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

Active Dates 8/1/2022-7/31/2025
Program Area Atmospheric System Research
Project Description
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 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  such that we can confidently assess cloud feedbacks on future climate change.

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  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 loading, introduces a new physical pathway for the shift to surface-driven  convection. 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.
Award Recipient(s)
  • University of California, Los Angeles (PI: Smalley, Mark)
  • Naval Postgraduate School (PI: Witte, Mikael)