A recent paper by Hans Mohrmann, a graduate student in the Department of Atmospheric Sciences, takes a look at the cause of seasonal changes in tiny atmospheric particles called aerosols that can form cloud droplets over the northeast Pacific Ocean between Hawai’i and California. Understanding the interactions between these aerosol particles and clouds is important because of  how clouds may change in response to different aerosol emissions is one of the largest sources of uncertainty in measuring how much human activities are changing the climate.

Published in the Journal of Geophysical Research: Atmospheres, Hans’s research makes use of observations from a container ship sailing between Hawai’i and California during a field campaign called MAGIC, which is a veritable matryoshka doll of acronyms: Marine ARM (Atmospheric Radiation Measurement, program of the U.S. Department of Energy) GPCI (GCSS (GEWEX (Global Energy and Water Cycle Exchanges) Cloud System Study) Pacific Cross-section Intercomparison) Investigation of Clouds. Estimates of aerosols from above the clouds that eventually get mixed down, or “entrained,” into the cloudy layer of air just above the ocean come from another field campaign, this one involving aircraft flying between Sacramento, California, and Kona, Hawai’i. Hans participated in this campaign, called the Cloud System Evolution in the Trades (CSET) study, helping to plan flight paths and set the plane’s altitude in-flight.

Hans and his coauthors conclude that it’s not the source of aerosols that matters most when thinking about summer-winter differences, but the sink, or loss process, due to precipitation. Different patterns of aerosol transport from North America (a source term) are the next-most important factor, although previous work had focused on this transport as the primary driver of summer-winter differences. The finding that precipitation plays the primary role in the seasonal cycle of aerosol over the northeast Pacific will help inform and improve future modeling studies of this region and similar areas across the globe.