Comparing Ship-Track Droplet Sizes Inferred from Terra and Aqua MODIS Data

In this study of ship tracks, Moderate Resolution Imaging Spectroradiometer (MODIS) measurements from late-morning (Terra) and early-afternoon (Aqua) Earth Observing System platforms are analyzed in five separate geographically distributed cases to compare estimates of the sizes (and their changes in time) of droplets associated with ship exhaust. Ship tracks are readily detected in near-infrared imagery as bright features, especially in 2.13-mm observations. The Terra ‘‘MOD06’’ and Aqua ‘‘MYD06’’ cloud products are used to determine the effective radius of the ship-track droplets; droplet age (time in the atmosphere) is estimated as a function of the distance from the ship. Terra and Aqua MODIS estimates of droplet sizes in ship-track plumes are found to be in agreement, with a correlation greater than 0.90; for the cases studied, droplet sizes in the ship plumes are between 6 and 18 mm. Moreover, the droplets’ size growth rates inferred from the length of the ship track were found to average between 0.5 and 1.0 mm h21.