A new field campaign at UC San Diego’s Scripps Institution of Oceanography will provide unprecedented four-season data on the properties of coastal marine clouds, including their radiative effects and the role human-made particles play.
Scientists behind the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) have already deployed instruments at the Ellen Browning Scripps Memorial Pier, which juts straight out over the sea, and on Mount Soledad, about a mile from the coast.
At an elevation of 250 meters (820 feet), Mount Soledad is high enough to allow collection of data from within the clouds themselves.
Cloud radars on Mount Soledad will have a clear view of the coast to the north and west. During cloud events there, instruments can also sample tiny cloud droplets, which are as much as a million times smaller than raindrops.
EPCAPE is a multi-institutional campaign, with 17 co-investigators from seven universities, three U.S. Department of Energy (DOE) national laboratories, NASA, and Environment and Climate Canada. The core instruments and support for EPCAPE are supplied by DOE’s Atmospheric Radiation Measurement (ARM) user facility.
ARM operates three fixed and three mobile atmospheric observatories in climate-critical regions of the world.
Scientists say it is crucial to understand marine stratocumulus clouds. At any given time, on average, thin decks of these bright, rounded, low-lying masses cover half of the world’s ocean surfaces. They are much brighter than the ocean surface they cover. That means marine stratocumulus clouds reflect more solar radiation than the ocean surface would, creating a net cooling effect. Their global ubiquity and cooling properties make such clouds an important influence on the earth’s energy balance.
EPCAPE data will help inform and validate the numerical models used to represent ocean-cloud processes.
EPCAPE’s principal investigator is atmospheric chemist Lynn Russell, a Scripps Oceanography professor of climate sciences.
The coast around La Jolla has plenty of offshore marine stratocumulus clouds to study. They interact with coastal land features such as Mount Soledad and persist overhead for more than six months a year.
Representative of other coastal cloud regimes in the world, stratocumulus formations around La Jolla are influenced by a large, dynamic range of aerosol concentrations and a variety of aerosol sources.
Atmospheric aerosols, Russell’s chief research interest, are tiny particles in the air that make cloud formation and precipitation possible. They can originate in nature, including as bits of soil or sea spray. Aerosols also originate as gases and soot from fossil-fuel burning or other human-caused emissions.
Russell has a special interest in the gray, misty marine clouds that hug urbanized coastlines, where the mix of aerosols is complex.
Despite the climate significance of marine stratocumulus, there are few long-term observational data on the properties of such clouds, which appear in regions with upwelling cold water from the deep ocean.
During the EPCAPE study, Russell and other aerosol researchers will team with cloud experts to build a year-long portrait of coastal stratocumulus cloud properties. They will investigate how cloud layers form and how related processes are affected by human-made particles. Such particles come from ports in the Los Angeles/Long Beach area to the north.
The EPCAPE team also aims to answer science questions related to cloud-aerosol climatology, cloud radiative fluxes, and aerosol-cloud interactions.
“The kind of clouds we have near San Diego―offshore that then often come ashore―are some of the most important” to climate, said Russell, since they “represent a large fraction of daytime cloud cover.”
From April through June 2023, the first of two EPCAPE intensive operational periods will focus on cloud chemistry, including cloud droplet sampling and investigations of aerosol formation. Historically, during this time of year, Mount Soledad is shrouded in cloud cover.
The second intensive operational period, on radiation, will take place from July through September 2023. Among other things, researchers will look for photochemical effects that contribute to aerosols and affect the cloud cover observed on Mount Soledad.
EPCAPE co-investigators said they are excited about the potential for observations that could provide new scientific insights, including:
- a better understanding of biases within the models that simulate coastal marine clouds
- more details on aerosol sources
- new perspectives on cloud drizzle and turbulence, and on how aerosol interactions influence solar-power resources
- more information on atmospheric circulations near coasts
- deeper knowledge of cloud chemistry.
EPCAPE will also convene many collaborations intended to meet its scientific objectives. These include projects supported by DOE’s Atmospheric System Research (ASR) to collect airborne and surface data on aerosol sizes and other properties.
Other researchers from UCLA and UC San Diego and North Carolina State University, funded by the National Science Foundation, will use instruments on Mount Soledad to investigate cloud chemistry.
