PAOC Colloquium: Dan Cziczo (MIT)

BETTER UNDERSTANDING CLIMATE, PRECIPITATION & ATMOSPHERIC CHEMISTRY BY UNDERSTANDING THE FORMATION OF MIXED PHASE CLOUDS

Speaker: Dan Cziczo, Associate Professor of Atmospheric Chemistry

Field and laboratory measurements using cloud chambers have been used to understand the atmospheric abundance of droplet and ice nucleating particles and to, in turn, construct parameterizations for mixed-phase and completely glaciated clouds in weather and climate models. This seminar investigates the measurement of droplet and cloud concentration and sizing using cloud chambers. We show here that assumptions of ideal instrument behavior are not supported by measurements. Non-ideal instrument behavior, which is likely inherent to varying degrees in all cloud chambers, is caused by exposure of particles to different humidities and/or temperatures than predicated from theory. This can result in a systematic, and variable, underestimation of reported cloud element concentrations. We further investigate the effects of undercounting on simulated cloud forcing in a global climate model. For ice clouds in particular, we find that simulated long wave iceboating cloud forcing in a global climate model can vary up to 0.8 W/m2 and can change sign from positive to negative within the experimentally constrained bias range. We’ll conclude with a discussion of possible instrument improvements and how these can improve our understanding of climate, precipitation and atmospheric chemistry.

About the Speaker

It has been known for over a century that greenhouse gases such as carbon dioxide and methane warm the planet by trapping heat. What is not as well known is that particles can cool the planet by reflecting sunlight into space and by acting as the seeds on which clouds form. Particles and clouds are also of contemporary interest because it has been suggested they might affect climate by interacting with cosmic rays or be used to manipulate the Earth’s temperature.

Cziczo's research group is interested in the interrelationship of particulate matter and cloud formation. His team utilizes laboratory and field studies to elucidate how small particles interact with water vapor to form droplets and ice crystals, which are important players in the Earth’s climate system. Experiments include using small cloud chambers in the laboratory to mimic atmospheric conditions that lead to cloud formation and observing clouds in situ from remote mountaintop sites or through the use of research aircraft.

The PAOC Colloquium is a weekly interdisciplinary seminar series that brings together the whole PAOC community. Seminar topics include all research concerning the physics, chemistry, and biology of the atmospheres, oceans and climate, but also talks about e.g. societal impacts of climatic processes. The seminars generally take place on Monday from 12-1pm. Lunch is provided to encourage students and post-docs to meet with the speaker. Besides the seminar and lunch, individual meetings with professors, post-docs, and students are arranged.