28 Nov 2023

Coastlines: Unsung heroes in Hawaiʻi’s rainfall creation

Tiny aerosols, like sea salt, dust and ash, might be minuscule, but they play a giant role in shaping weather and climate. A new study from the University of Hawaiʻi at Mānoa takes a deeper dive into a common aerosol in Hawaiʻi, giant sea salt aerosol, revealing that coastal areas can release up to five times more particles than the open ocean.

Why does it matter? These particles impact cloud formation and rainfall, making them key players in Hawaiʻi’s weather game.

“Aerosol-cloud interactions and rainfall are among the biggest uncertainties in projections of future climate,” said Katherine Ackerman, lead author of the study and atmospheric sciences doctoral candidate in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST). “Because freshwater is essential to the sustainability of life on the Hawaiian Islands, it is imperative to accurately predict where and how much it will rain as our climate changes.”

Two instrument measure aerosols off the R/V Kilo Moana. (Photo credit: K. Ackerman)

“The day-to-day rain from cumulus clouds is critical to replenishing our fresh groundwater,” added Ackerman. “But the transition from cloud to raining cloud is still poorly understood, and therefore difficult to predict. We hope this research helps improve predictions of rain generated by Hawaiʻi’s tradewind cumulus clouds so that we can better prepare Hawaiʻi in this changing world.”

Alison Nugent, associate professor of atmospheric sciences and senior author on the study, and her team developed a lightweight and affordable 3D-printed instrument that measures aerosol size distributions rather than only bulk aerosol concentration or mass, like many other methods of sampling. They attached the device to kites, fishing rods and drones to sample how giant sea salt size distributions vary across different altitudes, locations from the coastline, and a wide variety of environmental conditions.

Improving weather prediction

This finding can be used to improve numerical weather prediction of nearshore cloud formation and rainfall patterns across the Hawaiian islands.

“Regular in situ (in the original place) observation of clouds is difficult because of the effort involved with reaching them,” said Nugent. “So instead, much of our current research has focused on the giant sea salt particles that Hawaiʻi’s clouds form on, and understanding the specific mechanisms that influence how these particles are produced, and where they are able to travel after production.”

Waves breaking on the shoreline produce a mist made up of water droplets and giant sea salt aerosol. Those particles get carried towards the mountains by on-shore winds.

“Anyone who goes to the beach in Hawaiʻi has probably seen these processes in action,” said Ackerman. “Even when you can’t see them anymore, the particles are still there, moving with the wind, and eventually up to the clouds.”

How waves shape the sky

“We identified the coast as a major source of giant sea salt particles which is important in understanding how they impact our coastal clouds,” said Ackerman. “Their large size means they fall out of the atmosphere a lot faster than other aerosol types, so identifying their production locations is really important to quantifying their potential impacts on the local atmosphere.”

The researchers also found that wave heights were the biggest predictor of giant sea salt aerosol concentrations.

“This observation feels intuitive—larger waves breaking along the shore will produce more ocean mist,” said Ackerman. “However, most computer models have been using wind speeds to predict sea salt aerosol concentrations over the open ocean, and almost none recognize coastlines as an additional source.”