Simulating Aerosol Impacts on Warm Low Clouds

Here, I introduce my recent peer-reviewed study in plain language.

Source: Professor Rob Wood's presentation

Background

Low marine clouds, typically Stratocumulus clouds, are the most widespread clouds on Earth, and they significantly affect the Earth’s radiation balance by strongly reflecting sunlight.

As Stratocumulus clouds are transported westward and equatorward by Trade winds, the warmer ocean water enhances surface latent heat fluxes, causing the formation of shallow cumulus clouds and the dissipation of Stratocumulus clouds. This process is called stratocumulus-to-cumulus transition.

Top: Sattelite image over Eastern Pacific Ocean obtained from NASA Worldview website. Bottom: diagram of transition taken from Albrecht’s article.

Challenges

1. Warm clouds are predominant over remote parts of subtropical oceans and therefore available reliable measurements are very rare.
2. It is very challenging for weather and climate models to accurately simulate the stratocumulus clouds and their transition, because of the complex set of physical mechanisms and feedbacks.
3. Aerosol-cloud interactions for warm low clouds are not well understood.

Solutions

1. Intensive observational field campaigns of warm clouds and their transition provides a valuable source of measurements. A recent field campaign, the Cloud System Evolution in the Trades (CSET), was conducted over the Northeast Pacific in the summer of 2015.
2. Large-Eddy Simulation (LES) is a useful tool for studying transitions due to its ability to resolve turbulence and cloud processes in the atmospheric boundary layer.
3. LES needs to interact with an aerosol model in order to include a treatment of the aerosol lifecycle to explore the aerosol-cloud-precipitation interactions.

What’s new?

A recent study addresses the aforementioned issues by using an LES model coupled with an aerosol model to study the response of clouds to initial and boundary aerosol perturbations in two well-observed case studies.

Top: Sattelite images over the Eastern Pacific Ocean downloaded from the NASA Worldview website. Bottom: A few observations include 2D surface temperature, pressure, and wind. The thin lines show the aircraft paths, whereas the circle markers show the evolution of cloud fraction. Taken from Erfani’s article.

A few results

The LES model is able to simulate the transition from stratocumulus to cumulus in both cases. The different timing of transition shows differing meteorological and aerosol conditions among the two case studies.

Evolution of simulated low clouds for two case studies. Taken from Erfani’s article.

Although the interactive aerosol model within the LES model adds new degrees of freedom, the simulated results agree well with satellite and aircraft observations.

Macrophysical cloud properties for the first case from the simulations and observations. as shown by time series of multiple variables. Taken from Erfani’s article.

A positive precipitation-aerosol feedback exists during the transition and it leads to the removal of aerosols from the atmosphere and the formation of ultra-clean layers near the top of low clouds. Also, precipitation regulates the cloud radiative forcing.

Left: snapshots of simulated precipitation and clouds for the first case study. Right: Vertical cross-sections of aerosols, clouds, and precipitation. Cross-sections are at the black lines shown on the left. Taken from Erfani’s article.

TL;DR

A new study shows that the modeling of low warm clouds in a Large-Eddy simulation (LES) coupled with an interactive aerosol model is essential to accurately understand aerosol-cloud-precipitation interactions.

References:

Albrecht, B. et al. (2019). Cloud System Evolution in the Trades — CSET Following the Evolution of Boundary Layer Cloud Systems with the NSF/NCAR GV. Bull. Amer. Meteor. Soc., 100, 93–121.

Erfani, E., Blossey, P., Wood, R., Mohrmann, J., Doherty, S. J., Wyant, M., & O, K. (2022). Simulating aerosol lifecycle impacts on the subtropical stratocumulus-to-cumulus transition using large-eddy simulations. Journal of Geophysical Research: Atmospheres, 127, e2022JD037258. https://doi.org/10.1029/2022JD037258

Wood, R. (2017). Challenges for Quantifying Climate Effects of Aerosol-Cloud Interactions in Warm Low Clouds, American Meteorological Society Annual Meeting, Seattle, WA, Jan. 2017.