Minimal recipes for global cloudiness

Abstract

Clouds are one of the most fascinating, important, and complex components of Earth's climate system (Siebesma et al., 2020). Despite their importance, theoretical understanding of what controls planetary-wide cloudiness is largely absent. For example, while we have a good understanding of how clouds form and interact with radiation (Cotton et al., 2014;Houze, 2014;Siebesma et al., 2020), it is difficult to use these theories to make claims about global cloudiness. Earth System Models (ESMs) and other bottom-up approaches do couple simple models of cloud formation to the global circulation. However, so far they have not been proven effective in constraining global cloudiness variability (Sherwood et al., 2020;Zelinka et al., 2020). This makes it difficult to transparently establish links between variability in global cloudiness and Earth's energy balance, or how such a link would change in a changing climate.Conceptual models could be useful in elucidating how clouds, circulation, and energy balance, are tied together. Existing theoretical work has linked cloudiness to circulation, and most examples of such work focus on particular circulation systems, like the tropical overturning circulation (Betts & Ridgway, 1989;Pierrehumbert & Swanson, 1995), or the Walker cell (Peters & Bretherton, 2005), or individual cyclones (Carlson, 1980). What is missing is a conceptual framework that both closes the top-of-atmosphere energy budget (and hence by necessity considers the planet as a whole), but also includes clouds. A suitable candidate for such a framework would be an energy balance model (Budyko, 1969;Ghil, 1981;North & Kim, 2017;Sellers, 1969) that explicitly represents dynamic cloudiness, likely as an implicit function of circulation measures or other state variables.