between 2001 and 2009.
As a whole, the tropical atmosphere remained close to RCE over the 9-year period, their analysis reveals. Clouds played a key role in maintaining RCE, which occurs frequently at 5,000 by 5,000 square kilometers or larger. It occurs most often when low clouds are widespread and there are a few convection hot spots: places where hot, moist air rapidly ascends and produces large amounts of rainfall. The two areas are connected through atmospheric circulation, which in turn provides the conditions for the different cloud types to exist.
RCE occurred less than 20% of the time in regions of the atmosphere smaller than 1,000 by 1,000 square kilometers, the team found. Many computer models used to study clouds focus on RCE for areas smaller than this scale when the real atmosphere is not likely to be in equilibrium. The finding could improve scientists’ understanding of the interaction of clouds and circulation, a complex and poorly understood factor in climate change. (Journal of Geophysical Research: Atmospheres, https://doi.org/10.1029/2018JD030092, 2019)
—Emily Underwood, Freelance Writer