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Microphysical and Macrophysical Responses of Marine Stratocumulus Polluted by Underlying Ships

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May 1, 2012
Matt Christensen
Hosted by Graeme Stephens (advisor), Sue van den Heever, Chris Kummerow, Steven Reising (Electrical and Computer Engineering)


Ship tracks serve as a distinct manifestation of the radiative response due to the interaction of aerosols and clouds. Exhaust plumes from underlying ships strongly influence cloud microphysics (effective radius) and, to a largely undetermined extent, cloud macrophysics (e.g., liquid water path, cloud coverage, cloud depth, precipitation, and longevity). Because cloud macrophysics strongly influences the albedo of clouds, the factors that control these properties were examined using multiple sensors from the A-train constellation of satellites. The locations of over one thousand ship tracks were meticulously logged by hand using the Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. While a decrease in droplet size was commonly observed in ship tracks, cloud macrophysical properties rarely remained the same between the polluted and nearby unpolluted clouds. Ship plumes ingested into a broken field of open cell stratocumulus often resulted in significantly deeper and brighter clouds with higher liquid water amounts and rain rates. The opposite was generally true when the plumes interacted with an overcast deck of closed cell stratus. One implication for these differences is that the local aerosol indirect radiative forcing was more than five times larger when the clouds resembled an open cellular structure (-59 W m-2) compared to when the cloud field was composed of closed cells (-12 W m-2).

In the final part, the focus shifted to the climate scale to examine the impact of shipping on the Earth’s radiation budget. Two studies were employed, in the first; changes to the radiative properties of low-clouds were examined in response to the substantial decreases in ship traffic that resulted from the recent world economic recession in 2008. Differences in the cloud properties between 2007 and 2009 did not manifest as a clear response in the climate system and was probably masked either due to competing aerosol cloud feedbacks or by interannual climate variability. In the second study, a method was developed to estimate the radiative forcing from shipping by convolving lanes of densely populated ships onto the global distributions of closed and open cell stratocumulus clouds. Closed cells were observed more than twice as often as open cells. Despite the smaller abundance of open cells, a significant portion of the radiative forcing from shipping was claimed by this regime due to the highly susceptible nature of these clouds to changes in aerosol concentration. On the whole, the global radiative forcing from ship tracks was small (approximately -0.45 mW m-2) compared to the radiative forcing associated with the atmospheric buildup of anthropogenic CO2.