Response of polluted marine stratocumulus to additional particle pollution

Ship tracks have proven to be an invaluable laboratory for studying the response of marine stratocumulus to the increase in particles from ships passing beneath the clouds. Here the response of already polluted marine stratocumulus to additional particles is examined by studying the clouds where two ship tracks cross. Nearly one hundred such crossings were collected and analyzed using Terra and Aqua MODIS multispectral imagery for the daytime passes off the west coast of the U.S during summer months. In order to reduce biases in the retrieved cloud properties caused by subpixel spatial structure within the clouds, the results have been limited to ship tracks found in regions overcast by extensive layers of marine stratus. Of the two ship tracks that cross, one track exhibits much larger changes in droplet radii when compared with the surrounding unpolluted clouds. This track appears to be relatively fresh and typically narrow, sometimes only a few kilometers in width. For this track, the crossing is relatively close to the position of the ship. The second ship track appears to be relatively old and wide, with widths typically on the order of 10 km. For this track, the crossing is often much further from the ship than is the case for the dominant track. The clouds at the crossing generally exhibit properties that are closer to those of the dominant, fresher track than to those of the subordinate, older track. To determine whether the additional particles at the crossing affect the dominant track, local gradients in the retrieved cloud properties near the crossing were determined for both ship tracks. Based on the gradients, the clouds at the junction were found to have significantly larger optical depths than would be predicted based on the optical depths of the clouds in both ship tracks on either side of the crossing. The droplet radii for both ship tracks were reduced at the junction but the reduction for the dominant track was not statistically significant at the 95% confidence level. In addition, the liquid water paths of the clouds at the junction were increased for both tracks but the increases were statistically significant for neither of the ship tracks. The lack of statistical significance might be remedied through the collection of a much larger ensemble of crossings, but several hundred additional crossings would be required. The results suggest that the droplet concentrations generally increase when additional particles are added to marine stratocumulus that have already responded to enhanced particle loading. Comparing the response of the clouds to particle loading at the crossing and elsewhere along the ship tracks reveals that, as expected, the response is diminished for clouds that have already responded to increased particle concentrations.