Response of marine stratocumulus to haze pollution as deduced from MODIS observations of ship tracks

1-km MODIS observations of ship tracks off the west coast of the U.S. are used to characterize changes in droplet effective radius, cloud visible optical depth, pixel-scale cloud cover fraction, cloud altitude, and cloud liquid water column amount as low-level marine clouds respond to particle pollution from underlying ships. Earlier studies, based on 1-km AVHRR observations, showed that for pixels overcast by marine stratus, polluted clouds averaged 15 – 20% less liquid water than the nearby unpolluted clouds. In addition to using MODIS instead of AVHRR radiances, this new study uses a retrieval scheme that accounts for the effects of partial cloudiness within the 1-km pixels on the retrieved cloud properties. The new study also employs an improved automated scheme for identifying clouds polluted by underlying ships that allows unpolluted clouds to be identified within a couple of kilometers of the polluted clouds. Results obtained with the partly cloudy pixel retrieval scheme show that the pixel-scale cloud cover within the ship track is almost always greater than that found in the surrounding region containing the unpolluted clouds. In addition, when restricted to overcast pixels, as in earlier studies, results from the Terra MODIS indicate that cloud liquid water amount is 10-15% smaller for the polluted clouds than for the nearby uncontaminated clouds. The liquid water amount is smaller for the polluted clouds regardless of which MODIS channel is used to retrieve droplet radius. Results from both the Terra and Aqua MODIS instruments, and for the sensitivity of polluted clouds to additional particle loading, as occurs when ship tracks cross, will be presented.