Collaborative Research: Process Mechanics of Cloudiness Transitions in Subtropical Marine Boundary Layers

Clouds are one of the key components of the climate system because they influence the amount of sunlight that reaches the Earth’s surface and the amount of energy that the Earth radiates back to space. The most prominent clouds on the planet are thin layered clouds in the lower atmosphere over the oceans, which are known collectively as marine stratocumulus. The climate system is particularly sensitive to the coverage of marine stratocumulus clouds because they reflect almost all the incoming sunlight that would otherwise reach and warm the ocean surface. As marine stratocumuli in the mid-latitudes move south toward the tropics, the ocean surface beneath them warms and they undergo a structural transition that leads to breaks in the clouds. The location, and the details of this transition significantly impact the over-ocean energy budget. While some of the physical mechanisms that determine when and how the marine stratocumulus evolves into a more broken state are known, weather forecast and climate models, which are of societal importance, do not accurately reproduce these cloud structural transitions. This is primarily because the complicated combination of physical processes that produce these transitions are not fully understood. Marine stratocumulus transitions over the Eastern North Atlantic (ENA) are of particular importance because cloud cover over this region has decreased over the past 30-years and the ENA lies downstream of a rapidly warming Arctic. In addition, ocean circulations in the region are known to be sensitive to the input of meltwater as Arctic ice coverage declines. To facilitate a deeper understanding of processes and interactions responsible for marine stratocumulus transitions, the focus of this research is to investigate and understand the shifting balance of driving forces in various transition stages. These stages include transitions between patches of single layer stratocumulus, patches of cumulus, and hybrid regions containing coexisting stratocumulus and cumulus, a structure often referred to as “cumulus-coupled” stratocumulus. Cumulus-coupled stratocumulus present in two configurations: one in which small, random cumulus rise into stratocumulus and another in which the cumulus exhibits extensive mesoscale organization covering tens of kilometers, a configuration referred to as Marine Boundary Layer Convective Complexes (MBLCC). This project capitalizes on modern high-resolution computer simulations and a newly developed causal framework in which key drivers for stratocumulus cloud transitions will be used to build causal webs, illustrating the pathways of underlying processes and interactions. This new framework works for nonlinear systems, and importantly, allows multiple variables to work in concert, beyond concentrating only on independent influences of variables. Research results from this project are expected to address current shortcomings in model representations of marine stratocumulus in models of all types and to provide a new mantra for diagnosing interactions in other atmospheric systems. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.