Cumulus Parameterization: Those Who Can Remember the Past Are Condemned to Repeat It

Abstract

Moist convection plays a leading role in the dynamics and energy budget of Earth's tropics and influences the sensitivity of Earth's climate to greenhouse gas increases. Because individual convective cells are much smaller than the gridboxes of 3-dimensional global climate models (GCMs), these models parameterize the effects of an ensemble of moist convective updrafts and downdrafts on the environment. Cumulus parameterization has been a focus of the terrestrial meteorology community for half a century. Only in past decade, however, have GCMs with moist convective physics been applied to other planets. Given our lack of detailed knowledge about convective clouds except on Earth, planetary GCMs are often designed with very simple approaches to cumulus parameterization, adopted from the earliest generations of terrestrial GCMs. These parameterizations were based on breakthroughs in understanding of convection in their time. However, at the same time that planetary GCMs have begun to emerge, a quiet revolution in how we think about terrestrial convection has started to influence the design of terrestrial GCMs. In this paper we review how some of the assumptions in the classical cumulus parameterizations used in planetary GCMs (and still in some terrestrial GCMs) have given way in recent years to a deeper understanding of how convection operates and why it matters for atmospheric dynamics and climate - on any planet.