For Critical Marine Low Clouds, a Research and Observation Plan

Newswise — Marine low clouds hover in the lowest couple of kilometers above the world’s oceans. They produce little but drizzle, and could never match their deeper mid-continent cousin clouds for dramatic weather and severe storms. But marine low clouds are vastly important to the world’s climate and energy balance. They are a major determinant of the Earth’s albedo, a measure of the amount of solar energy reflected from the Earth back into space. And they also have a big impact on global energy and hydrologic cycles.

Still, there remain gaps in our understanding of the processes that regulate marine clouds, such that they are a challenge to properly represent in climate models, and remain a source of the greatest uncertainties in these simulations of future climate states. They are subject to complex processes that control their coverage and condensate loading and that determine their microphysical and radiative properties. Most marine clouds are also remote, which adds significant observational challenges.

All this importance and all these challenges add up to why marine low clouds received major scientific attention in 2016: a January workshop at Brookhaven National Laboratory, sponsored by the U.S. Department of Energy’s Atmospheric System Research (ASR) Program; an associated June workshop report; and a meeting summary in the September issue of the Bulletin of the American Meteorological Society.

In all three cases, in varying degrees of detail, the same imperative was laid out: Develop an action plan for the next five to 15 years in order to close the gaps in our current knowledge and to improve simulation capabilities. That means improving our understanding of the key processes that make and alter marine low clouds and improving ways of quantifying them.

Four research themes emerged: the interactions of aerosols and cloud droplets, precipitation rates, entrainment (how cloudy and clear air mix), and cloud organization on scales of 5 to 100 kilometers.

These research themes emphasized the importance of better representing these clouds in climate models. Workshop participants and report authors pointed to the contributions that will be made by ASR scientists and the Atmospheric Radiation Measurement (ARM) Climate Research Facility.

Among the research needs, one recurrent theme emerged: the need for measurements at both long-term sites, such as ARM’s Eastern North Atlantic observation facility, and the need for measurements from aircraft. It is thought that airborne instrument platforms could fill in data gaps related to aerosol composition, particle size, and variations in cloud fields and trace gases.

To meet this need, the ARM Facility will be conducting the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) in June 2017, which is being led by ASR researcher Jian Wang of Brookhaven National Laboratory.