Particles in the atmosphere play a key role in cloud formation, acting as nuclei for water droplets. Clouds play an important role in absorbing and reflecting heat, hence they can potentially mitigate or exacerbate global warming. Aerosol-cloud radiative interactions are widely held to be the largest single source of uncertainty in climate model projections of future climate change due to increasing anthropogenic emissions (IPCC, 2007). The underlying causes of this uncertainty among modeled predictions of climate are the gaps in our fundamental understanding of cloud processes. There has been significant progress with both observations and models on these important questions. However, the quantitative representation of these processes is nontrivial and limits our ability to represent them in global climate models (GCMs), resulting in the largest uncertainties in predictions of future climate. Given the timeliness of these questions for advancing GCMs, it is essential to address the unanswered questions in cloud dynamical response to aerosol perturbations.
Intellectual merit. This research is a targeted aircraft campaign with embedded modeling studies to inform the experiment planning and to facilitate the interpretation of the results. The study will use the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft in July 2011 off the coast of Monterey, California, with a full payload of instruments to measure particle and cloud number, mass and composition distributions. The research is composed of three novel and important additional, climate-focused studies: 1. Controlled release and atmospheric distribution of three different size ranges of particles in flight and on or by a dedicated ship; 2. Large Eddy Simulations and Aerosol-Cloud Parcel modeling studies constrained by the observations to test our ability to quantitatively predict the dynamical response to increases in particle concentrations in the natural atmosphere; 3. Satellite analyses of marine stratocumulus to constrain the radiative properties of the natural, perturbed, and regional cloud systems.
Broader impacts. The broader scientific impacts of the research will be the improved understanding of fundamental aerosol-cloud processes that can be incorporated in global climate models to better inform decision makers. The broader educational impacts of the research will be realized through: (1) Promotion of teaching, training and learning through development and piloting of an informal science education program targeting an underserved audience; (2) Broadened participation of underrepresented groups - in this case, retired and elderly people - in research as well as in outreach; (3) Enhancement of infrastructure for teaching through partnerships with an established educational organization (Osher Lifelong Learning Institute); (4) Broad dissemination of results through presentations, peer-reviewed publications and via the web; and (5) Societal benefits in terms of improved understanding of climate science and the related ethical issues.
That the Earth is warming as a result of emissions of CO2 to the atmosphere is unequivocal. Earth’s clouds provide a cooling mechanism on climate by reflecting sunlight back to space. Clouds form on particles in the air, and it is well established that more numerous airborne particles lead to clouds with more numerous droplets. Clouds with more numerous droplets, all else being equal, reflect more sunlight back to space and cool the Earth even more. Along with CO2, the concentration of particles in the air has increased since preindustrial time. This increased level of airborne particles is thought to lead to a larger cooling component that offsets a portion of the CO2-induced warming. The actual amount of cloud cooling that has resulted from the increase in airborne particle levels since preindustrial time is quite uncertain because actual particle levels at that time are unknown. Still, a leading scientific problem associated with climate is the actual physical mechanisms by which changing aerosol levels impact clouds. Marine stratocumulus clouds, the low-level clouds found over most of the Earth’s oceans, provide, of all Earth’s clouds, the greatest amount of climate cooling, so these clouds have received the most attention in terms of how they respond to changes in airborne particle levels. Aircraft measurement studies have been carried out over areas of the ocean where stratocumulus clouds are prevalent and where particle levels vary due to both marine and anthropogenic influence. In studies of this type, the aircraft must seek areas where the clouds are influenced by varying aerosol levels. In the NSF project, Collaborative Research: Using Controlled Aerosol Perturbations to Improve Understanding of Cloud Responses for Climate, Award Numbers AGS-1013423, AGS-1013381, AGS-1008848, and AGS-1013319, a team of researchers from the University of California, San Diego Scripps Institution of Oceanography, the California Institute of Technology, the University of Arizona, the University of Miami, the Georgia Institute of Technology, and the Naval Postgraduate School employed the Research/Vessel Point Sur to serve as a platform for emitting well-characterized organic smoke to produce a uniquely identifiable cloud signature. The project, call the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE), carried out in July and August 2011 off the coast of California, also measured the effect on clouds of exhaust from container ships transiting across the study region as well as salt particles released from the aircraft on which the airborne particle and cloud measurements were being made. The investigators quantified the effect of the tailor-made particles, as well as those from the exhaust of container ships, on the properties of the overlying clouds and compared the measured effect with that predicted by cloud physics models. This collaboration has contributed to 14 peer-reviewed publications (10 accepted as of 9/15/14, and 4 additional papers submitted or in preparation) by the four participating investigators that helped improve the representation of aerosol-cloud behavior in climate models. Because making the Earth cloudier is one way to offset CO2-induced global warming, it has been proposed that fleets of ships emitting sea spray into the air could intentionally thicken marine clouds, in activities termed climate engineering or geo-engineering. While the present NSF project was not specifically directed at geo-engineering, data from it could be useful if geo-engineering by brightening of marine clouds were to be seriously considered.