Scientists from Dalhousie University and Environment and Climate Change Canada will pull in ground-instrument data on cloud droplets, volatile organic compounds, and aerosol absorption properties.
In June 2023, researchers will participate in airborne campaigns to study marine cloud chemistry and aerosol-cloud interactions upwind of EPCAPE.
Funders for these campaigns include the Office of Naval Research and ASR (for SCILLA, the Southern California Investigation of Low cloud and Land Aerosol) and NOAA (for AEROMMA, the Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas campaign).
To get the kind of cloud and aerosol data they need, Russell and her EPCAPE co-investigators will rely heavily on ARM instruments.
To get ready, ARM technicians set up an Aerosol Observing System in December 2022 at Scripps Pier. The system will collect data on aerosol particle size, concentration, size distribution, backscattered radiation, and absorption.
During the same setup phase, ARM emplaced lidars and radars near the middle of the pier.
Included were Doppler lidars, which among other functions measure drizzle in a way that separates it from other motions in the air. Meanwhile, a large, bowl-like ARM radar will profile the full vertical structure of clouds, from the top of the troposphere to a few hundred meters above the ground.
The radars, lidars, and aerosol instruments are all part of the first ARM Mobile Facility. This portable observatory also includes shelters and communications and data systems.
On Mount Soledad, cloud and precipitation instrumentation will take measurements from within cloud cover; other instruments will scan the pier site. The Mount Soledad installation also includes instruments designed to sample the chemical composition of in-cloud droplets.
In all, EPCAPE researchers will have access to a two-site, multidimensional view of cloud and aerosol activity across all seasons.
From these two complementary observation points, “the radars will have a really good view of marine conditions out on the pier and a good scanning view with the Soledad site,” said Russell. “The combination will allow for better retrievals of cloud properties than have been possible before.”
In 2005, ARM conducted a seven-month field campaign at Point Reyes, California, just north of San Francisco. Marine Stratus Radiation Aerosol and Drizzle (MASRAD) included the first deployment of an ARM Mobile Facility.
MASRAD’s lead scientist was Mark Miller of Rutgers University. Eighteen years later, Miller returns to similar science questions as an EPCAPE co-investigator.
“I’m thrilled to be part of a mission I believe in,” said Miller, “and people I really like working with.”
Russell has had a longtime interest in coastal stratocumulus, “one of the things we’re just starting to get a good grasp of,” she said.
Over the open ocean, Russell said there is a “fairly well-behaved boundary layer” because the ocean surface is homogenous over hundreds of kilometers.
“Then you hit the coast,” she said, where cloud and aerosol properties become more complex because of coastal land features and urban air influences.
To illustrate the radical change in conditions from the open ocean to the coast, Russell points out that historical levels of carbon dioxide around the Scripps Pier show “clean marine” conditions only about five percent of the time.
Going back more than a decade, Russell was part of other efforts to collect data on coastal cloud-aerosol properties in the eastern north Pacific. For instance, she designed and led three aerosol composition experiments on the same pier EPCAPE will use.
A May 2010 study looked at the wintertime prevalence of submicron organic particles linked to inland urban emissions. A paper in November 2010 used data on particles transported downwind from wildfires in Northern California. A third study, in August 2011, investigated ozone-related particles associated with fossil-fuel combustion.
All three Scripps Pier studies—based on three data sets from 2008 and 2009—were warmups for EPCAPE.
Russell’s vision for the campaign is that the study will combine two things: what she calls ARM’s “amazingly comprehensive view of coastal stratocumulus clouds” and a new, more precise chemical characterization of aerosol and cloud droplets affected by pollution from urban centers.
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Adapted from ARM user facility
ARM is a DOE Office of Science user facility operated by nine DOE national laboratories.
About Scripps Oceanography
Scripps Institution of Oceanography at the University of California San Diego is one of the world’s most important centers for global earth science research and education. In its second century of discovery, Scripps scientists work to understand and protect the planet, and investigate our oceans, Earth, and atmosphere to find solutions to our greatest environmental challenges. Scripps offers unparalleled education and training for the next generation of scientific and environmental leaders through its undergraduate, master’s and doctoral programs. The institution also operates a fleet of four oceanographic research vessels, and is home to Birch Aquarium at Scripps, the public exploration center that welcomes 500,000 visitors each year.
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