Mesoscale cloud patterns in the trade-wind boundary layer

Summary

Greenhouse gas emissions are warming the planet, but how quickly? Uncertainties in Earth’s warming rate, as estimated by climate models, depends on how low-lying clouds over the subtropical oceans react to warming, and feed back on the warming. These clouds, so-called trade cumuli, are typically organised in horizontal patterns of tens to hundreds of kilometres across. In this thesis we use observations from space to characterise trade-cumulus patterns, and we study their formation through simulations that resolve clouds in more detail than climate models. The simulations indicate that many patterns form through spontaneously growing “circulations” of air, generated by the clouds themselves. Although different models are sometimes inconsistent with each other, they can accurately predict the statistics of observed circulations. Therefore, we find the detailed simulations to be a valuable tool, which indicates that the circulations and cloudiness are resilient to changes in their environment. In other words, mesoscale trade-cumulus patterns will most likely keep cooling our planet at similar rates in a warmer future as today.