Effects of Additional Particles on Already Polluted Marine Stratus

The response of already polluted marine stratocumulus to additional particles was examined by studying the clouds where two ship tracks cross. Nearly 100 such crossings were collected and analyzed using Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) multispectral imagery for the daytime passes off the western coast of the United States during the summer months of 4 years. To reduce biases in the retrieved cloud properties caused by the subpixel spatial structure of the clouds, results are presented only for ship tracks found in regions overcast by extensive layers of marine stratus. When two ship tracks cross, one of the tracks exhibits much larger changes in droplet radii when compared with the surrounding unpolluted clouds and is referred to as the dominant ship track. The clouds at the crossing typically exhibit properties that are closer to those of the dominant than to those of the subordinate ship 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 smaller droplet radii and significantly larger column droplet number concentrations than were predicted based on their values in both ship tracks on either side of the crossing. Comparing the effects of particle loading at the crossings and elsewhere along the ship tracks revealed that the effects decreased as the column droplet number concentration of the clouds being affected increased.