The aerosol-cold pool connection: impacts on marine low cloud morphology

Low clouds over the subtropical oceans cool global climate substantially. The magnitude of this cooling strongly depends on the cloud coverage, which in turn depends on the spatial organization of clouds into closed cell high clouds and open cell low clouds. Beyond their instantaneous radiative effects, an emerging body of work on climate feedbacks

climate feedback

Processes that can either amplify or reduce the effects of climate drivers (e.g., solar radiation; greenhouse gas emissions; and aerosols, dust, smoke, and soot). A feedback that increases an initial warming is called a "positive feedback." A feedback that reduces an initial warming and then returns the system to it's original state is a "negative feedback."

Source: NASA Global Climate Change Source

shows that closed cell stratocumulus clouds are much more sensitive to future warming than open cells and shallow cumulus. This implies that it is critical to accurately represent the physical processes that determine cloud morphology 

morphology

The science of the form and structure of organisms (plants, animals, and other forms of life).

Source: National Cancer Institute Source

such that we can confidently assess cloud feedbacks on future climate change.

climate change

A change in the average conditions — such as temperature and rainfall — in a region over a long period of time.

Source: NASA Climate Kids Source

The transition from closed to open cellular modes is primarily dictated by gradients in environmental parameters such as sea surface temperature (SST), which controls surface flux 

flux

The amount of some type of particle or energy crossing a unit area per unit time.

Source: NRC Library Source

magnitude, and lower tropospheric stability, which impacts the ability of clouds to grow upward and interact with the overlying free troposphere. The traditional view is that buoyancy generated at the surface by a warming SST induces more vigorous updrafts, which slowly erode the stable layer capping the clouds. But the onset of precipitation, which is modulated by background aerosol

aerosols

Small particles or droplets, such as wildfire smoke, volcanic gases, or salty sea spray, that float in the air. They are emitted by both natural events and human activities.

Source: NASA Global Climate Change Source

loading, introduces a new physical pathway for the shift to surface-driven  convection.

convection

The rising of warm air and the sinking of cool air. Convection can cause local breezes, winds, and thunderstorms.

Source: NASA earth observatory Source

Evaporating rain below cloud base causes local cooling which further accelerates rain-laden downdrafts. As these downdrafts near the surface, they diverge and cause gust fronts known as cold pools. Collisions of cold pools mechanically induce new updrafts that then generate new precipitation, which removes more aerosol, generating stronger precipitation and more intense cold pools, finally catalyzing the transition away from closed cell cloud organization.

Recent studies have shown that a model must include some representation of aerosol removal by precipitation to simulate this physical pathway as well as a mechanism to generate cold pools. While fine-scale models can represent these complicated aerosol-precipitation-dynamics feedbacks, global circulation models cannot. Thus, despite the modeling evidence presented to date, it is unclear whether aerosol impacts on convective organization are globally important or tractable from a global modeling perspective. This project therefore proposes a set of experiments to:

(1) identify the importance of aerosol-cold pool feedbacks on long-term observations of transition cases at the Atmospheric Radiation Measurement program Eastern North Atlantic (ENA) site; and

(2) determine the minimum complexity required to represent the relevant physical mechanisms in coarse-gridded atmospheric models using idealized simulations and realistic case studies at ENA.

These experiments will lead to improved understanding of feedbacks among aerosols, clouds and convective processes as well as improved simulation of the radiative effects of organized shallow convection. The first objective will be accomplished by analyzing observations to understand the synoptic and aerosol conditions associated with low cloud transitions and observable cold pools at ENA, while the second objective will involve a set of idealized and observationally-forced large eddy simulations using a "process denial" approach.