Clouds at different heights sometimes overlap, stacking one above the other. This overlap strongly affects how much heat enters or escapes the atmosphere, impacting Earth’s climate. However, the fine-scale resolution needed to study cloud layers hinders detailed models of the overlap.
Most cloud overlap studies have adopted a wide-angle approach, focusing on either the whole troposphere or on tall cumulonimbus clouds, which are responsible for thunderstorms. Now Corbetta et al. have validated a 2011 fine-scale simulation of overlap among low-lying, cotton ball–like cumulus clouds.
For different times of day between April and August 2013, the authors collected cumulus cloud composition and position data at the Jülich Observatory for Cloud Evolution in western Germany. They calculated the ratio between per pixel cloud volume and per pixel cloud area at different altitudes. A higher ratio indicates more overlap between clouds at different heights.
The daily volume-to-area ratios observed for the cumulus clouds matched nicely with simulated ratios, and the scientists also found that both observed and simulated cloud overlap behaviors fit the same type of mathematical function.
The close match between the observations and simulated data suggests that computer models could be used to reliably predict cumulus overlap behavior and that incorporating them into global climate models could improve forecasts of weather and climate.
The simulations were made using the large eddy simulation (LES) technique, which models turbulence in the atmosphere. Following their success here, the researchers call for further verification of the usefulness of the LES method for virtually exploring cloud overlap. (Geophysical Research Letters, doi:10.1002/2015GL065140, 2015)
—Sarah Stanley, Freelance Writer
Citation: Stanley, S. (2015), Cloud overlap observations put simulations to the test, Eos, 96, doi:10.1029/2015EO040437. Published on 2 December 2015.