Aerosol Particles from Arctic “Frost Flowers.”

Summary

Aerosol particles in the atmosphere can impact cloud formation, the number of droplets in a cloud, and radiative impacts of the clouds. In the Arctic, sea salt particles from the ocean are an important source of aerosols and are included in climate model simulations. During Arctic winters, there is much less open water due to sea ice formation, so models assume much smaller concentrations of aerosol particles from sea salt. However, recent studies have shown that “frost flowers” may be an important source of aerosol particles during winter and early spring in the Arctic. Frost flowers are clusters of salty ice crystals growing on newly formed sea ice or frozen lakes. They wick brine from the surface of the sea ice, resulting in a high saline content. As the frost flowers sublime and blow away in the wind, they contribute to aerosol concentrations in the atmosphere. Current climate models do not include this source of aerosol particles.

To examine how the neglect of aerosol particles from frost flowers impacts model simulations, U.S. Department of Energy scientists developed the first observationally based parameterization to include estimates of sea salt production from frost flowers. The researchers found that the particle flux from frost flowers can be quite large, but is highly localized to regions of new sea ice formation and highly dependent on wind speed. The new parameterization was implemented into a regional model to examine the impact of this missing aerosol source on cloud properties. The simulations showed that the addition of sea salt aerosol emissions from frost flowers increases averaged sea salt aerosol number concentrations (improving agreement with observations) and subsequent cloud droplet numbers. This change of cloud droplet number concentration increased downward longwave, cloud radiative forcing through enhanced cloud optical depth and emissivity. However, the effect of sea salt aerosols from frost flowers contributed only a small amount to surface warming in Barrow, Alaska, in the model scenarios studied.  Although the new parameterization showed only small effects on average surface warming, the model studies conducted here did not include coupling to the cryospheric system, which may be important. The developed parameterization can be used in coupled Earth system models to explore these interactions between sea ice formation, aerosol production, and cloud properties.

References

Xu, L., L. M. Russell, R. C. Somerville, and P. K. Quinn. 2013. “Frost Flower Aerosol Effects on Arctic Wintertime Longwave Cloud Radiative Forcing,” Journal of Geophysical Research – Atmospheres 118(23), 13,282-291. DOI:10.1002/2013JD020554.