MODIS observations of ship tracks in summertime stratus off the west coast of the United States

Three years of I-km Moderate Resolution Imaging Spectroradiometer (MODIS) observations for morning (Terra) and afternoon (Aqua) summertime marine stratus off the west coast of the United States were analyzed to determine the response of the clouds to aerosols in the plumes of underlying ships. The polluted clouds were identified in pixels with elevated 2.1-mu m reflectivities and 3.7-mu m radiances when compared with nearby pixels that contained unpolluted clouds. An automated routine was developed to identify which of the pixels contained clouds polluted by underlying ships and then to select unpolluted control pixels on both sides of the ship track as near as was feasible to the polluted clouds, often with a separation of as little as 1 pixel. While no requirements were placed on the cloud cover fraction-within the selected pixels, to facilitate comparisons with an earlier study based on 1-km Advanced Very High Resolution Radiometer (AVHRR) imagery, the results presented here are restricted to 20-pixel segments along the ship tracks in which all of the pixels identified as polluted and all of the pixels selected as controls on both sides of the tracks were found to be overcast. The pollution caused about a 20% reduction in droplet radius in both the MODIS and the AVHRR observations. In the MODIS observations, the increase in optical depth was about 15% and statistically significant at a confidence level greater than 99.5%. These changes were much larger than the 2% and statistically negligible increases found for the AVHRR observations. Like the AVHRR observations, the MODIS observations indicated that the majority of clouds (60%) lost liquid water when polluted, but consistent with the different outcomes for the optical depths, the losses for the MODIS observations were about half the nearly 20% loss found in the AVHRR observations. Cloud altitudes were unaffected by the pollution for both the MODIS and the AVHRR observations. For both morning and afternoon clouds, those with the largest increases in albedo in response to the pollution also showed the largest increases in cloud liquid water amount. The changes in droplet radii and liquid water amounts retrieved using the MODIS observations suggested that droplet growth was suppressed in the polluted clouds and that the rate of growth from cloud base to cloud top was also reduced when compared with nearby unpolluted clouds.